path: root/source4/heimdal/doc/standardisation/draft-yu-krb-wg-kerberos-extensions-02.txt

INTERNET-DRAFT                                                    Tom Yu
draft-yu-krb-wg-kerberos-extensions-02.txt                           MIT
Expires: 25 April 2005                                   25 October 2004


        The Kerberos Network Authentication Service (Version 5)


Status of This Memo


   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.


   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.


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Copyright Notice


   Copyright (C) The Internet Society (2004).  All Rights Reserved.


Abstract


   This document describes version 5 of the Kerberos network
   authentication protocol.  It describes a framework to allow for
   extensions to be made to the protocol without creating
   interoperability problems.


Key Words for Requirements


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", and "MAY" in this document are
   to be interpreted as described in RFC 2119.





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Table of Contents


   Status of This Memo ..............................................  1
   Copyright Notice .................................................  1
   Abstract .........................................................  1
   Key Words for Requirements .......................................  1
   Table of Contents ................................................  2
   1.  Introduction .................................................  5
   1.1.  Kerberos Protocol Overview .................................  5
   1.2.  Document Organization ......................................  6
   2.  Compatibility Considerations .................................  6
   2.1.  Extensibility ..............................................  7
   2.2.  Compatibility with RFC 1510 ................................  7
   2.3.  Backwards Compatibility ....................................  7
   2.4.  1.4.2. Sending Extensible Messages .........................  8
   2.5.  Criticality ................................................  8
   2.6.  Authenticating Cleartext Portions of Messages ..............  9
   2.7.  Capability Negotiation ..................................... 10
   3.  Use of ASN.1 in Kerberos ..................................... 10
   3.1.  Module Header .............................................. 11
   3.2.  Top-Level Type ............................................. 11
   3.3.  Previously Unused ASN.1 Notation (informative) ............. 12
   3.3.1.  Parameterized Types ...................................... 12
   3.3.2.  COMPONENTS OF Notation ................................... 12
   3.3.3.  Constraints .............................................. 12
   3.4.  New Types .................................................. 13
   4.  Basic Types .................................................. 14
   4.1.  Constrained Integer Types .................................. 14
   4.2.  KerberosTime ............................................... 15
   4.3.  KerberosString ............................................. 15
   4.4.  Language Tags .............................................. 16
   4.5.  KerberosFlags .............................................. 16
   4.6.  Typed Holes ................................................ 16
   4.7.  HostAddress and HostAddresses .............................. 17
   4.7.1.  Internet (IPv4) Addresses ................................ 17
   4.7.2.  Internet (IPv6) Addresses ................................ 17
   4.7.3.  DECnet Phase IV addresses ................................ 18
   4.7.4.  Netbios addresses ........................................ 18
   4.7.5.  Directional Addresses .................................... 18
   5.  Principals ................................................... 19
   5.1.  Name Types ................................................. 19
   5.2.  Principal Type Definition .................................. 19
   5.3.  Principal Name Reuse ....................................... 20
   5.4.  Realm Names ................................................ 20
   5.5.  Printable Representations of Principal Names ............... 21
   5.6.  Ticket-Granting Service Principal .......................... 21
   5.6.1.  Cross-Realm TGS Principals ............................... 21
   6.  Types Relating to Encryption ................................. 21
   6.1.  Assigned Numbers for Encryption ............................ 22
   6.1.1.  EType .................................................... 22
   6.1.2.  Key Usages ............................................... 22


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   6.2.  Which Key to Use ........................................... 23
   6.3.  EncryptionKey .............................................. 24
   6.4.  EncryptedData .............................................. 24
   6.5.  Checksums .................................................. 25
   6.5.1.  ChecksumOf ............................................... 26
   6.5.2.  Signed ................................................... 27
   7.  Tickets ...................................................... 27
   7.1.  Timestamps ................................................. 28
   7.2.  Ticket Flags ............................................... 28
   7.2.1.  Flags Relating to Initial Ticket Acquisition ............. 29
   7.2.2.  Invalid Tickets .......................................... 29
   7.2.3.  OK as Delegate ........................................... 30
   7.2.4.  Renewable Tickets ........................................ 30
   7.2.5.  Postdated Tickets ........................................ 31
   7.2.6.  Proxiable and Proxy Tickets .............................. 32
   7.2.7.  Forwarded and Forwardable Tickets ........................ 33
   7.3.  Transited Realms ........................................... 34
   7.4.  Authorization Data ......................................... 34
   7.4.1.  AD-IF-RELEVANT ........................................... 35
   7.4.2.  AD-KDCIssued ............................................. 35
   7.4.3.  AD-AND-OR ................................................ 37
   7.4.4.  AD-MANDATORY-FOR-KDC ..................................... 37
   7.5.  Encrypted Part of Ticket ................................... 37
   7.6.  Cleartext Part of Ticket ................................... 38
   8.  Credential Acquisition ....................................... 40
   8.1.  KDC-REQ .................................................... 40
   8.2.  PA-DATA .................................................... 46
   8.3.  KDC-REQ Processing ......................................... 46
   8.3.1.  Handling Replays ......................................... 46
   8.3.2.  Request Validation ....................................... 47
   8.3.2.1.  AS-REQ Authentication .................................. 47
   8.3.2.2.  TGS-REQ Authentication ................................. 47
   8.3.2.3.  Principal Validation ................................... 47
   8.3.2.4.  Checking For Revoked or Invalid Tickets ................ 48
   8.3.3.  Timestamp Handling ....................................... 48
   8.3.3.1.  AS-REQ Timestamp Processing ............................ 49
   8.3.3.2.  TGS-REQ Timestamp Processing ........................... 49
   8.3.4.  Handling Transited Realms ................................ 50
   8.3.5.  Address Processing ....................................... 50
   8.3.6.  Ticket Flag Processing ................................... 51
   8.3.7.  Key Selection ............................................ 52
   8.3.7.1.  Reply Key and Session Key Selection .................... 52
   8.3.7.2.  Ticket Key Selection ................................... 52
   8.4.  KDC-REP .................................................... 52
   8.5.  Reply Validation ........................................... 55
   8.6.  IP Transports .............................................. 55
   8.6.1.  UDP/IP transport ......................................... 55
   8.6.2.  TCP/IP transport ......................................... 56
   8.6.3.  KDC Discovery on IP Networks ............................. 57
   8.6.3.1.  DNS vs. Kerberos - Case Sensitivity of Realm Names ..... 57
   8.6.3.2.  DNS SRV records for KDC location ....................... 58


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   8.6.3.3.  KDC Discovery for Domain Style Realm Names on IP
                Networks ............................................ 58
   9.  Errors ....................................................... 58
   10.  Session Key Exchange ........................................ 61
   10.1.  AP-REQ .................................................... 61
   10.2.  AP-REP .................................................... 63
   11.  Session Key Use ............................................. 65
   11.1.  KRB-SAFE .................................................. 65
   11.2.  KRB-PRIV .................................................. 65
   11.3.  KRB-CRED .................................................. 66
   12.  Security Considerations ..................................... 67
   12.1.  Time Synchronization ...................................... 67
   12.2.  Replays ................................................... 67
   12.3.  Principal Name Reuse ...................................... 67
   12.4.  Password Guessing ......................................... 67
   12.5.  Forward Secrecy ........................................... 67
   12.6.  Authorization ............................................. 68
   12.7.  Login Authentication ...................................... 68
   13.  IANA Considerations ......................................... 68
   14.  Acknowledgments ............................................. 69
   Appendices ....................................................... 69
   A.  ASN.1 Module (Normative) ..................................... 69
   B.  Kerberos and Character Encodings (Informative) ...............103
   C.  Kerberos History (Informative) ...............................104
   D.  Notational Differences from [KCLAR] ..........................105
   Normative References .............................................106
   Informative References ...........................................106
   Author's Address .................................................108
   Full Copyright Statement .........................................108
























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1.  Introduction


   The Kerberos network authentication protocol is a trusted-third-party
   protocol utilizing symmetric-key cryptography.  It assumes that all
   communications between parties in the protocol may be arbitrarily
   tampered with or monitored, and that the security of the overall
   system depends only on the effectiveness of the cryptographic
   techniques and the secrecy of the cryptographic keys used.  The
   Kerberos protocol authenticates an application client's identity to
   an application server, and likewise authenticates the application
   server's identity to the application client.  These assurances are
   made possible by the client and the server sharing secrets with the
   trusted third party: the Kerberos server, also known as the Key
   Distribution Center (KDC).  In addition, the protocol establishes an
   ephemeral shared secret (the session key) between the client and the
   server, allowing the protection of further communications between
   them.


   The Kerberos protocol, as originally specified, provides insufficient
   means for extending the protocol in a backwards-compatible way.  This
   deficiency has caused problems for interoperability.  This document
   describes a framework which enables backwards-compatible extensions
   to the Kerberos protocol.


1.1.  Kerberos Protocol Overview


   Kerberos comprises three main sub-protocols: credentials acquisition,
   session key exchange, and session key usage.  A client acquires
   credentials by asking the KDC for a credential for a service; the KDC
   issues the credential, which contains a ticket and a session key.
   The ticket, containing the client's identity, timestamps, expiration
   time, and a session key, is a encrypted in a key known to the
   application server.  The KDC encrypts the credential using a key
   known to the client, and transmits the credential to the client.


   There are two means of requesting credentials: the Authentication
   Service (AS) exchange, and the Ticket-Granting Service (TGS)
   exchange.  In the typical AS exchange, a client uses a password-
   derived key to decrypt the response.  In the TGS exchange, the KDC
   behaves as an application server; the client authenticates to the TGS
   by using a Ticket-Granting Ticket (TGT).  The client usually obtains
   the TGT by using the AS exchange.


   Session key exchange consists of the client transmitting the ticket
   to the application server, accompanied by an authenticator.  The
   authenticator contains a timestamp and additional data encrypted
   using the ticket's session key.  The application server decrypts the
   ticket, extracting the session key.  The application server then uses
   the session key to decrypt the authenticator.  Upon successful
   decryption of the authenticator, the application server knows that
   the data in the authenticator were sent by the client named in the


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   associated ticket.  Additionally, since authenticators expire more
   quickly than tickets, the application server has some assurance that
   the transaction is not a replay.  The application server may send an
   encrypted acknowledgment to the client, verifying its identity to the
   client.


   Once session key exchange has occurred, the client and server may use
   the established session key to protect further traffic.  This
   protection may consist of protection of integrity only, or of
   protection of confidentiality and integrity.  Additional measures
   exist for a client to securely forward credentials to a server.


   The entire scheme depends on loosely synchronized clocks.
   Synchronization of the clock on the KDC with the application server
   clock allows the application server to accurately determine whether a
   credential is expired.  Likewise, synchronization of the clock on the
   client with the application server clock prevents replay attacks
   utilizing the same credential.  Careful design of the application
   protocol may allow replay prevention without requiring client-server
   clock synchronization.


   After establishing a session key, application client and the
   application server can exchange Kerberos protocol messages that use
   the session key to protect the integrity or confidentiality of
   communications between the client and the server.  Additionally, the
   client may forward credentials to the application server.


   The credentials acquisition protocol takes place over specific,
   defined transports (UDP and TCP).  Application protocols define which
   transport to use for the session key establishment protocol and for
   messages using the session key; the application may choose to perform
   its own encapsulation of the Kerberos messages, for example.


1.2.  Document Organization


   The remainder of this document begins by describing the general
   frameworks for protocol extensibility, including whether to interpret
   unknown extensions as critical.  It then defines the protocol
   messages and exchanges.


   The definition of the Kerberos protocol uses Abstract Syntax Notation
   One (ASN.1) [X680], which specifies notation for describing the
   abstract content of protocol messages.  This document defines a
   number of base types using ASN.1; these base types subsequently
   appear in multiple types which define actual protocol messages.
   Definitions of principal names and of tickets, which are central to
   the protocol, also appear preceding the protocol message definitions.






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2.  Compatibility Considerations


2.1.  Extensibility


   In the past, significant interoperability problems have resulted from
   conflicting assumptions about how the Kerberos protocol can be
   extended.  As the deployed base of Kerberos grows, the ability to
   extend the Kerberos protocol becomes more important.  In order to
   ensure that vendors and the IETF can extend the protocol while
   maintaining backwards compatibility, this document outlines a
   framework for extending Kerberos.


   Kerberos provides two general mechanisms for protocol extensibility.
   Many protocol messages, including some defined in RFC 1510, contain
   typed holes--sub-messages containing an octet string along with an
   integer that identifies how to interpret the octet string.  The
   integer identifiers are registered centrally, but can be used both
   for vendor extensions and for extensions standardized in the IETF.
   This document adds typed holes to a number of messages which
   previously lacked typed holes.


   Many new messages defined in this document also contain ASN.1
   extension markers.  These markers allow future revisions of this
   document to add additional elements to messages, for cases where
   typed holes are inadequate for some reason.  Because tag numbers and
   position in a sequence need to be coordinated in order to maintain
   interoperability, implementations MUST NOT include ASN.1 extensions
   except when those extensions are specified by IETF standards-track
   documents.


2.2.  Compatibility with RFC 1510


   Implementations of RFC 1510 did not use extensible ASN.1 types.
   Sending additional fields not in RFC 1510 to these implementations
   results in undefined behavior.  Examples of this behavior are known
   to include discarding messages with no error indications.


   Where messages have been changed since RFC 1510, ASN.1 CHOICE types
   are used; one alternative of the CHOICE provides a message which is
   wire-encoding compatible with RFC 1510, and the other alternative
   provides the new, extensible message.


   Implementations sending new messages MUST ensure that the recipient
   supports these new messages.  Along with each extensible message is a
   guideline for when that message MAY be used.  IF that guideline is
   followed, then the recipient is guaranteed to understand the message.


2.3.  Backwards Compatibility


   This document describes two sets (for the most part) of ASN.1 types.
   The first set of types is wire-encoding compatible with RFC 1510 and


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   [KCLAR].  The second set of types is the set of types enabling
   extensibility.  This second set may be referred to as
   "extensibility-enabled types". [ need to make this consistent
   throughout? ]


   A major difference between the new extensibility-enabled types and
   the types for RFC 1510 compatibility is that the extensibility-
   enabled types allow for the use of UTF-8 encodings in various
   character strings in the protocol.  Each party in the protocol must
   have some knowledge of the capabilities of the other parties in the
   protocol.  There are methods for establishing this knowledge without
   necessarily requiring explicit configuration.


   An extensibility-enabled client can detect whether a KDC supports the
   extensibility-enabled types by requesting an extensibility-enabled
   reply.  If the KDC replies with an extensibility-enabled reply, the
   client knows that the KDC supports extensibility.  If the KDC issues
   an extensibility-enabled ticket, the client knows that the service
   named in the ticket is extensibility-enabled.


2.4.  1.4.2. Sending Extensible Messages


   Care must be taken to make sure that old implementations can
   understand messages sent to them even if they do not understand an
   extension that is used.  Unless the sender knows the extension is
   supported, the extension cannot change the semantics of the core
   message or previously defined extensions.


   For example, an extension including key information necessary to
   decrypt the encrypted part of a KDC-REP could only be used in
   situations where the recipient was known to support the extension.
   Thus when designing such extensions it is important to provide a way
   for the recipient to notify the sender of support for the extension.
   For example in the case of an extension that changes the KDC-REP
   reply key, the client could indicate support for the extension by
   including a padata element in the AS-REQ sequence.  The KDC should
   only use the extension if this padata element is present in the AS-
   REQ.  Even if policy requires the use of the extension, it is better
   to return an error indicating that the extension is required than to
   use the extension when the recipient may not support it; debugging
   why implementations do not interoperate is easier when errors are
   returned.


2.5.  Criticality


   Recipients of unknown message extensions (including typed holes, new
   flags, and ASN.1 extension elements) should preserve the encoding of
   the extension but otherwise ignore the presence of the extension;
   i.e., unknown extensions SHOULD be treated as non-critical.  If a
   copy of the message is used later--for example, when a Ticket
   received in a KDC-REP is later used in an AP-REQ--then the unknown


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   extensions MUST be included.


   An implementation SHOULD NOT reject a request merely because it does
   not understand some element of the request.  As a related
   consequence, implementations SHOULD handle communicating with other
   implementations which do not implement some requested options.  This
   may require designers of options to provide means to determine
   whether an option has been rejected, not understood, or (perhaps
   maliciously) deleted or modified in transit.


   There is one exception to non-criticality described above: if an
   unknown authorization data element is received by a server either in
   an AP-REQ or in a Ticket contained in an AP-REQ, then the
   authentication SHOULD fail.  Authorization data is intended to
   restrict the use of a ticket.  If the service cannot determine
   whether the restriction applies to that service then a security
   weakness may result if authentication succeeds.  Authorization
   elements meant to be truly optional can be enclosed in the AD-IF-
   RELEVANT element.


   Many RFC 1510 implementations ignore unknown authorization data
   elements.  Depending on these implementations to honor authorization
   data restrictions may create a security weakness.


2.6.  Authenticating Cleartext Portions of Messages


   Various denial of service attacks and downgrade attacks against
   Kerberos are possible unless plaintexts are somehow protected against
   modification.  An early design goal of Kerberos Version 5 was to
   avoid encrypting more of the authentication exchange that was
   required.  (Version 4 doubly-encrypted the encrypted part of a ticket
   in a KDC reply, for example.)  This minimization of encryption
   reduces the load on the KDC and busy servers.  Also, during the
   initial design of Version 5, the existence of legal restrictions on
   the export of cryptography made it desirable to minimize of the
   number of uses of encryption in the protocol.  Unfortunately,
   performing this minimization created numerous instances of
   unauthenticated security-relevant plaintext fields.


   The extensible variants of the messages described in this document
   wrap the actual message in an ASN.1 sequence containing a keyed
   checksum of the contents of the message.  Guidelines in [XXX] section
   3 specify when the checksum MUST be included and what key MUST be
   used.  Guidelines on when to include a checksum are never ambiguous:
   a particular PDU is never correct both with and without a checksum.
   With the exception of the KRB-ERROR message, receiving
   implementations MUST reject messages where a checksum is included and
   not expected or where a checksum is expected but not included.  The
   receiving implementation does not always have sufficient information
   to know whether a KRB-ERROR should contain a checksum.  Even so,
   KRB-ERROR messages with invalid checksums MUST be rejected and


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   implementations MAY consider the presence or absence of a checksum
   when evaluating whether to trust the error.


   This authenticated cleartext protection is provided only in the
   extensible variants of the messages; it is never used when
   communicating with an RFC 1510 implementation.


2.7.  Capability Negotiation


   Kerberos is a three-party protocol.  Each of the three parties
   involved needs a means of detecting the capabilities supported by the
   others.  Two of the parties, the KDC and the application server, do
   not communicate directly in the Kerberos protocol.  Communicating
   capabilities from the KDC to the application server requires using a
   ticket as an intermediary.


   The main capability requiring negotiation is the support of the
   extensibility framework described in this document.  Negotiation of
   this capability while remaining compatible with RFC 1510
   implementations is possible.  The main complication is that the
   client needs to know whether the application server supports the
   extensibility framework prior to sending any message to the
   application server.  This can be accomplished if the KDC has
   knowledge of whether an application server supports the extensibility
   framework.


   Client software advertizes its capabilities when requesting
   credentials from the KDC.  If the KDC recognizes the capabilities, it
   acknowledges this fact to the client in its reply.  In addition, if
   the KDC has knowledge that the application server supports certain
   capabilities, it also communicates this knowledge to the client in
   its reply.  The KDC can encode its own capabilities in the ticket so
   that the application server may discover these capabilities.  The
   client advertizes its capabilities to the application server when it
   initiates authentication to the application server.


3.  Use of ASN.1 in Kerberos


   Kerberos uses the ASN.1 Distinguished Encoding Rules (DER) [X690].
   Even though ASN.1 theoretically allows the description of protocol
   messages to be independent of the encoding rules used to encode the
   messages, Kerberos messages MUST be encoded with DER.  Subtleties in
   the semantics of the notation, such as whether tags carry any
   semantic content to the application, may cause the use of other ASN.1
   encoding rules to be problematic.


   Implementors not using existing ASN.1 tools (e.g., compilers or
   support libraries) are cautioned to thoroughly read and understand
   the actual ASN.1 specification to ensure correct implementation
   behavior.  There is more complexity in the notation than is
   immediately obvious, and some tutorials and guides to ASN.1 are


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   misleading or erroneous.  Recommended tutorials and guides include
   [Dub00], [Lar99], though there is still no substitute for reading the
   actual ASN.1 specification.


3.1.  Module Header


   The type definitions in this document assume an ASN.1 module
   definition of the following form:


      KerberosV5Spec3 {
          iso(1) identified-organization(3) dod(6) internet(1)
          security(5) kerberosV5(2) modules(4) krb5spec3(4)
      } DEFINITIONS EXPLICIT TAGS ::= BEGIN


      -- Rest of definitions here


      END


   This specifies that the tagging context for the module will be
   explicit and that automatic tagging is not done.


   Some other publications [RFC1510] [RFC1964] erroneously specify an
   object identifier (OID) having an incorrect value of "5" for the
   "dod" component of the OID.  In the case of RFC 1964, use of the
   "correct" OID value would result in a change in the wire protocol;
   therefore, the RFC 1964 OID remains unchanged for now.


3.2.  Top-Level Type


   The ASN.1 type "KRB-PDU" is a CHOICE over all the types (Protocol
   Data Units or PDUs) which an application may directly reference.
   Applications SHOULD NOT transmit any types other than those which are
   alternatives of the KRB-PDU CHOICE.




















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      -- top-level type
      --
      -- Applications should not directly reference any types
      -- other than KRB-PDU and its component types.
      --
      KRB-PDU         ::= CHOICE {
          ticket      Ticket,
          as-req      AS-REQ,
          as-rep      AS-REP,
          tgs-req     TGS-REQ,
          tgs-rep     TGS-REP,
          ap-req      AP-REQ,
          ap-rep      AP-REP,
          krb-safe    KRB-SAFE,
          krb-priv    KRB-PRIV,
          krb-cred    KRB-CRED,
          tgt-req     TGT-REQ,
          krb-error   KRB-ERROR,
          ...
      }



3.3.  Previously Unused ASN.1 Notation (informative)


   Some aspects of ASN.1 notation used in this document were not used in
   [KCLAR], and may be unfamiliar to some readers.  This subsection is
   informative; for normative definitions of the notation, see the
   actual ASN.1 specifications [X680], [X682], [X683].


3.3.1.  Parameterized Types


   This document uses ASN.1 parameterized types [X683] to make
   definitions of types more readable.  For some types, some or all of
   the parameters are advisory, i.e., they are not encoded in any form
   for transmission in a protocol message.  These advisory parameters
   can describe implementation behavior associated with the type.


3.3.2.  COMPONENTS OF Notation


   The "COMPONENTS OF" notation, used to define the RFC 1510
   compatibility types, extracts all of the component types of an ASN.1
   SEQUENCE type.  The extension marker (the "..." notation) and any
   extension components are not extracted by "COMPONENTS OF".  The
   "COMPONENTS OF" notation permits concise definition of multiple types
   which have many components in common with each other.


3.3.3.  Constraints


   This document uses ASN.1 constraints, including the
   "UserDefinedConstraint" notation [X682].  Some constraints can be
   handled automatically by tools that can parse them.  Uses of the


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   "UserDefinedConstraint" notation (the "CONSTRAINED BY" notation) will
   typically need to have behavior manually coded; the notation provides
   a formalized way of conveying intended implementation behavior.


   The "WITH COMPONENTS" constraint notation allows constraints to apply
   to component types of a SEQUENCE type.  This constraint notation
   effectively allows constraints to "reach into" a type to constrain
   its component types.


3.4.  New Types


   This document defines a number of ASN.1 types which are new since
   [KCLAR].  The names of these types will typically have a suffix like
   "Ext", indicating that the types are intended to support
   extensibility.  Types original to RFC 1510 and [KCLAR] have been
   renamed to have a suffix like "1510".  The "Ext" and "1510" types
   often contain a number of common elements; these common elements have
   a suffix like "Common".  The "1510" types have various ASN.1
   constraints applied to them; the constraints limit the possible
   values of the "1510" types to those permitted by RFC 1510 or by
   [KCLAR].  The "Ext" types may have different constraints, typically
   to force string values to be sent as UTF-8.


   For example, consider


      -- example "common" type
      Foo-Common      ::= SEQUENCE {
          a           [0] INTEGER,
          b           [1] OCTET STRING,
          ...,
          c           [2] INTEGER,
          ...
      }


      -- example "RFC 1510 compatibility" type
      Foo-1510        ::= SEQUENCE {
          -- the COMPONENTS OF notation drops the extension marker
          -- and all extension additions.
          COMPONENTS OF Foo-Common
      }


      -- example "extensibility-enabled" type
      Foo-Ext         ::= Foo-Common


   where "Foo-Common" is a common type used to define both the "1510"
   and "Ext" versions of a type.  "Foo-1510" is the RFC 1510 version of
   the type, while "Foo-Ext" is the extensible version.  "Foo-1510" does
   not contain the extension marker, nor does it contain the extension
   component "c".  The type "Foo-1510" is equivalent to "Foo-1510-
   Equiv", shown below.



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      -- example type equivalent to Foo-1510
      Foo-1510-Equiv  ::= SEQUENCE {
          a           [0] INTEGER,
          b           [1] OCTET STRING
      }



4.  Basic Types


   These "basic" Kerberos ASN.1 types appear in multiple other Kerberos
   types.


4.1.  Constrained Integer Types


   In RFC 1510, many types contained references to the unconstrained
   INTEGER type.  Since an unconstrained INTEGER can contain almost any
   possible abstract integer value, most of the unconstrained references
   to INTEGER in RFC 1510 were constrained to 32 bits or smaller in
   [KCLAR].


      -- signed values representable in 32 bits
      --
      -- These are often used as assigned numbers for various things.
      Int32           ::= INTEGER (-2147483648..2147483647)


   The "Int32" type often contains an assigned number identifying the
   contents of a typed hole.  Unless otherwise stated, non-negative
   values are registered, and negative values are available for local
   use.


      -- unsigned 32 bit values
      UInt32          ::= INTEGER (0..4294967295)


   The "UInt32" type is used in some places where an unsigned 32-bit
   integer is needed.


      -- unsigned 64 bit values
      UInt64          ::= INTEGER (0..18446744073709551615)


   The "UInt64" type is used in places where 32 bits of precision may
   provide inadequate security.


      -- sequence numbers
      SeqNum          ::= UInt64


   Sequence numbers were constrained to 32 bits in [KCLAR], but this
   document allows for 64-bit sequence numbers.


      -- nonces
      Nonce           ::= UInt64



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   Likewise, nonces were constrained to 32 bits in [KCLAR], but expanded
   to 64 bits here.


      -- microseconds
      Microseconds    ::= INTEGER (0..999999)


   The "microseconds" type is intended to carry the microseconds part of
   a time value.


4.2.  KerberosTime


      KerberosTime    ::= GeneralizedTime (CONSTRAINED BY {
                              -- MUST NOT include fractional seconds
      })


   The timestamps used in Kerberos are encoded as GeneralizedTimes.  A
   KerberosTime value MUST NOT include any fractional portions of the
   seconds.  As required by the DER, it further MUST NOT include any
   separators, and it specifies the UTC time zone (Z).  Example: The
   only valid format for UTC time 6 minutes, 27 seconds after 9 pm on 6
   November 1985 is "19851106210627Z".


4.3.  KerberosString


      -- used for names and for error messages
      KerberosString  ::= CHOICE {
          ia5         GeneralString (IA5String),
          utf8        UTF8String,
          ...         -- no extension may be sent
                      -- to an rfc1510 implementation --
      }


   The KerberosString type is used for character strings in various
   places in the Kerberos protocol.  For compatibility with RFC 1510,
   GeneralString values constrained to IA5String (US-ASCII) are
   permitted in messages exchanged with RFC 1510 implementations.  The
   new protocol messages contain strings encoded as UTF-8, and these
   strings MUST be normalized using [SASLPREP].  KerberosString values
   are present in principal names and in error messages.  Control
   characters SHOULD NOT be used in principal names, and used with
   caution in error messages.


      -- IA5 choice only; useful for constraints
      KerberosStringIA5       ::= KerberosString
          (WITH COMPONENTS { ia5 PRESENT })


      -- IA5 excluded; useful for constraints
      KerberosStringExt       ::= KerberosString
          (WITH COMPONENTS { ia5 ABSENT })


   KerberosStringIA5 requires the use of the "ia5" alternative, while


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   KerberosStringExt forbids it.  These types appear in ASN.1
   constraints on messages.


   For detailed background regarding the history of character string use
   in Kerberos, as well as discussion of some compatibility issues, see
   Appendix B.


4.4.  Language Tags


      -- used for language tags
      LangTag ::= PrintableString
          (FROM ("A".."Z" | "a".."z" | "0".."9" | "-"))


   The "LangTag" type is used to specify language tags for localization
   purposes, using the [RFC3066] format.


4.5.  KerberosFlags


   For several message types, a specific constrained bit string type,
   KerberosFlags, is used.


      KerberosFlags { NamedBits } ::= BIT STRING (SIZE (32..MAX))
          (CONSTRAINED BY {
          -- MUST be a valid value of -- NamedBits
          -- but if the value to be sent would be truncated to shorter
          -- than 32 bits according to DER, the value MUST be padded
          -- with trailing zero bits to 32 bits.  Otherwise, no
          -- trailing zero bits may be present.


      })



   The actual bit string type encoded in Kerberos messages does not use
   named bits.  The advisory parameter of the KerberosFlags type names a
   bit string type defined using named bits, whose value is encoded as
   if it were a bit string with unnamed bits.  This practice is
   necessary because the DER require trailing zero bits to be removed
   when encoding bit strings defined using named bits.  Existing
   implementations of Kerberos send exactly 32 bits rather than
   truncating, so the size constraint requires the transmission of at
   least 32 bits.  Trailing zero bits beyond the first 32 bits are
   truncated.


4.6.  Typed Holes


      -- Typed hole identifiers
      TH-id           ::= CHOICE {
          int32               Int32,
          rel-oid             RELATIVE-OID
      }



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   The "TH-id" type is a generalized means to identify the contents of a
   typed hole.  The "int32" alternative may be used for simple integer
   assignments, in the same manner as "Int32", while the "rel-oid"
   alternative may be used for hierarchical delegation.


4.7.  HostAddress and HostAddresses


      AddrType        ::= Int32


      HostAddress     ::= SEQUENCE  {
          addr-type   [0] AddrType,
          address     [1] OCTET STRING
      }


      -- NOTE: HostAddresses is always used as an OPTIONAL field and
      -- should not be a zero-length SEQUENCE OF.
      --
      -- The extensible messages explicitly constrain this to be
      -- non-empty.
      HostAddresses   ::= SEQUENCE OF HostAddress



   addr-type
        This field specifies the type of address that follows.


   address
        This field encodes a single address of the type identified by
        "addr-type".


   All negative values for the host address type are reserved for local
   use.  All non-negative values are reserved for officially assigned
   type fields and interpretations.



      addr-type |     meaning
      __________|______________
              2 |   IPv4
              3 |   directional
             12 |   DECnet
             20 |   NetBIOS
             24 |   IPv6




4.7.1.  Internet (IPv4) Addresses


   Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded in
   MSB order (most significant byte first). The IPv4 loopback address
   SHOULD NOT appear in a Kerberos PDU. The type of IPv4 addresses is
   two (2).



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4.7.2.  Internet (IPv6) Addresses


   IPv6 addresses [RFC2373] are 128-bit (16-octet) quantities, encoded
   in MSB order (most significant byte first). The type of IPv6
   addresses is twenty-four (24). The following addresses MUST NOT
   appear in any Kerberos PDU:


      * the Unspecified Address


      * the Loopback Address


      * Link-Local addresses


   This restriction applies to the inclusion in the address fields of
   Kerberos PDUs, but not to the address fields of packets that might
   carry such PDUs.  The restriction is necessary because the use of an
   address with non-global scope could allow the acceptance of a message
   sent from a node that may have the same address, but which is not the
   host intended by the entity that added the restriction.  If the
   link-local address type needs to be used for communication, then the
   address restriction in tickets must not be used (i.e. addressless
   tickets must be used).


   IPv4-mapped IPv6 addresses MUST be represented as addresses of type
   2.


4.7.3.  DECnet Phase IV addresses


   DECnet Phase IV addresses are 16-bit addresses, encoded in LSB order.
   The type of DECnet Phase IV addresses is twelve (12).


4.7.4.  Netbios addresses


   Netbios addresses are 16-octet addresses typically composed of 1 to
   15 alphanumeric characters and padded with the US-ASCII SPC character
   (code 32).  The 16th octet MUST be the US-ASCII NUL character (code
   0).  The type of Netbios addresses is twenty (20).


4.7.5.  Directional Addresses


   In many environments, including the sender address in KRB-SAFE and
   KRB-PRIV messages is undesirable because the addresses may be changed
   in transport by network address translators. However, if these
   addresses are removed, the messages may be subject to a reflection
   attack in which a message is reflected back to its originator. The
   directional address type provides a way to avoid transport addresses
   and reflection attacks.  Directional addresses are encoded as four
   byte unsigned integers in network byte order.  If the message is
   originated by the party sending the original AP-REQ message, then an
   address of 0 SHOULD be used. If the message is originated by the
   party to whom that AP-REQ was sent, then the address 1 SHOULD be


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   used.  Applications involving multiple parties can specify the use of
   other addresses.


   Directional addresses MUST only be used for the sender address field
   in the KRB-SAFE or KRB-PRIV messages.  They MUST NOT be used as a
   ticket address or in a AP-REQ message.  This address type SHOULD only
   be used in situations where the sending party knows that the
   receiving party supports the address type.  This generally means that
   directional addresses may only be used when the application protocol
   requires their support.  Directional addresses are type (3).


5.  Principals


   Principals are participants in the Kerberos protocol.  A "realm"
   consists of principals in one administrative domain, served by one
   KDC (or one replicated set of KDCs).  Each principal name has an
   arbitrary number of components, though typical principal names will
   only have one or two components.  A principal name is meant to be
   readable by and meaningful to humans, especially in a realm lacking a
   centrally adminstered authorization infrastructure.


5.1.  Name Types


   Each PrincipalName has NameType indicating what sort of name it is.
   The name-type SHOULD be treated as a hint.  Ignoring the name type,
   no two names can be the same (i.e., at least one of the components,
   or the realm, must be different).


      -- assigned numbers for name types (used in principal names)
      NameType        ::= Int32


      -- Name type not known
      nt-unknown              NameType ::= 0
      -- Just the name of the principal as in DCE, or for users
      nt-principal            NameType ::= 1
      -- Service and other unique instance (krbtgt)
      nt-srv-inst             NameType ::= 2
      -- Service with host name as instance (telnet, rcommands)
      nt-srv-hst              NameType ::= 3
      -- Service with host as remaining components
      nt-srv-xhst             NameType ::= 4
      -- Unique ID
      nt-uid                  NameType ::= 5
      -- Encoded X.509 Distingished name [RFC 2253]
      nt-x500-principal       NameType ::= 6
      -- Name in form of SMTP email name (e.g. user@foo.com)
      nt-smtp-name            NameType ::= 7
      -- Enterprise name - may be mapped to principal name
      nt-enterprise           NameType ::= 10




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5.2.  Principal Type Definition


      PrincipalName   ::= SEQUENCE {
          name-type   [0] NameType,
          -- May have zero elements, or individual elements may be
          -- zero-length, but this is NOT RECOMMENDED.
          name-string [1] SEQUENCE OF KerberosString
      }



   name-type
        hint of the type of name that follows


   name-string
        The "name-string" encodes a sequence of components that form a
        name, each component encoded as a KerberosString.  Taken
        together, a PrincipalName and a Realm form a principal
        identifier.  Most PrincipalNames will have only a few components
        (typically one or two).


5.3.  Principal Name Reuse


   Realm administrators SHOULD use extreme caution when considering
   reusing a principal name.  A service administrator might explicitly
   enter principal names into a local access control list (ACL) for the
   service.  If such local ACLs exist independently of a centrally
   administered authorization infrastructure, realm administrators
   SHOULD NOT reuse principal names until confirming that all extant ACL
   entries referencing that principal name have been updated.  Failure
   to perform this check can result in a security vulnerability, as a
   new principal can inadvertently inherit unauthorized privileges upon
   receiving a reused principal name.  An organization whose Kerberos-
   authenticated services all use a centrally-adminstered authorization
   infrastructure may not need to take these precautions regarding
   principal name reuse.


5.4.  Realm Names


      Realm           ::= KerberosString


      -- IA5 only
      RealmIA5        ::= Realm (KerberosStringIA5)


      -- IA5 excluded
      RealmExt        ::= Realm (KerberosStringExt)



   Kerberos realm names are KerberosStrings.  Realms MUST NOT contain a
   character with the code 0 (the US-ASCII NUL).  Most realms will
   usually consist of several components separated by periods (.), in
   the style of Internet Domain Names, or separated by slashes (/) in


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   the style of X.500 names.


5.5.  Printable Representations of Principal Names


   [ perhaps non-normative? ]


   The printable form of a principal name consists of the concatenation
   of components of the PrincipalName value using the slash character
   (/), followed by an at-sign (@), followed by the realm name.  Any
   occurrence of a backslash (\), slash (/) or at-sign (@) in the
   PrincipalName value is quoted by a backslash.


5.6.  Ticket-Granting Service Principal


   The PrincipalName value corresponding to a ticket-granting service
   has two components: the first component is the string "krbtgt", and
   the second component is the realm name of the TGS which will accept a
   ticket-granting ticket having this service principal name.  The realm
   name of service always indicates which realm issued the ticket.  A
   ticket-granting ticket issued by "A.EXAMPLE.COM" which is valid for
   obtaining tickets in the same realm would have the following ASN.1
   values for its "realm" and "sname" components, respectively:


      -- Example Realm and PrincipalName for a TGS


      tgtRealm1       Realm ::= ia5 : "A.EXAMPLE.COM"


      tgtPrinc1       PrincipalName ::= {
          name-type nt-srv-inst,
          name-string { ia5 : "krbtgt", ia5 : "A.EXAMPLE.COM" }
      }


   Its printable representation would be written as
   "krbtgt/A.EXAMPLE.COM@A.EXAMPLE.COM".


5.6.1.  Cross-Realm TGS Principals


   It is possible for a principal in one realm to authenticate to a
   service in another realm.  A KDC can issue a cross-realm ticket-
   granting ticket to allow one of its principals to authenticate to a
   service in a foreign realm.  For example, the TGS principal
   "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM" is a principal that permits a
   client principal in the realm A.EXAMPLE.COM to authenticate to a
   service in the realm B.EXAMPLE.COM.  When the KDC for B.EXAMPLE.COM
   issues a ticket to a client originating in A.EXAMPLE.COM, the
   client's realm name remains "A.EXAMPLE.COM", even though the service
   principal will have the realm "B.EXAMPLE.COM".






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6.  Types Relating to Encryption


   Many Kerberos protocol messages contain encrypted encodings of
   various data types.  Some Kerberos protocol messages also contain
   checksums (signatures) of encodings of various types.


6.1.  Assigned Numbers for Encryption


   Encryption algorithm identifiers and key usages both have assigned
   numbers, described in [KCRYPTO].


6.1.1.  EType


   EType is the integer type for assigned numbers for encryption
   algorithms.  Defined in [KCRYPTO].


      -- Assigned numbers denoting encryption mechanisms.
      EType ::= Int32


      -- assigned numbers for encryption schemes
      et-des-cbc-crc                  EType ::= 1
      et-des-cbc-md4                  EType ::= 2
      et-des-cbc-md5                  EType ::= 3
      --     [reserved]                         4
      et-des3-cbc-md5                 EType ::= 5
      --     [reserved]                         6
      et-des3-cbc-sha1                EType ::= 7
      et-dsaWithSHA1-CmsOID           EType ::= 9
      et-md5WithRSAEncryption-CmsOID  EType ::= 10
      et-sha1WithRSAEncryption-CmsOID EType ::= 11
      et-rc2CBC-EnvOID                EType ::= 12
      et-rsaEncryption-EnvOID         EType ::= 13
      et-rsaES-OAEP-ENV-OID           EType ::= 14
      et-des-ede3-cbc-Env-OID         EType ::= 15
      et-des3-cbc-sha1-kd             EType ::= 16
      -- AES
      et-aes128-cts-hmac-sha1-96      EType ::= 17
      -- AES
      et-aes256-cts-hmac-sha1-96      EType ::= 18
      -- Microsoft
      et-rc4-hmac                     EType ::= 23
      -- Microsoft
      et-rc4-hmac-exp                 EType ::= 24
      -- opaque; PacketCable
      et-subkey-keymaterial           EType ::= 65



6.1.2.  Key Usages


   KeyUsage is the integer type for assigned numbers for key usages.
   Key usage values are inputs to the encryption and decryption


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   functions described in [KCRYPTO].


      -- Assigned numbers denoting key usages.
      KeyUsage ::= UInt32


      --
      -- Actual identifier names are provisional and subject to
      -- change.
      --
      ku-pa-enc-ts                    KeyUsage ::= 1
      ku-Ticket                       KeyUsage ::= 2
      ku-EncASRepPart                 KeyUsage ::= 3
      ku-TGSReqAuthData-sesskey       KeyUsage ::= 4
      ku-TGSReqAuthData-subkey        KeyUsage ::= 5
      ku-pa-TGSReq-cksum              KeyUsage ::= 6
      ku-pa-TGSReq-authenticator      KeyUsage ::= 7
      ku-EncTGSRepPart-sesskey        KeyUsage ::= 8
      ku-EncTGSRepPart-subkey         KeyUsage ::= 9
      ku-Authenticator-cksum          KeyUsage ::= 10
      ku-APReq-authenticator          KeyUsage ::= 11
      ku-EncAPRepPart                 KeyUsage ::= 12
      ku-EncKrbPrivPart               KeyUsage ::= 13
      ku-EncKrbCredPart               KeyUsage ::= 14
      ku-KrbSafe-cksum                KeyUsage ::= 15
      ku-ad-KDCIssued-cksum           KeyUsage ::= 19



      -- The following numbers are provisional...
      -- conflicts may exist elsewhere.
      ku-Ticket-cksum                 KeyUsage ::= 25
      ku-ASReq-cksum                  KeyUsage ::= 26
      ku-TGSReq-cksum                 KeyUsage ::= 27
      ku-ASRep-cksum                  KeyUsage ::= 28
      ku-TGSRep-cksum                 KeyUsage ::= 29
      ku-APReq-cksum                  KeyUsage ::= 30
      ku-APRep-cksum                  KeyUsage ::= 31
      ku-KrbCred-cksum                KeyUsage ::= 32
      ku-KrbError-cksum               KeyUsage ::= 33
      ku-KDCRep-cksum                 KeyUsage ::= 34



6.2.  Which Key to Use











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      -- KeyToUse identifies which key is to be used to encrypt or
      -- sign a given value.
      --
      -- Values of KeyToUse are never actually transmitted over the
      -- wire, or even used directly by the implementation in any
      -- way, as key usages are; it exists primarily to identify
      -- which key gets used for what purpose.  Thus, the specific
      -- numeric values associated with this type are irrelevant.
      KeyToUse        ::= ENUMERATED {
          -- unspecified
          key-unspecified,
          -- server long term key
          key-server,
          -- client long term key
          key-client,
          -- key selected by KDC for encryption of a KDC-REP
          key-kdc-rep,
          -- session key from ticket
          key-session,
          -- subsession key negotiated via AP-REQ/AP-REP
          key-subsession,
          ...
      }



6.3.  EncryptionKey


   The "EncryptionKey" type holds an encryption key.


      EncryptionKey   ::= SEQUENCE {
          keytype     [0] EType,
          keyvalue    [1] OCTET STRING
      }



   keytype
        This "EType" identifies the encryption algorithm, described in
        [KCRYPTO].


   keyvalue
        Contains the actual key.


6.4.  EncryptedData


   The "EncryptedData" type contains the encryption of another data
   type.  The recipient uses fields within EncryptedData to determine
   which key to use for decryption.






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      -- "Type" specifies which ASN.1 type is encrypted to the
      -- ciphertext in the EncryptedData.  "Keys" specifies a set of
      -- keys of which one key may be used to encrypt the data.
      -- "KeyUsages" specifies a set of key usages, one of which may
      -- be used to encrypt.
      --
      -- None of the parameters is transmitted over the wire.
      EncryptedData {
          Type, KeyToUse:Keys, KeyUsage:KeyUsages
      } ::= SEQUENCE {
          etype       [0] EType,
          kvno        [1] UInt32 OPTIONAL,
          cipher      [2] OCTET STRING (CONSTRAINED BY {
              -- must be encryption of --
              OCTET STRING (CONTAINING Type),
              -- with one of the keys -- KeyToUse:Keys,
              -- with key usage being one of --
              KeyUsage:KeyUsages
          }),
          ...
      }




   KeyUsages
        Advisory parameter indicating which key usage to use when
        encrypting the ciphertext.  If "KeyUsages" indicate multiple
        "KeyUsage" values, the detailed description of the containing
        message will indicate which key to use under which conditions.


   Type
        Advisory parameter indicating the ASN.1 type whose DER encoding
        is the plaintext encrypted into the EncryptedData.


   Keys
        Advisory parameter indicating which key to use to perform the
        encryption.  If "Keys" indicate multiple "KeyToUse" values, the
        detailed description of the containing message will indicate
        which key to use under which conditions.


   KeyUsages
        Advisory parameter indicating which "KeyUsage" value is used to
        encrypt.  If "KeyUsages" indicates multiple "KeyUsage" values,
        the detailed description of the containing message will indicate
        which key usage to use under which conditions.


6.5.  Checksums


   Several types contain checksums (actually signatures) of data.



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      CksumType       ::= Int32


      -- The parameters specify which key to use to produce the
      -- signature, as well as which key usage to use.  The
      -- parameters are not actually sent over the wire.
      Checksum {
          KeyToUse:Keys, KeyUsage:KeyUsages
      } ::= SEQUENCE {
          cksumtype   [0] CksumType,
          checksum    [1] OCTET STRING (CONSTRAINED BY {
              -- signed using one of the keys --
              KeyToUse:Keys,
              -- with key usage being one of --
              KeyUsage:KeyUsages
          })
      }



   CksumType
        Integer type for assigned numbers for signature algorithms.
        Defined in [KCRYPTO]


   Keys
        As in EncryptedData


   KeyUsages
        As in EncryptedData


   cksumtype
        Signature algorithm used to produce the signature.


   checksum
        The actual checksum.


6.5.1.  ChecksumOf


   ChecksumOf is similar to "Checksum", but specifies which type is
   signed.


      -- a Checksum that must contain the checksum
      -- of a particular type
      ChecksumOf {
          Type, KeyToUse:Keys, KeyUsage:KeyUsages
      } ::= Checksum { Keys, KeyUsages } (WITH COMPONENTS {
          ...,
          checksum (CONSTRAINED BY {
              -- must be checksum of --
              OCTET STRING (CONTAINING Type)
          })
      })



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   Type
        Indicates the ASN.1 type whose DER encoding is signed.


6.5.2.  Signed


   Signed is similar to "ChecksumOf", but contains an actual instance of
   the type being signed in addition to the signature.


      -- parameterized type for wrapping authenticated plaintext
      Signed {
          InnerType, KeyToUse:Keys, KeyUsage:KeyUsages
      } ::= SEQUENCE {
          cksum       [0] ChecksumOf {
              InnerType, Keys, KeyUsages
          } OPTIONAL,
          inner       [1] InnerType,
          ...
      }



7.  Tickets


   [ A large number of items described here are duplicated in the
   sections describing KDC-REQ processing.  Should find a way to avoid
   this duplication. ]


   A ticket binds a principal name to a session key.  The important
   fields of a ticket are in the encrypted part.  The components in
   common between the RFC 1510 and the extensible versions are


      EncTicketPartCommon ::= SEQUENCE {
          flags               [0] TicketFlags,
          key                 [1] EncryptionKey,
          crealm              [2] Realm,
          cname               [3] PrincipalName,
          transited           [4] TransitedEncoding,
          authtime            [5] KerberosTime,
          starttime           [6] KerberosTime OPTIONAL,
          endtime             [7] KerberosTime,
          renew-till          [8] KerberosTime OPTIONAL,
          caddr               [9] HostAddresses OPTIONAL,
          authorization-data  [10] AuthorizationData OPTIONAL,
          ...
      }



   crealm
        This field contains the client's realm.


   cname
        This field contains the client's name.


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   caddr
        This field lists the network addresses (if absent, all addresses
        are permitted) from which the ticket is valid.


   Descriptions of the other fields appear in the following sections.


7.1.  Timestamps


   Three of the ticket timestamps may be requested from the KDC.  The
   timestamps may differ from those requested, depending on site policy.


   authtime
        The time at which the client authenticated, as recorded by the
        KDC.


   starttime
        The earliest time when the ticket is valid.  If not present, the
        ticket is valid starting at the authtime.  This is requested as
        the "from" field of KDC-REQ-BODY-COMMON.


   endtime
        This time is requested in the "till" field of KDC-REQ-BODY-
        COMMON.  Contains the time after which the ticket will not be
        honored (its expiration time).  Note that individual services
        MAY place their own limits on the life of a ticket and MAY
        reject tickets which have not yet expired.  As such, this is
        really an upper bound on the expiration time for the ticket.


   renew-till
        This time is requested in the "rtime" field of KDC-REQ-BODY-
        COMMON.  It is only present in tickets that have the "renewable"
        flag set in the flags field.  It indicates the maximum endtime
        that may be included in a renewal.  It can be thought of as the
        absolute expiration time for the ticket, including all renewals.


7.2.  Ticket Flags


   A number of flags may be set in the ticket, further defining some of
   its capabilities.  Some of these flags map to flags in a KDC request.














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      TicketFlags     ::= KerberosFlags { TicketFlagsBits }


      TicketFlagsBits ::= BIT STRING {
          reserved            (0),
          forwardable         (1),
          forwarded           (2),
          proxiable           (3),
          proxy               (4),
          may-postdate        (5),
          postdated           (6),
          invalid             (7),
          renewable           (8),
          initial             (9),
          pre-authent         (10),
          hw-authent          (11),
          transited-policy-checked (12),
          ok-as-delegate      (13),
          anonymous           (14),
          cksummed-ticket     (15)
      }



7.2.1.  Flags Relating to Initial Ticket Acquisition


   [ adapted KCLAR 2.1. ]


   Several flags indicate the details of how the initial ticket was
   acquired.


   initial
        The "initial" flag indicates that a ticket was issued using the
        AS protocol, rather than issued based on a ticket-granting
        ticket.  Application servers (e.g., a password-changing program)
        requiring a client's definite knowledge of its secret key can
        insist that this flag be set in any tickets they accept, thus
        being assured that the client's key was recently presented to
        the application client.


   pre-authent
        The "pre-authent" flag indicates that some sort of pre-
        authentication was used during the AS exchange.


   hw-authent
        The "hw-authent" flag indicates that some sort of hardware-based
        pre-authentication occurred during the AS exchange.


   Both the "pre-authent" and the "hw-authent" flags may be present with
   or without the "initial" flag; such tickets with the "initial" flag
   clear are ones which are derived from initial tickets with the "pre-
   authent" or "hw-authent" flags set.



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7.2.2.  Invalid Tickets


   [ KCLAR 2.2. ]


   The "invalid" flag indicates that a ticket is invalid.  Application
   servers MUST reject tickets which have this flag set.  A postdated
   ticket will be issued in this form.  Invalid tickets MUST be
   validated by the KDC before use, by presenting them to the KDC in a
   TGS request with the "validate" option specified.  The KDC will only
   validate tickets after their starttime has passed.  The validation is
   required so that postdated tickets which have been stolen before
   their starttime can be rendered permanently invalid (through a hot-
   list mechanism -- see Section 8.3.2.4).


7.2.3.  OK as Delegate


   [ KCLAR 2.8. ]


   The "ok-as-delegate" flag provides a way for a KDC to communicate
   local realm policy to a client regarding whether the service for
   which the ticket is issued is trusted to accept delegated
   credentials.  For some applications, a client may need to delegate
   credentials to a service to act on its behalf in contacting other
   services.  The ability of a client to obtain a service ticket for a
   service conveys no information to the client about whether the
   service should be trusted to accept delegated credentials.


   The copy of the ticket flags visible to the client may have the "ok-
   as-delegate" flag set to indicate to the client that the service
   specified in the ticket has been determined by policy of the realm to
   be a suitable recipient of delegation.  A client can use the presence
   of this flag to help it make a decision whether to delegate
   credentials (either grant a proxy or a forwarded ticket-granting
   ticket) to this service.  It is acceptable to ignore the value of
   this flag.  When setting this flag, an administrator should consider
   the security and placement of the server on which the service will
   run, as well as whether the service requires the use of delegated
   credentials.


7.2.4.  Renewable Tickets


   [ adapted KCLAR 2.3. ]


   The "renewable" flag indicates whether the ticket may be renewed.


   Renewable tickets can be used to mitigate the consequences of ticket
   theft.  Applications may desire to hold credentials which can be
   valid for long periods of time.  However, this can expose the
   credentials to potential theft for equally long periods, and those
   stolen credentials would be valid until the expiration time of the
   ticket(s).  Simply using short-lived tickets and obtaining new ones


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   periodically would require the application to have long-term access
   to the client's secret key, which is an even greater risk.


   Renewable tickets have two "expiration times": the first is when the
   current instance of the ticket expires, and the second is the latest
   permissible value for an individual expiration time.  An application
   client must periodically present an unexpired renewable ticket to the
   KDC, setting the "renew" option in the KDC request.  The KDC will
   issue a new ticket with a new session key and a later expiration
   time.  All other fields of the ticket are left unmodified by the
   renewal process.  When the latest permissible expiration time
   arrives, the ticket expires permanently.  At each renewal, the KDC
   MAY consult a hot-list to determine if the ticket had been reported
   stolen since its last renewal; it will refuse to renew such stolen
   tickets, and thus the usable lifetime of stolen tickets is reduced.


   The "renewable" flag in a ticket is normally only interpreted by the
   ticket-granting service.  It can usually be ignored by application
   servers.  However, some particularly careful application servers MAY
   disallow renewable tickets.


   If a renewable ticket is not renewed by its expiration time, the KDC
   will not renew the ticket.  The "renewable" flag is clear by default,
   but a client can request it be set by setting the "renewable" option
   in the AS-REQ message.  If it is set, then the "renew-till" field in
   the ticket contains the time after which the ticket may not be
   renewed.


7.2.5.  Postdated Tickets


   postdated
        indicates a ticket which has been postdated


   may-postdate
        indicates that postdated tickets may be issued based on this
        ticket


   [ KCLAR 2.4. ]


   Applications may occasionally need to obtain tickets for use much
   later, e.g., a batch submission system would need tickets to be valid
   at the time the batch job is serviced.  However, it is dangerous to
   hold valid tickets in a batch queue, since they will be on-line
   longer and more prone to theft.  Postdated tickets provide a way to
   obtain these tickets from the KDC at job submission time, but to
   leave them "dormant" until they are activated and validated by a
   further request of the KDC.  If a ticket theft were reported in the
   interim, the KDC would refuse to validate the ticket, and the thief
   would be foiled.




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   The "may-postdate" flag in a ticket is normally only interpreted by
   the TGS.  It can be ignored by application servers.  This flag MUST
   be set in a ticket-granting ticket in order for the KDC to issue a
   postdated ticket based on the presented ticket.  It is reset by
   default; it MAY be requested by a client by setting the "allow-
   postdate" option in the AS-REQ [?also TGS-REQ?] message.  This flag
   does not allow a client to obtain a postdated ticket-granting ticket;
   postdated ticket-granting tickets can only by obtained by requesting
   the postdating in the AS-REQ message.  The life (endtime minus
   starttime) of a postdated ticket will be the remaining life of the
   ticket-granting ticket at the time of the request, unless the
   "renewable" option is also set, in which case it can be the full life
   (endtime minus starttime) of the ticket-granting ticket.  The KDC MAY
   limit how far in the future a ticket may be postdated.


   The "postdated" flag indicates that a ticket has been postdated.  The
   application server can check the authtime field in the ticket to see
   when the original authentication occurred.  Some services MAY choose
   to reject postdated tickets, or they may only accept them within a
   certain period after the original authentication.  When the KDC
   issues a "postdated" ticket, it will also be marked as "invalid", so
   that the application client MUST present the ticket to the KDC for
   validation before use.


7.2.6.  Proxiable and Proxy Tickets


   proxy
        indicates a proxy ticket


   proxiable
        indicates that proxy tickets may be issued based on this ticket


   [ KCLAR 2.5. ]


   It may be necessary for a principal to allow a service to perform an
   operation on its behalf.  The service must be able to take on the
   identity of the client, but only for a particular purpose.  A
   principal can allow a service to take on the principal's identity for
   a particular purpose by granting it a proxy.


   The process of granting a proxy using the "proxy" and "proxiable"
   flags is used to provide credentials for use with specific services.
   Though conceptually also a proxy, users wishing to delegate their
   identity in a form usable for all purposes MUST use the ticket
   forwarding mechanism described in the next section to forward a
   ticket-granting ticket.


   The "proxiable" flag in a ticket is normally only interpreted by the
   ticket-granting service.  It can be ignored by application servers.
   When set, this flag tells the ticket-granting server that it is OK to
   issue a new ticket (but not a ticket-granting ticket) with a


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   different network address based on this ticket.  This flag is set if
   requested by the client on initial authentication.  By default, the
   client will request that it be set when requesting a ticket-granting
   ticket, and reset when requesting any other ticket.


   This flag allows a client to pass a proxy to a server to perform a
   remote request on its behalf (e.g. a print service client can give
   the print server a proxy to access the client's files on a particular
   file server in order to satisfy a print request).


   In order to complicate the use of stolen credentials, Kerberos
   tickets may contain a set of network addresses from which they are
   valid.  When granting a proxy, the client MUST specify the new
   network address from which the proxy is to be used, or indicate that
   the proxy is to be issued for use from any address.


   The "proxy" flag is set in a ticket by the TGS when it issues a proxy
   ticket.  Application servers MAY check this flag and at their option
   they MAY require additional authentication from the agent presenting
   the proxy in order to provide an audit trail.


7.2.7.  Forwarded and Forwardable Tickets


   forwarded
        indicates a forwarded ticket


   forwardable
        indicates that forwarded tickets may be issued based on this
        ticket


   [ KCLAR 2.6. ]


   Authentication forwarding is an instance of a proxy where the service
   that is granted is complete use of the client's identity.  An example
   where it might be used is when a user logs in to a remote system and
   wants authentication to work from that system as if the login were
   local.


   The "forwardable" flag in a ticket is normally only interpreted by
   the ticket-granting service.  It can be ignored by application
   servers.  The "forwardable" flag has an interpretation similar to
   that of the "proxiable" flag, except ticket-granting tickets may also
   be issued with different network addresses.  This flag is reset by
   default, but users MAY request that it be set by setting the
   "forwardable" option in the AS request when they request their
   initial ticket-granting ticket.


   This flag allows for authentication forwarding without requiring the
   user to enter a password again.  If the flag is not set, then
   authentication forwarding is not permitted, but the same result can
   still be achieved if the user engages in the AS exchange specifying


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   the requested network addresses and supplies a password.


   The "forwarded" flag is set by the TGS when a client presents a
   ticket with the "forwardable" flag set and requests a forwarded
   ticket by specifying the "forwarded" KDC option and supplying a set
   of addresses for the new ticket.  It is also set in all tickets
   issued based on tickets with the "forwarded" flag set.  Application
   servers may choose to process "forwarded" tickets differently than
   non-forwarded tickets.


   If addressless tickets are forwarded from one system to another,
   clients SHOULD still use this option to obtain a new TGT in order to
   have different session keys on the different systems.


7.3.  Transited Realms


   [ KCLAR 2.7., plus new stuff ]


7.4.  Authorization Data


   [ KCLAR 5.2.6. ]


      ADType          ::= TH-id


      AuthorizationData       ::= SEQUENCE OF SEQUENCE {
          ad-type             [0] ADType,
          ad-data             [1] OCTET STRING
      }



   ad-type
        This field identifies the contents of the ad-data.  All negative
        values are reserved for local use.  Non-negative values are
        reserved for registered use.


   ad-data
        This field contains authorization data to be interpreted
        according to the value of the corresponding ad-type field.


   Each sequence of ADType and OCTET STRING is referred to as an
   authorization element.  Elements MAY be application specific,
   however, there is a common set of recursive elements that should be
   understood by all implementations.  These elements contain other
   AuthorizationData, and the interpretation of the encapsulating
   element determines which enclosed elements must be interpreted, and
   which may be ignored.


   Depending on the meaning of the encapsulating element, the
   encapsulated AuthorizationData may be ignored, interpreted as issued
   directly by the KDC, or be stored in a separate plaintext part of the
   ticket.  The types of the encapsulating elements are specified as


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   part of the Kerberos protocol because behavior based on these
   container elements should be understood across implementations, while
   other elements need only be understood by the applications which they
   affect.


   Authorization data elements are considered critical if present in a
   ticket or authenticator.  Unless encapsulated in a known
   authorization data element modifying the criticality of the elements
   it contains, an application server MUST reject the authentication of
   a client whose AP-REQ or ticket contains an unrecognized
   authorization data element.  Authorization data is intended to
   restrict the use of a ticket.  If the service cannot determine
   whether it is the target of a restriction, a security weakness may
   exist if the ticket can be used for that service.  Authorization
   elements that are truly optional can be enclosed in AD-IF-RELEVANT
   element.



      ad-type |           contents of ad-data
      ________|_______________________________________
            1 |   DER encoding of AD-IF-RELEVANT
            4 |   DER encoding of AD-KDCIssued
            5 |   DER encoding of AD-AND-OR
            8 |   DER encoding of AD-MANDATORY-FOR-KDC




7.4.1.  AD-IF-RELEVANT


      ad-if-relevant          ADType ::= int32 : 1


      -- Encapsulates another AuthorizationData.
      -- Intended for application servers; receiving application servers
      -- MAY ignore.
      AD-IF-RELEVANT          ::= AuthorizationData


   AD elements encapsulated within the if-relevant element are intended
   for interpretation only by application servers that understand the
   particular ad-type of the embedded element. Application servers that
   do not understand the type of an element embedded within the if-
   relevant element MAY ignore the uninterpretable element. This element
   promotes interoperability across implementations which may have local
   extensions for authorization.  The ad-type for AD-IF-RELEVANT is (1).


7.4.2.  AD-KDCIssued








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      -- KDC-issued privilege attributes
      ad-kdcissued            ADType ::= int32 : 4


      AD-KDCIssued            ::= SEQUENCE {
          ad-checksum [0] ChecksumOf {
                              AuthorizationData, { key-session },
                              { ku-ad-KDCIssued-cksum }},
          i-realm     [1] Realm OPTIONAL,
          i-sname     [2] PrincipalName OPTIONAL,
          elements    [3] AuthorizationData
      }



   ad-checksum
        A cryptographic checksum computed over the DER encoding of the
        AuthorizationData in the "elements" field, keyed with the
        session key.  Its checksumtype is the mandatory checksum type
        for the encryption type of the session key, and its key usage
        value is 19.


   i-realm, i-sname
        The name of the issuing principal if different from the KDC
        itself.  This field would be used when the KDC can verify the
        authenticity of elements signed by the issuing principal and it
        allows this KDC to notify the application server of the validity
        of those elements.


   elements
        AuthorizationData issued by the KDC.


   The KDC-issued ad-data field is intended to provide a means for
   Kerberos credentials to embed within themselves privilege attributes
   and other mechanisms for positive authorization, amplifying the
   privileges of the principal beyond what it would have if using
   credentials without such an authorization-data element.


   This amplification of privileges cannot be provided without this
   element because the definition of the authorization-data field allows
   elements to be added at will by the bearer of a TGT at the time that
   they request service tickets and elements may also be added to a
   delegated ticket by inclusion in the authenticator.


   For KDC-issued elements this is prevented because the elements are
   signed by the KDC by including a checksum encrypted using the
   server's key (the same key used to encrypt the ticket -- or a key
   derived from that key).  AuthorizationData encapsulated with in the
   AD-KDCIssued element MUST be ignored by the application server if
   this "signature" is not present.  Further, AuthorizationData
   encapsulated within this element from a ticket-granting ticket MAY be
   interpreted by the KDC, and used as a basis according to policy for
   including new signed elements within derivative tickets, but they


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   will not be copied to a derivative ticket directly.  If they are
   copied directly to a derivative ticket by a KDC that is not aware of
   this element, the signature will not be correct for the application
   ticket elements, and the field will be ignored by the application
   server.


   This element and the elements it encapsulates MAY be safely ignored
   by applications, application servers, and KDCs that do not implement
   this element.


   The ad-type for AD-KDC-ISSUED is (4).


7.4.3.  AD-AND-OR


      ad-and-or               ADType ::= int32 : 5


      AD-AND-OR               ::= SEQUENCE {
          condition-count     [0] INTEGER,
          elements            [1] AuthorizationData
      }



   When restrictive AD elements are encapsulated within the and-or
   element, the and-or element is considered satisfied if and only if at
   least the number of encapsulated elements specified in condition-
   count are satisfied.  Therefore, this element MAY be used to
   implement an "or" operation by setting the condition-count field to
   1, and it MAY specify an "and" operation by setting the condition
   count to the number of embedded elements. Application servers that do
   not implement this element MUST reject tickets that contain
   authorization data elements of this type.


   The ad-type for AD-AND-OR is (5).


7.4.4.  AD-MANDATORY-FOR-KDC


      -- KDCs MUST interpret any AuthorizationData wrapped in this.
      ad-mandatory-for-kdc            ADType ::= int32 : 8
      AD-MANDATORY-FOR-KDC            ::= AuthorizationData


   AD elements encapsulated within the mandatory-for-kdc element are to
   be interpreted by the KDC.  KDCs that do not understand the type of
   an element embedded within the mandatory-for-kdc element MUST reject
   the request.


   The ad-type for AD-MANDATORY-FOR-KDC is (8).


7.5.  Encrypted Part of Ticket


   The complete definition of the encrypted part is



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      -- Encrypted part of ticket
      EncTicketPart ::= CHOICE {
          rfc1510     [APPLICATION 3] EncTicketPart1510,
          ext         [APPLICATION 5] EncTicketPartExt
      }


   with the common portion being


      EncTicketPartCommon ::= SEQUENCE {
          flags               [0] TicketFlags,
          key                 [1] EncryptionKey,
          crealm              [2] Realm,
          cname               [3] PrincipalName,
          transited           [4] TransitedEncoding,
          authtime            [5] KerberosTime,
          starttime           [6] KerberosTime OPTIONAL,
          endtime             [7] KerberosTime,
          renew-till          [8] KerberosTime OPTIONAL,
          caddr               [9] HostAddresses OPTIONAL,
          authorization-data  [10] AuthorizationData OPTIONAL,
          ...
      }



   The encrypted part of the backwards-compatibility form of a ticket
   is:


      EncTicketPart1510 ::= SEQUENCE {
          COMPONENTS OF EncTicketPartCommon
      } (WITH COMPONENTS {
          ...,
          -- explicitly force IA5 in strings
          crealm (RealmIA5),
          cname (PrincipalNameIA5)
      })


   The encrypted part of the extensible form of a ticket is:


      EncTicketPartExt ::= EncTicketPartCommon (WITH COMPONENTS {
          ...,
          -- explicitly force UTF-8 in strings
          crealm (RealmExt),
          cname (PrincipalNameExt),
          -- explicitly constrain caddr to be non-empty if present
          caddr (SIZE (1..MAX)),
          -- forbid empty authorization-data encodings
          authorization-data (SIZE (1..MAX))
      })





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7.6.  Cleartext Part of Ticket


   The complete definition of Ticket is:


      Ticket          ::= CHOICE {
          rfc1510     [APPLICATION 1] Ticket1510,
          ext         [APPLICATION 4] Signed {
              TicketExt, { key-server }, { ku-Ticket-cksum }
          }
      }


   with the common portion being:


      -- takes a parameter specifying which type gets encrypted
      TicketCommon { EncPart } ::= SEQUENCE {
          tkt-vno     [0] INTEGER (5),
          realm       [1] Realm,
          sname       [2] PrincipalName,
          enc-part    [3] EncryptedData {
              EncPart, { key-server }, { ku-Ticket }
          },
          ...,
          extensions          [4] TicketExtensions OPTIONAL,
          ...
      }



   The "sname" field provides the name of the target service principal
   in cleartext, as a hint to aid the server in choosing the correct
   decryption key.


   The backwards-compatibility form of Ticket is:


      Ticket1510 ::= SEQUENCE {
          COMPONENTS OF TicketCommon { EncTicketPart1510 }
      } (WITH COMPONENTS {
          ...,
          -- explicitly force IA5 in strings
          realm (RealmIA5),
          sname (PrincipalNameIA5)
      })


   The extensible form of Ticket is:










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      -- APPLICATION tag goes inside Signed{} as well as outside,
      -- to prevent possible substitution attacks.
      TicketExt ::= [APPLICATION 4] TicketCommon {
          EncTicketPartExt
      } (WITH COMPONENTS {
          ...,
          -- explicitly force UTF-8 in strings
          realm (RealmExt),
          sname (PrincipalNameExt)
      })



   TicketExtensions, which may only be present in the extensible form of
   Ticket, are a cleartext typed hole for extension use.
   AuthorizationData already provides an encrypted typed hole.


      TEType                  ::= TH-id


      -- ticket extensions: for TicketExt only
      TicketExtensions        ::= SEQUENCE (SIZE (1..MAX)) OF SEQUENCE {
          te-type             [0] TEType,
          te-data             [1] OCTET STRING
      }



   A client will only receive an extensible Ticket if the application
   server supports extensibility.


8.  Credential Acquisition


   There are two exchanges that can be used for acquiring credentials:
   the AS exchange and the TGS exchange.  These exchanges have many
   similarities, and this document describes them in parallel, noting
   which behaviors are specific to one type of exchange.  The AS request
   (AS-REQ) and TGS request (TGS-REQ) are both forms of KDC requests
   (KDC-REQ).  Likewise, the AS reply (AS-REP) and TGS reply (TGS-REP)
   are forms of KDC replies (KDC-REP).


   The credentials acquisition protocol operates over specific
   transports.  Additionally, specific methods exist to permit a client
   to discover the KDC host with which to communicate.


8.1.  KDC-REQ


   The KDC-REQ has a large number of fields in common between the RFC
   1510 and the extensible versions.  The KDC-REQ type itself is never
   directly encoded; it is always a part of a AS-REQ or a TGS-REQ.






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      KDC-REQ-COMMON  ::= SEQUENCE {
      -- NOTE: first tag is [1], not [0]
          pvno        [1] INTEGER (5),
          msg-type    [2] INTEGER (10 -- AS-REQ.rfc1510 --
                                   | 12 -- TGS-REQ.rfc1510 --
                                   | 6 -- AS-REQ.ext --
                                   | 8 -- TGS-REQ.ext -- ),
          padata      [3] SEQUENCE OF PA-DATA OPTIONAL
          -- NOTE: not empty
      }



      KDC-REQ-BODY-COMMON     ::= SEQUENCE {
          kdc-options         [0] KDCOptions,
          cname               [1] PrincipalName OPTIONAL
          -- Used only in AS-REQ --,


          realm               [2] Realm
          -- Server's realm; also client's in AS-REQ --,


          sname               [3] PrincipalName OPTIONAL,
          from                [4] KerberosTime OPTIONAL,
          till                [5] KerberosTime OPTIONAL
          -- was required in rfc1510;
          -- still required for compat versions
          -- of messages --,


          rtime               [6] KerberosTime OPTIONAL,
          nonce               [7] Nonce,
          etype               [8] SEQUENCE OF EType
          -- in preference order --,


          addresses           [9] HostAddresses OPTIONAL,
          enc-authorization-data      [10] EncryptedData {
              AuthorizationData, { key-session | key-subsession },
              { ku-TGSReqAuthData-subkey |
                ku-TGSReqAuthData-sesskey }
          } OPTIONAL,


          additional-tickets  [11] SEQUENCE OF Ticket OPTIONAL
          -- NOTE: not empty --,
          ...
          lang-tags   [5] SEQUENCE (SIZE (1..MAX)) OF
                              LangTag OPTIONAL,
          ...
      }



   Many fields of KDC-REQ-BODY-COMMON correspond directly to fields of
   an EncTicketPartCommon.  The KDC copies most of them unchanged,
   provided that the requested values meet site policy.


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   kdc-options
        These flags do not correspond directly to "flags" in
        EncTicketPartCommon.


   cname
        This field is copied to the "cname" field in
        EncTicketPartCommon.  The "cname" field is required in an AS-
        REQ; the client places its own name here.  In a TGS-REQ, the
        "cname" in the ticket in the AP-REQ takes precedence.


   realm
        This field is the server's realm, and also holds the client's
        realm in an AS-REQ.


   sname
        The "sname" field indicates the server's name.  It may be absent
        in a TGS-REQ which requests user-to-user authentication, in
        which case the "sname" of the issued ticket will be taken from
        the included additional ticket.


   The "from", "till", and "rtime" fields correspond to the "starttime",
   "endtime", and "renew-till" fields of EncTicketPartCommon.


   addresses
        This field corresponds to the "caddr" field of
        EncTicketPartCommon.


   enc-authorization-data
        For TGS-REQ, this field contains authorization data encrypted
        using either the TGT session key or the AP-REQ subsession key;
        the KDC may copy these into the "authorization-data" field of
        EncTicketPartCommon if policy permits.


   lang-tags
        Only present in the extensible messages.  Specifies the set of
        languages which the client is willing to accept in error
        messages.


   KDC options used in a KDC-REQ are:














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      KDCOptions      ::= KerberosFlags { KDCOptionsBits }


      KDCOptionsBits  ::= BIT STRING {
          reserved            (0),
          forwardable         (1),
          forwarded           (2),
          proxiable           (3),
          proxy               (4),
          allow-postdate      (5),
          postdated           (6),
          unused7             (7),
          renewable           (8),
          unused9             (9),
          unused10            (10),
          unused11            (11),
          unused12            (12),
          unused13            (13),
          requestanonymous    (14),
          canonicalize        (15),
          disable-transited-check (26),
          renewable-ok        (27),
          enc-tkt-in-skey     (28),
          renew               (30),
          validate            (31)
          -- XXX need "need ticket1" flag?
      }


   Different options apply to different phases of KDC-REQ processing.


   The backwards-compatibility form of a KDC-REQ is:


      KDC-REQ-1510    ::= SEQUENCE {
          COMPONENTS OF KDC-REQ-COMMON,
          req-body    [4] KDC-REQ-BODY-1510
      } (WITH COMPONENTS { ..., msg-type (10 | 12) })


   The extensible form of a KDC-REQ is:


      -- APPLICATION tag goes inside Signed{} as well as outside,
      -- to prevent possible substitution attacks.
      KDC-REQ-EXT     ::= SEQUENCE {
          COMPONENTS OF KDC-REQ-COMMON,
          req-body    [4] KDC-REQ-BODY-EXT,
          ...
      } (WITH COMPONENTS {
          ...,
          msg-type (6 | 8),
          padata (SIZE (1..MAX))
      })


   The backwards-compatibility form of a KDC-REQ-BODY is:


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      KDC-REQ-BODY-1510 ::= SEQUENCE {
          COMPONENTS OF KDC-REQ-BODY-COMMON
      } (WITH COMPONENTS {
          ...,
          cname (PrincipalNameIA5),
          realm (RealmIA5),
          sname (PrincipalNameIA5),
          till PRESENT,
          nonce (UInt32)
      })


   The extensible form of a KDC-REQ-BODY is:


      KDC-REQ-BODY-EXT        ::= KDC-REQ-BODY-COMMON
      (WITH COMPONENTS {
          ...,
          cname (PrincipalNameExt),
          realm (RealmExt),
          sname (PrincipalNameExt),
          addresses (SIZE (1..MAX)),
          enc-authorization-data (EncryptedData {
              AuthorizationData (SIZE (1..MAX)),
              { key-session | key-subsession },
              { ku-TGSReqAuthData-subkey |
                ku-TGSReqAuthData-sesskey }
          }),
          additional-tickets (SIZE (1..MAX))
      })


   The AS-REQ is:























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      AS-REQ  ::= CHOICE {
          rfc1510     [APPLICATION 10] KDC-REQ-1510
          (WITH COMPONENTS {
              ...,
              msg-type (10),
              -- AS-REQ must include client name
              req-body (WITH COMPONENTS { ..., cname PRESENT })
          }),
          ext         [APPLICATION 6]  Signed {
              -- APPLICATION tag goes inside Signed{} as well as
              -- outside, to prevent possible substitution attacks.
              [APPLICATION 6] KDC-REQ-EXT,
              -- not sure this is correct key to use; do we even want
              -- to sign AS-REQ?
              { key-client },
              { ku-ASReq-cksum }
          }
          (WITH COMPONENTS {
              ...,
              msg-type  (6),
              -- AS-REQ must include client name
              req-body (WITH COMPONENTS { ..., cname PRESENT })
          })
      }


   A client SHOULD NOT send the extensible AS-REQ alternative to a KDC
   if the client does not know that the KDC supports the extensibility
   framework.  A client SHOULD send the extensible AS-REQ alternative in
   a PA-AS-REQ PA-DATA.  A KDC supporting extensibility will treat the
   AS-REQ contained within the PA-AS-REQ as the actual AS-REQ.  [ XXX
   what if their contents conflict? ]


   The TGS-REQ is:




















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      TGS-REQ ::= CHOICE {
          rfc1510     [APPLICATION 12] KDC-REQ-1510
          (WITH COMPONENTS {
              ...,
              msg-type (12),
              -- client name optional in TGS-REQ
              req-body (WITH COMPONENTS { ..., cname ABSENT })
          }),
          ext         [APPLICATION 8]  Signed {
              -- APPLICATION tag goes inside Signed{} as well as
              -- outside, to prevent possible substitution attacks.
              [APPLICATION 8] KDC-REQ-EXT,
              { key-session }, { ku-TGSReq-cksum }
          }
          (WITH COMPONENTS {
              ...,
              msg-type  (8),
              -- client name optional in TGS-REQ
              req-body (WITH COMPONENTS { ..., cname ABSENT })
          })
      }



8.2.  PA-DATA


   PA-DATA have multiple uses in the Kerberos protocol.  They may pre-
   authenticate an AS-REQ; they may also modify several of the
   encryption keys used in a KDC-REP.  PA-DATA may also provide "hints"
   to the client about which long-term key (usually password-derived) to
   use.  PA-DATA may also include "hints" about which pre-authentication
   mechanisms to use, or include data for input into a pre-
   authentication mechanism.


   [ XXX enumerate standard padata here ]


8.3.  KDC-REQ Processing


   Processing of a KDC-REQ proceeds through several steps.  An
   implementation need not perform these steps exactly as described, as
   long as it behaves as if the steps were performed as described.  The
   KDC performs replay handling upon receiving the request; it then
   validates the request, adjusts timestamps, and selects the keys used
   in the reply.  It copies data from the request into the issued
   ticket, adjusting the values to conform with its policies.  The KDC
   then transmits the reply to the client.


8.3.1.  Handling Replays


   Because Kerberos can run over unreliable transports such as UDP, the
   KDC MUST be prepared to retransmit responses in case they are lost.
   If a KDC receives a request identical to one it has recently


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   successfully processed, the KDC MUST respond with a KDC-REP message
   rather than a replay error.  In order to reduce the amount of
   ciphertext given to a potential attacker, KDCs MAY send the same
   response generated when the request was first handled.  KDCs MUST
   obey this replay behavior even if the actual transport in use is
   reliable.  If the AP-REQ which authenticates a TGS-REQ is a replay,
   and the entire request is not identical to a recently successfully
   processed request, the KDC SHOULD return "krb-ap-err-repeat", as is
   appropriate for AP-REQ processing.


8.3.2.  Request Validation


8.3.2.1.  AS-REQ Authentication


   Site policy determines whether a given client principal is required
   to provide some pre-authentication prior to receiving an AS-REP.
   Since the default reply key is typically the client's long-term
   password-based key, an attacker may easily request known plaintext
   (in the form of an AS-REP) upon which to mount a dictionary attack.
   Pre-authentication can limit the possibility of such an attack.


   If site policy requires pre-authentication for a client principal,
   and no pre-authentication is provided, the KDC returns the error
   "kdc-err-preauth-required".  Accompanying this error are "e-data"
   which include hints telling the client which pre-authentication
   mechanisms to use, or data for input to pre-authentication mechanisms
   (e.g., input to challenge-response systems).  If pre-authentication
   fails, the KDC returns the error "kdc-err-preauth-failed".


   [ may need additional changes based on Sam's preauth draft ]


8.3.2.2.  TGS-REQ Authentication


   A TGS-REQ has an accompanying AP-REQ, which is included in the "pa-
   tgs-req".  The KDC MUST validate the checksum in the Authenticator of
   the AP-REQ, which is computed over the KDC-REQ-BODY-1510 or KDC-REQ-
   BODY-EXT (for TGS-REQ-1510 or TGS-REQ-EXT, respectively) of the
   request.  [ padata not signed by authenticator! ] Any error from the
   AP-REQ validation process SHOULD be returned in a KRB-ERROR message.
   The service principal in the ticket of the AP-REQ may be a ticket-
   granting service principal, or a normal application service
   principal.  A ticket which is not a ticket-granting ticket MUST NOT
   be used to issue a ticket for any service other than the one named in
   the ticket.  In this case, the "renew", "validate", or "proxy" [?also
   forwarded?]  option must be set in the request.


8.3.2.3.  Principal Validation


   If the client principal in an AS-REQ is unknown, the KDC returns the
   error "kdc-err-c-principal-unknown".  If the server principal in a
   KDC-REQ is unknown, the KDC returns the error "kdc-err-s-principal-


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   unknown".


8.3.2.4.  Checking For Revoked or Invalid Tickets


   [ KCLAR 3.3.3.1 ]


   Whenever a request is made to the ticket-granting server, the
   presented ticket(s) is(are) checked against a hot-list of tickets
   which have been canceled.  This hot-list might be implemented by
   storing a range of issue timestamps for "suspect tickets"; if a
   presented ticket had an authtime in that range, it would be rejected.
   In this way, a stolen ticket-granting ticket or renewable ticket
   cannot be used to gain additional tickets (renewals or otherwise)
   once the theft has been reported to the KDC for the realm in which
   the server resides.  Any normal ticket obtained before it was
   reported stolen will still be valid (because they require no
   interaction with the KDC), but only until their normal expiration
   time.  If TGTs have been issued for cross-realm authentication, use
   of the cross-realm TGT will not be affected unless the hot-list is
   propagated to the KDCs for the realms for which such cross-realm
   tickets were issued.


   If a TGS-REQ ticket has its "invalid" flag set, the KDC MUST NOT
   issue any ticket unless the TGS-REQ requests the "validate" option.


8.3.3.  Timestamp Handling


   [ some aspects of timestamp handling, especially regarding postdating
   and renewal, are difficult to read in KCLAR... needs closer
   examination here ]


   Processing of "starttime" (requested in the "from" field) differs
   depending on whether the "postdated" option is set in the request.
   If the "postdated" option is not set, and the requested "starttime"
   is in the future beyond the window of acceptable clock skew, the KDC
   returns the error "kdc-err-cannot-postdate".  If the "postdated"
   option is not set, and the requested "starttime" is absent or does
   not indicate a time in the future beyond the acceptable clock skew,
   the KDC sets the "starttime" to the KDC's current time.  The
   "postdated" option MUST NOT be honored if the ticket is being
   requested by TGS-REQ and the "may-postdate" is not set in the TGT.
   Otherwise, if the "postdated" option is set, and site policy permits,
   the KDC sets the "starttime" as requested, and sets the "invalid"
   flag in the new ticket.


   The "till" field is required in the RFC 1510 version of the KDC-REQ.
   If the "till" field is equal to "19700101000000Z" (midnight, January
   1, 1970), the KDC SHOULD behave as if the "till" field were absent.


   The KDC MUST NOT issue a ticket whose "starttime", "endtime", or
   "renew-till" time is later than the "renew-till" time of the ticket


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   from which it is derived.


8.3.3.1.  AS-REQ Timestamp Processing


   In the AS exchange, the "authtime" of a ticket is set to the local
   time at the KDC.


   The "endtime" of the ticket will be set to the earlier of the
   requested "till" time and a time determined by local policy, possibly
   determined using factors specific to the realm or principal.  For
   example, the expiration time MAY be set to the earliest of the
   following:


      * the expiration time ("till" value) requested


      * the ticket's start time plus the maximum allowable lifetime
        associated with the client principal from the authentication
        server's database


      * the ticket's start time plus the maximum allowable lifetime
        associated with the server principal


      * the ticket's start time plus the maximum lifetime set by the
        policy of the local realm


   If the requested expiration time minus the start time (as determined
   above) is less than a site-determined minimum lifetime, an error
   message with code "kdc-err-never-valid" is returned.  If the
   requested expiration time for the ticket exceeds what was determined
   as above, and if the "renewable-ok" option was requested, then the
   "renewable" flag is set in the new ticket, and the "renew-till" value
   is set as if the "renewable" option were requested.


   If the "renewable" option has been requested or if the "renewable-ok"
   option has been set and a renewable ticket is to be issued, then the
   "renew-till" field MAY be set to the earliest of:


      * its requested value


      * the start time of the ticket plus the minimum of the two maximum
        renewable lifetimes associated with the principals' database
        entries


      * the start time of the ticket plus the maximum renewable lifetime
        set by the policy of the local realm


8.3.3.2.  TGS-REQ Timestamp Processing


   In the TGS exchange, the KDC sets the "authtime" to that of the
   ticket in the AP-REQ authenticating the TGS-REQ.  [?application
   server can print a ticket for itself with a spoofed authtime.


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   security issues for hot-list?] [ MIT implementation may change
   authtime of renewed tickets; needs check... ]


   If the TGS-REQ has a TGT as the ticket in its AP-REQ, and the TGS-REQ
   requests an "endtime" (in the "till" field), then the "endtime" of
   the new ticket is set to the minimum of


      * the requested "endtime" value,


      * the "endtime" in the TGT, and


      * an "endtime" determined by site policy on ticket lifetimes.


   If the new ticket is a renewal, the "endtime" of the new ticket is
   bounded by the minimum of


      * the requested "endtime" value,


      * the value of the "renew-till" value of the old,


      * the "starttime" of the new ticket plus the lifetime (endtime
        minus starttime) of the old ticket, i.e., the lifetime of the
        new ticket may not exceed that of the ticket being renewed [
        adapted from KCLAR 3.3.3. ], and


      * an "endtime" determined by site policy on ticket lifetimes.


   When handling a TGS-REQ, a KDC MUST NOT issue a postdated ticket with
   a "starttime", "endtime", or "renew-till" time later than the
   "renew-till" time of the TGT.


8.3.4.  Handling Transited Realms


   The KDC checks the ticket in a TGS-REQ against site policy, unless
   the "disable-transited-check" option is set in the TGS-REQ.  If site
   policy permits the transit path in the TGS-REQ ticket, the KDC sets
   the "transited-policy-checked" flag in the issued ticket.  If the
   "disable-transited-check" option is set, the issued ticket will have
   the "transited-policy-checked" flag cleared.


8.3.5.  Address Processing The requested "addresses" in the KDC-REQ are
   copied into the issued ticket.  If the "addresses" field is absent or
   empty in a TGS-REQ, the KDC copies addresses from the ticket in the
   TGS-REQ into the issued ticket, unless the either "forwarded" or
   "proxy" option is set.  If the "forwarded" option is set, and the
   ticket in the TGS-REQ has its "forwardable" flag set, the KDC copies
   the addresses from the TGS-REQ, not the from TGS-REQ ticket, into the
   issued ticket.  The KDC behaves similarly if the "proxy" option is
   set in the TGS-REQ and the "proxiable" flag is set in the ticket.
   The "proxy" option will not be honored on requests for additional
   ticket-granting tickets.


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8.3.6.  Ticket Flag Processing


   Many kdc-options request that the KDC set a corresponding flag in the
   issued ticket.  kdc-options marked with an asterisk (*) in the
   following table do not directly request the corresponding ticket flag
   and therefore require special handling.



             kdc-option       |    ticket flag affected
      ________________________|__________________________
      forwardable             |  forwardable
      forwarded               |  forwarded
      proxiable               |  proxiable
      proxy                   |  proxy
      allow-postdate          |  may-postdate
      postdated               |  postdated
      renewable               |  renewable
      requestanonymous        |  anonymous
      canonicalize            |  -
      disable-transited-check*|  transited-policy-checked
      renewable-ok*           |  renewable
      enc-tkt-in-skey         |  -
      renew                   |  -
      validate*               |  invalid




   forwarded
        The KDC sets the "forwarded" flag in the issued ticket if the
        "forwarded" option is set in the TGS-REQ and the "forwardable"
        flag is set in the TGS-REQ ticket.


   proxy
        The KDC sets the "proxy" flag in the issued ticket if the
        "proxy" option is set in the TGS-REQ and the "proxiable" flag is
        set in the TGS-REQ ticket.


   disable-transited-check
        The handling of the "disable-transited-check" kdc-option is
        described in Section 8.3.4.


   renewable-ok
        The handling of the "renewable-ok" kdc-option is described in
        Section 8.3.3.1.


   enc-tkt-in-skey
        This flag modifies ticket key selection to use the session key
        of an additional ticket included in the TGS-REQ, for the purpose
        of user-to-user authentication.




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   validate
        If the "validate" kdc-option is set in a TGS-REQ, and the
        "starttime" has passed, the KDC will clear the "invalid" bit on
        the ticket before re-issuing it.


8.3.7.  Key Selection


   Three keys are involved in creating a KDC-REP.  The reply key
   encrypts the encrypted part of the KDC-REP.  The session key is
   stored in the encrypted part of the ticket, and is also present in
   the encrypted part of the KDC-REP so that the client can retrieve it.
   The ticket key is used to encrypt the ticket.  These keys all have
   initial values for a given exchange; pre-authentication and other
   extension mechanisms may change the value used for any of these keys.


   [ again, may need changes based on Sam's preauth draft ]


8.3.7.1.  Reply Key and Session Key Selection


   The set of encryption types which the client will understand appears
   in the "etype" field of KDC-REQ-BODY-COMMON.  The KDC limits the set
   of possible reply keys and the set of session key encryption types
   based on the "etype" field.


   For the AS exchange, the reply key is initially a long-term key of
   the client, limited to those encryption types listed in the "etype"
   field.  The KDC SHOULD use the first valid strong "etype" for which
   an encryption key is available.  For the TGS exchange, the reply key
   is initially the subsession key of the Authenticator.  If the
   Authenticator subsesion key is absent, the reply key is initially the
   session key of the ticket used to authenticate the TGS-REQ.


   The session key is initially randomly generated, and has an
   encryption type which both the client and the server will understand.
   Typically, the KDC has prior knowledge of which encryption types the
   server will understand.  It selects the first valid strong "etype"
   listed the request which the server also will understand.


8.3.7.2.  Ticket Key Selection


   The ticket key is initially the long-term key of the service.  The
   "enc-tkt-in-skey" option requests user-to-user authentication, where
   the ticket encryption key of the issued ticket is set equal to the
   session key of the additional ticket in the request.


8.4.  KDC-REP


   The important parts of the KDC-REP are encrypted.





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      EncASRepPart1510        ::= [APPLICATION 25] EncKDCRepPart1510
      EncTGSRepPart1510       ::= [APPLICATION 26] EncKDCRepPart1510


      EncASRepPartExt         ::= [APPLICATION 32] EncKDCRepPartExt
      EncTGSRepPartExt        ::= [APPLICATION 33] EncKDCRepPartExt


      EncKDCRepPartCom        ::= SEQUENCE {
          key                 [0] EncryptionKey,
          last-req            [1] LastReq,
          nonce               [2] Nonce,
          key-expiration      [3] KerberosTime OPTIONAL,
          flags               [4] TicketFlags,
          authtime            [5] KerberosTime,
          starttime           [6] KerberosTime OPTIONAL,
          endtime             [7] KerberosTime,
          renew-till          [8] KerberosTime OPTIONAL,
          srealm              [9] Realm,
          sname               [10] PrincipalName,
          caddr               [11] HostAddresses OPTIONAL,
          ...
      }


      EncKDCRepPart1510       ::= SEQUENCE {
          COMPONENTS OF EncKDCRepPartCom
      } (WITH COMPONENTS {
          ...,
          srealm (RealmIA5),
          sname (PrincipalNameIA5),
          nonce UInt32 })


      EncKdcRepPartExt        ::= EncKDCRepPartCom (WITH COMPONENTS {
          ...,
          srealm (RealmExt),
          sname (PrincipalNameExt)
      })



   Most of the fields of EncKDCRepPartCom are duplicates of the
   corresponding fields in the returned ticket.














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      KDC-REP-COMMON { EncPart } ::= SEQUENCE {
          pvno        [0] INTEGER (5),
          msg-type    [1] INTEGER (11 -- AS-REP.rfc1510 -- |
                                   13 -- TGS.rfc1510 -- |
                                   7 -- AS-REP.ext -- |
                                   9 -- TGS-REP.ext -- ),
          padata      [2] SEQUENCE OF PA-DATA OPTIONAL,
          crealm      [3] Realm,
          cname       [4] PrincipalName,
          ticket      [5] Ticket,


          enc-part    [6] EncryptedData {
              EncPart,
              { key-reply },
              -- maybe reach into EncryptedData in AS-REP/TGS-REP
              -- definitions to apply constraints on key usages?
              { ku-EncASRepPart -- if AS-REP -- |
                ku-EncTGSRepPart-subkey -- if TGS-REP and
                                        -- using Authenticator
                                        -- session key -- |
                ku-EncTGSRepPart-sesskey -- if TGS-REP and using
                                         -- subsession key -- }
          },


          ...,
          -- In extensible version, KDC signs original request
          -- to avoid replay attacks against client.
          req-cksum   [7] ChecksumOf { CHOICE {
              as-req          AS-REQ,
              tgs-req         TGS-REQ
          }, { key-reply }, { ku-KDCRep-cksum }} OPTIONAL,
          lang-tag    [8] LangTag OPTIONAL,
          ...
      }



      KDC-REP-1510 { EncPart } ::= SEQUENCE {
          COMPONENTS OF KDC-REP-COMMON { EncPart }
      } (WITH COMPONENTS {
          ...,
          msg-type (11 | 13),
          crealm (RealmIA5),
          cname (PrincipalNameIA5)
      })









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      KDC-REP-EXT { EncPart } ::= KDC-REP-COMMON { EncPart }
      (WITH COMPONENTS {
          ...,
          msg-type (7 | 9),
          crealm (RealmExt),
          cname (PrincipalNameExt)
      })



   req-cksum
        Signature of the original request using the reply key, to avoid
        replay attacks against the client, among other things.  Only
        present in the extensible version of KDC-REP.


           AS-REP          ::= CHOICE {
               rfc1510     [APPLICATION 11] KDC-REP-1510 {
                   EncASRepPart1510
               } (WITH COMPONENTS { ..., msg-type (11) }),
               ext         [APPLICATION  7]  Signed {
                   [APPLICATION 7] KDC-REP-EXT { EncASRepPartExt },
                   { key-reply }, { ku-ASRep-cksum }
               } (WITH COMPONENTS { ..., msg-type (7) })
           }



           TGS-REP         ::= CHOICE {
               rfc1510     [APPLICATION 13] KDC-REP-1510 {
                   EncTGSRepPart1510
               } (WITH COMPONENTS { ..., msg-type (13) }),
               ext         [APPLICATION  9]  Signed {
                   [APPLICATION 9] KDC-REP-EXT { EncTGSRepPartExt },
                   { key-reply }, { ku-TGSRep-cksum }
               } (WITH COMPONENTS { ..., msg-type (9) })
           }



   The extensible versions of AS-REQ and TGS-REQ are signed with the
   reply key, to prevent an attacker from performing a delayed denial-
   of-service attack by substituting the ticket.


8.5.  Reply Validation


   [ signature verification ]


8.6.  IP Transports


   [ KCLAR 7.2 ]


   Kerberos defines two IP transport mechanisms for the credentials
   acquisition protocol: UDP/IP and TCP/IP.



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8.6.1.  UDP/IP transport


   Kerberos servers (KDCs) supporting IP transports MUST accept UDP
   requests and SHOULD listen for such requests on port 88 (decimal)
   unless specifically configured to listen on an alternative UDP port.
   Alternate ports MAY be used when running multiple KDCs for multiple
   realms on the same host.


   Kerberos clients supporting IP transports SHOULD support the sending
   of UDP requests. Clients SHOULD use KDC discovery (Section 8.6.3) to
   identify the IP address and port to which they will send their
   request.


   When contacting a KDC for a KRB_KDC_REQ request using UDP/IP
   transport, the client shall send a UDP datagram containing only an
   encoding of the request to the KDC. The KDC will respond with a reply
   datagram containing only an encoding of the reply message (either a
   KRB-ERROR or a KDC-REP) to the sending port at the sender's IP
   address. The response to a request made through UDP/IP transport MUST
   also use UDP/IP transport. If the response can not be handled using
   UDP (for example because it is too large), the KDC MUST return "krb-
   err-response-too-big", forcing the client to retry the request using
   the TCP transport.


8.6.2.  TCP/IP transport


   Kerberos servers (KDCs) supporting IP transports MUST accept TCP
   requests and SHOULD listen for such requests on port 88 (decimal)
   unless specifically configured to listen on an alternate TCP port.
   Alternate ports MAY be used when running multiple KDCs for multiple
   realms on the same host.


   Clients MUST support the sending of TCP requests, but MAY choose to
   initially try a request using the UDP transport. Clients SHOULD use
   KDC discovery (Section 8.6.3) to identify the IP address and port to
   which they will send their request.


   Implementation note: Some extensions to the Kerberos protocol will
   not succeed if any client or KDC not supporting the TCP transport is
   involved.  Implementations of RFC 1510 were not required to support
   TCP/IP transports.


   When the KDC-REQ message is sent to the KDC over a TCP stream, the
   response (KDC-REP or KRB-ERROR message) MUST be returned to the
   client on the same TCP stream that was established for the request.
   The KDC MAY close the TCP stream after sending a response, but MAY
   leave the stream open for a reasonable period of time if it expects a
   followup. Care must be taken in managing TCP/IP connections on the
   KDC to prevent denial of service attacks based on the number of open
   TCP/IP connections.



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   The client MUST be prepared to have the stream closed by the KDC at
   anytime after the receipt of a response.  A stream closure SHOULD NOT
   be treated as a fatal error.  Instead, if multiple exchanges are
   required (e.g., certain forms of pre-authentication) the client may
   need to establish a new connection when it is ready to send
   subsequent messages.  A client MAY close the stream after receiving a
   response, and SHOULD close the stream if it does not expect to send
   followup messages.


   A client MAY send multiple requests before receiving responses,
   though it must be prepared to handle the connection being closed
   after the first response.


   Each request (KDC-REQ) and response (KDC-REP or KRB-ERROR) sent over
   the TCP stream is preceded by the length of the request as 4 octets
   in network byte order. The high bit of the length is reserved for
   future expansion and MUST currently be set to zero.  If a KDC that
   does not understand how to interpret a set high bit of the length
   encoding receives a request with the high order bit of the length
   set, it MUST return a KRB-ERROR message with the error "krb-err-
   field-toolong" and MUST close the TCP stream.


   If multiple requests are sent over a single TCP connection, and the
   KDC sends multiple responses, the KDC is not required to send the
   responses in the order of the corresponding requests.  This may
   permit some implementations to send each response as soon as it is
   ready even if earlier requests are still being processed (for
   example, waiting for a response from an external device or database).


8.6.3.  KDC Discovery on IP Networks


   Kerberos client implementations MUST provide a means for the client
   to determine the location of the Kerberos Key Distribution Centers
   (KDCs).  Traditionally, Kerberos implementations have stored such
   configuration information in a file on each client machine.
   Experience has shown this method of storing configuration information
   presents problems with out-of-date information and scaling problems,
   especially when using cross-realm authentication. This section
   describes a method for using the Domain Name System [RFC 1035] for
   storing KDC location information.


8.6.3.1.  DNS vs. Kerberos - Case Sensitivity of Realm Names


   In Kerberos, realm names are case sensitive.  While it is strongly
   encouraged that all realm names be all upper case this recommendation
   has not been adopted by all sites.  Some sites use all lower case
   names and other use mixed case.  DNS, on the other hand, is case
   insensitive for queries.  Since the realm names "MYREALM", "myrealm",
   and "MyRealm" are all different, but resolve the same in the domain
   name system, it is necessary that only one of the possible
   combinations of upper and lower case characters be used in realm


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   names.


8.6.3.2.  DNS SRV records for KDC location


   KDC location information is to be stored using the DNS SRV RR [RFC
   2782].  The format of this RR is as follows:


      _Service._Proto.Realm TTL Class SRV Priority Weight Port Target


   The Service name for Kerberos is always "kerberos".


   The Proto can be one of "udp", "tcp". If these SRV records are to be
   used, both "udp" and "tcp" records MUST be specified for all KDC
   deployments.


   The Realm is the Kerberos realm that this record corresponds to.  The
   realm MUST be a domain style realm name.


   TTL, Class, SRV, Priority, Weight, and Target have the standard
   meaning as defined in RFC 2782.


   As per RFC 2782 the Port number used for "_udp" and "_tcp" SRV
   records SHOULD be the value assigned to "kerberos" by the Internet
   Assigned Number Authority: 88 (decimal) unless the KDC is configured
   to listen on an alternate TCP port.


   Implementation note: Many existing client implementations do not
   support KDC Discovery and are configured to send requests to the IANA
   assigned port (88 decimal), so it is strongly recommended that KDCs
   be configured to listen on that port.


8.6.3.3.  KDC Discovery for Domain Style Realm Names on IP Networks


   These are DNS records for a Kerberos realm EXAMPLE.COM. It has two
   Kerberos servers, kdc1.example.com and kdc2.example.com.  Queries
   should be directed to kdc1.example.com first as per the specified
   priority.  Weights are not used in these sample records.


      _kerberos._udp.EXAMPLE.COM.   IN   SRV   0 0 88 kdc1.example.com.
      _kerberos._udp.EXAMPLE.COM.   IN   SRV   1 0 88 kdc2.example.com.
      _kerberos._tcp.EXAMPLE.COM.   IN   SRV   0 0 88 kdc1.example.com.
      _kerberos._tcp.EXAMPLE.COM.   IN   SRV   1 0 88 kdc2.example.com.



9.  Errors


   The KRB-ERROR message is returned by the KDC if an error occurs
   during credentials acquisition.  It may also be returned by an
   application server if an error occurs during authentication.




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      ErrCode ::= Int32


      KRB-ERROR       ::= CHOICE {
          rfc1510     [APPLICATION 30] KRB-ERROR-1510,
          ext         [APPLICATION 23] Signed {
              KRB-ERROR-EXT, { ku-KrbError-cksum } }
      }



   The extensible KRB-ERROR is only signed if there has been a key
   negotiated with its recipient.  KRB-ERROR messages sent in response
   to AS-REQ messages will probably not be signed unless a
   preauthentication mechanism has negotiated a key.  (Signing using a
   client's long-term key can expose ciphertext to dictionary attacks.)


   The parts of a KRB-ERROR common to both the backwards-compatibility
   and the extensibile messages are


      KRB-ERROR-COMMON ::= SEQUENCE {
          pvno        [0] INTEGER (5),
          msg-type    [1] INTEGER (30 | 23),
          ctime       [2] KerberosTime OPTIONAL,
          cusec       [3] Microseconds OPTIONAL,
          stime       [4] KerberosTime,
          susec       [5] Microseconds,
          error-code  [6] ErrCode,
          crealm      [7] Realm OPTIONAL,
          cname       [8] PrincipalName OPTIONAL,
          realm       [9] Realm -- Correct realm --,
          sname       [10] PrincipalName -- Correct name --,
          e-text      [11] KerberosString OPTIONAL,
          e-data      [12] OCTET STRING OPTIONAL,
          ...,
          typed-data          [13] TYPED-DATA OPTIONAL,
          nonce               [14] Nonce OPTIONAL,
          lang-tag            [15] LangTag OPTIONAL,
          ...
      }



      KRB-ERROR-1510 ::= SEQUENCE {
          COMPONENTS OF KRB-ERROR-COMMON
      } (WITH COMPONENTS {
          ...,
          msg-type (30)
      })



      KRB-ERROR-EXT ::= [APPLICATION 23] KRB-ERROR-COMMON
          (WITH COMPONENTS { ..., msg-type (23) })



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   ctime, cusec
        Client's time, if known from a KDC-REQ or AP-REQ.


   stime, susec
        KDC or application server's current time.


   error-code
        Numeric error code designating the error.


   crealm, cname
        Client's realm and name, if known.


   realm, sname
        server's realm and name. [ XXX what if these aren't known? ]


   e-text
        Human-readable text providing additional details for the error.


   e-data
        This field contains additional data about the error for use by
        the client to help it recover from or handle the error. If the
        "error-code" is "kdc-err-preauth-required", then the e-data
        field will contain an encoding of a sequence of padata fields,
        each corresponding to an acceptable pre-authentication method
        and optionally containing data for the method:


           METHOD-DATA     ::= SEQUENCE OF PA-DATA



        For error codes defined in this document other than "kdc-err-
        preauth-required", the format and contents of the e-data field
        are implementation-defined.  Similarly, for future error codes,
        the format and contents of the e-data field are implementation-
        defined unless specified.


   lang-tag
        Indicates the language of the message in the "e-text" field.  It
        is only present in the extensible KRB-ERROR.


   nonce
        is the nonce from a KDC-REQ.  It is only present in the
        extensible KRB-ERROR.


   typed-data
        TYPED-DATA is a typed hole allowing for additional data to be
        returned in error conditions, since "e-data" is insufficiently
        flexible for some purposes.  TYPED-DATA is only present in the
        extensible KRB-ERROR.





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           TDType ::= TH-id


           TYPED-DATA      ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
               data-type   [0] TDType,
               data-value  [1] OCTET STRING OPTIONAL
           }



10.  Session Key Exchange


   Session key exchange consists of the AP-REQ and AP-REP messages.  The
   client sends the AP-REQ message, and the service responds with an
   AP-REP message if mutual authentication is desired.  Following
   session key exchange, the client and service share a secret session
   key, or possibly a subsesion key, which can be used to protect
   further communications.  Additionally, the session key exchange
   process can establish initial sequence numbers which the client and
   service can use to detect replayed messages.


10.1.  AP-REQ


   An AP-REQ message contains a ticket and a authenticator.  The
   authenticator is ciphertext encrypted with the session key contained
   in the ticket.  The plaintext contents of the authenticator are:


      -- plaintext of authenticator
      Authenticator   ::= [APPLICATION 2] SEQUENCE  {
          authenticator-vno   [0] INTEGER (5),
          crealm              [1] Realm,
          cname               [2] PrincipalName,
          cksum               [3] Checksum {{ key-session },
              { ku-Authenticator-cksum |
                ku-pa-TGSReq-cksum }} OPTIONAL,
          cusec               [4] Microseconds,
          ctime               [5] KerberosTime,
          subkey              [6] EncryptionKey OPTIONAL,
          seq-number          [7] SeqNum OPTIONAL,
          authorization-data  [8] AuthorizationData OPTIONAL
      }


   The common parts between the RFC 1510 and the extensible versions of
   the AP-REQ are:











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      AP-REQ-COMMON   ::= SEQUENCE {
          pvno                [0] INTEGER (5),
          msg-type            [1] INTEGER (14 | 18),
          ap-options          [2] APOptions,
          ticket              [3] Ticket,
          authenticator       [4] EncryptedData {
              Authenticator,
              { key-session },
              { ku-APReq-authenticator |
                ku-pa-TGSReq-authenticator }
          },
          ...,
          extensions          [5] ApReqExtensions OPTIONAL,
          lang-tag            [6] SEQUENCE (SIZE (1..MAX))
                                      OF LangTag OPTIONAL,
          ...
      }


   The complete definition of AP-REQ is:


      AP-REQ          ::= CHOICE {
          rfc1510     [APPLICATION 14] AP-REQ-1510,
          ext         [APPLICATION 18] Signed {
              AP-REQ-EXT, { key-session }, { ku-APReq-cksum }
          }
      }



      AP-REQ-COMMON   ::= SEQUENCE {
          pvno                [0] INTEGER (5),
          msg-type            [1] INTEGER (14 | 18),
          ap-options          [2] APOptions,
          ticket              [3] Ticket,
          authenticator       [4] EncryptedData {
              Authenticator,
              { key-session },
              { ku-APReq-authenticator |
                ku-pa-TGSReq-authenticator }
          },
          ...,
          extensions          [5] ApReqExtensions OPTIONAL,
          lang-tag            [6] SEQUENCE (SIZE (1..MAX))
                                      OF LangTag OPTIONAL,
          ...
      }








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      AP-REQ-1510 ::= SEQUENCE {
          COMPONENTS OF AP-REQ-COMMON
      } (WITH COMPONENTS {
          ...,
          msg-type (14),
          authenticator (EncryptedData {
              Authenticator (WITH COMPONENTS {
                  ...,
                  crealm (RealmIA5),
                  cname (PrincipalNameIA5),
                  seqnum (UInt32)
              }), { key-session }, { ku-APReq-authenticator }})
      })



      AP-REQ-EXT      ::= AP-REQ-COMMON
      (WITH COMPONENTS {
          ...,
          msg-type (18),
          -- The following constraints on Authenticator assume that
          -- we want to restrict the use of AP-REQ-EXT with TicketExt
          -- only, since that is the only way we can enforce UTF-8.
          authenticator (EncryptedData {
              Authenticator (WITH COMPONENTS {
                  ...,
                  crealm (RealmExt),
                  cname (PrincipalNameExt),
                  authorization-data (SIZE (1..MAX))
              }), { key-session }, { ku-APReq-authenticator }})
      })



      APOptions       ::= KerberosFlags { APOptionsBits }


      APOptionsBits ::= BIT STRING {
          reserved            (0),
          use-session-key     (1),
          mutual-required     (2)
      }



10.2.  AP-REP


   An AP-REP message provides mutual authentication of the service to
   the client.


      EncAPRepPart    ::= CHOICE {
          rfc1510     [APPLICATION 27] EncAPRepPart1510,
          ext         [APPLICATION 31] EncAPRepPartExt
      }



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      EncAPRepPartCom          ::= SEQUENCE {
          ctime               [0] KerberosTime,
          cusec               [1] Microseconds,
          subkey              [2] EncryptionKey OPTIONAL,
          seq-number          [3] SeqNum OPTIONAL,
          ...,
          authorization-data  [4] AuthorizationData OPTIONAL,
          ...
      }



      EncAPRepPart1510        ::= SEQUENCE {
          COMPONENTS OF ENCAPRepPartCom
      } (WITH COMPONENTS {
          ...,
          seq-number (UInt32),
          authorization-data ABSENT
      })



      EncAPRepPartExt         ::= EncAPRepPartCom



      AP-REP          ::= CHOICE {
          rfc1510     [APPLICATION 15] AP-REP-1510,
          ext         [APPLICATION 19] Signed {
              AP-REP-EXT,
              { key-session | key-subsession }, { ku-APRep-cksum }}
      }



      AP-REP-COMMON { EncPart }       ::= SEQUENCE {
          pvno        [0] INTEGER (5),
          msg-type    [1] INTEGER (15 | 19),
          enc-part    [2] EncryptedData {
              EncPart,
              { key-session | key-subsession }, { ku-EncAPRepPart }},
          ...,
          extensions          [3] ApRepExtensions OPTIONAL,
          ...
      }



      AP-REP-1510     ::= SEQUENCE {
          COMPONENTS OF AP-REP-COMMON { EncAPRepPart1510 }
      } (WITH COMPONENTS {
          ...,
          msg-type (15)
      })




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      AP-REP-EXT      ::= [APPLICATION 19] AP-REP-COMMON {
          EncAPRepPartExt
      } (WITH COMPONENTS { ..., msg-type (19) })



11.  Session Key Use


   Once a session key has been established, the client and server can
   use several kinds of messages to securely transmit data.  KRB-SAFE
   provides integrity protection for application data, while KRB-PRIV
   provides confidentiality along with integrity protection.  The KRB-
   CRED message provides a means of securely forwarding credentials from
   the client host to the server host.


11.1.  KRB-SAFE


   The KRB-SAFE message provides integrity protection for an included
   cleartext message.


      -- Do we chew up another tag for KRB-SAFE-EXT?  That would
      -- allow us to  make safe-body optional, allowing for a GSS-MIC
      -- sort of message.
      KRB-SAFE        ::= [APPLICATION 20] SEQUENCE {
          pvno        [0] INTEGER (5),
          msg-type    [1] INTEGER (20),
          safe-body   [2] KRB-SAFE-BODY,
          cksum       [3] ChecksumOf {
              KRB-SAFE-BODY,
              { key-session | key-subsession }, { ku-KrbSafe-cksum }},
          ...         -- ASN.1 extensions must be excluded
                      -- when sending to rfc1510 implementations
      }



      KRB-SAFE-BODY   ::= SEQUENCE {
          user-data   [0] OCTET STRING,
          timestamp   [1] KerberosTime OPTIONAL,
          usec        [2] Microseconds OPTIONAL,
          seq-number  [3] SeqNum OPTIONAL,
          s-address   [4] HostAddress,
          r-address   [5] HostAddress OPTIONAL,
          ...         -- ASN.1 extensions must be excluded
                      -- when sending to rfc1510 implementations
      }



11.2.  KRB-PRIV


   The KRB-PRIV message provides confidentiality and integrity
   protection.



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      KRB-PRIV        ::= [APPLICATION 21] SEQUENCE {
          pvno        [0] INTEGER (5),
          msg-type    [1] INTEGER (21),
          enc-part    [3] EncryptedData {
              EncKrbPrivPart,
              { key-session | key-subsession }, { ku-EncKrbPrivPart }},
          ...
      }



      EncKrbPrivPart  ::= [APPLICATION 28] SEQUENCE {
          user-data   [0] OCTET STRING,
          timestamp   [1] KerberosTime OPTIONAL,
          usec        [2] Microseconds OPTIONAL,
          seq-number  [3] SeqNum OPTIONAL,
          s-address   [4] HostAddress -- sender's addr --,
          r-address   [5] HostAddress OPTIONAL -- recip's addr --,
          ...         -- ASN.1 extensions must be excluded
                      -- when sending to rfc1510 implementations
      }



11.3.  KRB-CRED


   The KRB-CRED message provides a means of securely transferring
   credentials from the client to the service.


      KRB-CRED        ::= CHOICE {
          rfc1510     [APPLICATION 22] KRB-CRED-1510,
          ext         [APPLICATION 24] Signed {
              KRB-CRED-EXT,
              { key-session | key-subsession }, { ku-KrbCred-cksum }}
      }



      KRB-CRED-COMMON ::= SEQUENCE {
          pvno        [0] INTEGER (5),
          msg-type    [1] INTEGER (22 | 24),
          tickets     [2] SEQUENCE OF Ticket,
          enc-part    [3] EncryptedData {
              EncKrbCredPart,
              { key-session | key-subsession }, { ku-EncKrbCredPart }},
          ...
      }



      KRB-CRED-1510 ::= SEQUENCE {
          COMPONENTS OF KRB-CRED-COMMON
      } (WITH COMPONENTS { ..., msg-type (22) })




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      KRB-CRED-EXT    ::= [APPLICATION 24] KRB-CRED-COMMON
          (WITH COMPONENTS { ..., msg-type (24) })



      EncKrbCredPart  ::= [APPLICATION 29] SEQUENCE {
          ticket-info [0] SEQUENCE OF KrbCredInfo,
          nonce       [1] Nonce OPTIONAL,
          timestamp   [2] KerberosTime OPTIONAL,
          usec        [3] Microseconds OPTIONAL,
          s-address   [4] HostAddress OPTIONAL,
          r-address   [5] HostAddress OPTIONAL
      }



      KrbCredInfo     ::= SEQUENCE {
          key         [0] EncryptionKey,
          prealm      [1] Realm OPTIONAL,
          pname       [2] PrincipalName OPTIONAL,
          flags       [3] TicketFlags OPTIONAL,
          authtime    [4] KerberosTime OPTIONAL,
          starttime   [5] KerberosTime OPTIONAL,
          endtime     [6] KerberosTime OPTIONAL,
          renew-till  [7] KerberosTime OPTIONAL,
          srealm      [8] Realm OPTIONAL,
          sname       [9] PrincipalName OPTIONAL,
          caddr       [10] HostAddresses OPTIONAL
      }



12.  Security Considerations


12.1.  Time Synchronization


   Time synchronization between the KDC and application servers is
   necessary to prevent acceptance of expired tickets.


   Time synchronization is needed between application servers and
   clients to prevent replay attacks if a replay cache is being used.
   If negotiated subsession keys are used to encrypt application data,
   replay caches may not be necessary.


12.2.  Replays


12.3.  Principal Name Reuse


   See Section 5.3.


12.4.  Password Guessing





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12.5.  Forward Secrecy


   [from KCLAR 10.; needs some rewriting]


   The Kerberos protocol in its basic form does not provide perfect
   forward secrecy for communications.  If traffic has been recorded by
   an eavesdropper, then messages encrypted using the KRB-PRIV message,
   or messages encrypted using application-specific encryption under
   keys exchanged using Kerberos can be decrypted if any of the user's,
   application server's, or KDC's key is subsequently discovered.  This
   is because the session key used to encrypt such messages is
   transmitted over the network encrypted in the key of the application
   server, and also encrypted under the session key from the user's
   ticket-granting ticket when returned to the user in the TGS-REP
   message.  The session key from the ticket-granting ticket was sent to
   the user in the AS-REP message encrypted in the user's secret key,
   and embedded in the ticket-granting ticket, which was encrypted in
   the key of the KDC.  Application requiring perfect forward secrecy
   must exchange keys through mechanisms that provide such assurance,
   but may use Kerberos for authentication of the encrypted channel
   established through such other means.


12.6.  Authorization


   As an authentication service, Kerberos provides a means of verifying
   the identity of principals on a network.  Kerberos does not, by
   itself, provide authorization.  Applications SHOULD NOT accept the
   mere issuance of a service ticket by the Kerberos server as granting
   authority to use the service.


12.7.  Login Authentication


   Some applications, particularly those which provide login access when
   provided with a password, SHOULD NOT treat successful acquisition of
   credentials as sufficient proof of the user's identity.  An attacker
   posing as a user could generate an illegitimate KDC-REP message which
   decrypts properly.  To authenticate a user logging on to a local
   system, the credentials obtained SHOULD be used in a TGS exchange to
   obtain credentials for a local service.  Successful use of those
   credentials to authenticate to the local service assures that the
   initially obtained credentials are from a valid KDC.


13.  IANA Considerations


   [ needs more work ]


   Each use of Int32 in this document defines a number space.  [ XXX
   enumerate these ] Negative numbers are reserved for private use;
   local and experimental extensions should use these values.  Zero is
   reserved and may not be assigned.



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   Typed hole contents may be identified by either Int32 values or by
   RELATIVE-OID values.  Since RELATIVE-OIDs define a hierarchical
   namespace, assignments to the top level of the RELATIVE-OID space may
   be made on a first-come, first-served basis.


14.  Acknowledgments


   Much of the text here is adapted from draft-ietf-krb-wg-kerberos-
   clarifications-07.  The description of the general form of the
   extensibility framework was derived from text by Sam Hartman.


Appendices


A.  ASN.1 Module (Normative)


      KerberosV5Spec3 {
          iso(1) identified-organization(3) dod(6) internet(1)
          security(5) kerberosV5(2) modules(4) krb5spec3(4)
      } DEFINITIONS EXPLICIT TAGS ::= BEGIN



      -- OID arc for KerberosV5
      --
      -- This OID may be used to identify Kerberos protocol messages
      -- encapsulated in other protocols.
      --
      -- This OID also designates the OID arc for KerberosV5-related
      -- OIDs.
      --
      -- NOTE: RFC 1510 had an incorrect value (5) for "dod" in its
      -- OID.
      id-krb5         OBJECT IDENTIFIER ::= {
          iso(1) identified-organization(3) dod(6) internet(1)
          security(5) kerberosV5(2)
      }


















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      -- top-level type
      --
      -- Applications should not directly reference any types
      -- other than KRB-PDU and its component types.
      --
      KRB-PDU         ::= CHOICE {
          ticket      Ticket,
          as-req      AS-REQ,
          as-rep      AS-REP,
          tgs-req     TGS-REQ,
          tgs-rep     TGS-REP,
          ap-req      AP-REQ,
          ap-rep      AP-REP,
          krb-safe    KRB-SAFE,
          krb-priv    KRB-PRIV,
          krb-cred    KRB-CRED,
          tgt-req     TGT-REQ,
          krb-error   KRB-ERROR,
          ...
      }



      --
      -- *** basic types
      --



      -- signed values representable in 32 bits
      --
      -- These are often used as assigned numbers for various things.
      Int32           ::= INTEGER (-2147483648..2147483647)



      -- Typed hole identifiers
      TH-id           ::= CHOICE {
          int32               Int32,
          rel-oid             RELATIVE-OID
      }



      -- unsigned 32 bit values
      UInt32          ::= INTEGER (0..4294967295)



      -- unsigned 64 bit values
      UInt64          ::= INTEGER (0..18446744073709551615)



      -- sequence numbers
      SeqNum          ::= UInt64



Yu                         Expires: Apr 2005                   [Page 70]
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      -- nonces
      Nonce           ::= UInt64



      -- microseconds
      Microseconds    ::= INTEGER (0..999999)



      KerberosTime    ::= GeneralizedTime (CONSTRAINED BY {
                              -- MUST NOT include fractional seconds
      })



      -- used for names and for error messages
      KerberosString  ::= CHOICE {
          ia5         GeneralString (IA5String),
          utf8        UTF8String,
          ...         -- no extension may be sent
                      -- to an rfc1510 implementation --
      }



      -- IA5 choice only; useful for constraints
      KerberosStringIA5       ::= KerberosString
          (WITH COMPONENTS { ia5 PRESENT })


      -- IA5 excluded; useful for constraints
      KerberosStringExt       ::= KerberosString
          (WITH COMPONENTS { ia5 ABSENT })



      -- used for language tags
      LangTag ::= PrintableString
          (FROM ("A".."Z" | "a".."z" | "0".."9" | "-"))



















Yu                         Expires: Apr 2005                   [Page 71]
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      -- assigned numbers for name types (used in principal names)
      NameType        ::= Int32


      -- Name type not known
      nt-unknown              NameType ::= 0
      -- Just the name of the principal as in DCE, or for users
      nt-principal            NameType ::= 1
      -- Service and other unique instance (krbtgt)
      nt-srv-inst             NameType ::= 2
      -- Service with host name as instance (telnet, rcommands)
      nt-srv-hst              NameType ::= 3
      -- Service with host as remaining components
      nt-srv-xhst             NameType ::= 4
      -- Unique ID
      nt-uid                  NameType ::= 5
      -- Encoded X.509 Distingished name [RFC 2253]
      nt-x500-principal       NameType ::= 6
      -- Name in form of SMTP email name (e.g. user@foo.com)
      nt-smtp-name            NameType ::= 7
      -- Enterprise name - may be mapped to principal name
      nt-enterprise           NameType ::= 10



      PrincipalName   ::= SEQUENCE {
          name-type   [0] NameType,
          -- May have zero elements, or individual elements may be
          -- zero-length, but this is NOT RECOMMENDED.
          name-string [1] SEQUENCE OF KerberosString
      }



      -- IA5 only
      PrincipalNameIA5 ::= PrincipalName (WITH COMPONENTS {
          ...,
          name-string (WITH COMPONENT (KerberosStringIA5))
      })


      -- IA5 excluded
      PrincipalNameExt ::= PrincpalName (WITH COMPONENTS {
          ...,
          name-string (WITH COMPONENT (KerberosStringExt))
      })











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      Realm           ::= KerberosString


      -- IA5 only
      RealmIA5        ::= Realm (KerberosStringIA5)


      -- IA5 excluded
      RealmExt        ::= Realm (KerberosStringExt)



      KerberosFlags { NamedBits } ::= BIT STRING (SIZE (32..MAX))
          (CONSTRAINED BY {
          -- MUST be a valid value of -- NamedBits
          -- but if the value to be sent would be truncated to shorter
          -- than 32 bits according to DER, the value MUST be padded
          -- with trailing zero bits to 32 bits.  Otherwise, no
          -- trailing zero bits may be present.


      })



      AddrType        ::= Int32


      HostAddress     ::= SEQUENCE  {
          addr-type   [0] AddrType,
          address     [1] OCTET STRING
      }


      -- NOTE: HostAddresses is always used as an OPTIONAL field and
      -- should not be a zero-length SEQUENCE OF.
      --
      -- The extensible messages explicitly constrain this to be
      -- non-empty.
      HostAddresses   ::= SEQUENCE OF HostAddress



      --
      -- *** crypto-related types and assignments
      --















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      -- Assigned numbers denoting encryption mechanisms.
      EType ::= Int32


      -- assigned numbers for encryption schemes
      et-des-cbc-crc                  EType ::= 1
      et-des-cbc-md4                  EType ::= 2
      et-des-cbc-md5                  EType ::= 3
      --     [reserved]                         4
      et-des3-cbc-md5                 EType ::= 5
      --     [reserved]                         6
      et-des3-cbc-sha1                EType ::= 7
      et-dsaWithSHA1-CmsOID           EType ::= 9
      et-md5WithRSAEncryption-CmsOID  EType ::= 10
      et-sha1WithRSAEncryption-CmsOID EType ::= 11
      et-rc2CBC-EnvOID                EType ::= 12
      et-rsaEncryption-EnvOID         EType ::= 13
      et-rsaES-OAEP-ENV-OID           EType ::= 14
      et-des-ede3-cbc-Env-OID         EType ::= 15
      et-des3-cbc-sha1-kd             EType ::= 16
      -- AES
      et-aes128-cts-hmac-sha1-96      EType ::= 17
      -- AES
      et-aes256-cts-hmac-sha1-96      EType ::= 18
      -- Microsoft
      et-rc4-hmac                     EType ::= 23
      -- Microsoft
      et-rc4-hmac-exp                 EType ::= 24
      -- opaque; PacketCable
      et-subkey-keymaterial           EType ::= 65
























Yu                         Expires: Apr 2005                   [Page 74]
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      -- Assigned numbers denoting key usages.
      KeyUsage ::= UInt32


      --
      -- Actual identifier names are provisional and subject to
      -- change.
      --
      ku-pa-enc-ts                    KeyUsage ::= 1
      ku-Ticket                       KeyUsage ::= 2
      ku-EncASRepPart                 KeyUsage ::= 3
      ku-TGSReqAuthData-sesskey       KeyUsage ::= 4
      ku-TGSReqAuthData-subkey        KeyUsage ::= 5
      ku-pa-TGSReq-cksum              KeyUsage ::= 6
      ku-pa-TGSReq-authenticator      KeyUsage ::= 7
      ku-EncTGSRepPart-sesskey        KeyUsage ::= 8
      ku-EncTGSRepPart-subkey         KeyUsage ::= 9
      ku-Authenticator-cksum          KeyUsage ::= 10
      ku-APReq-authenticator          KeyUsage ::= 11
      ku-EncAPRepPart                 KeyUsage ::= 12
      ku-EncKrbPrivPart               KeyUsage ::= 13
      ku-EncKrbCredPart               KeyUsage ::= 14
      ku-KrbSafe-cksum                KeyUsage ::= 15
      ku-ad-KDCIssued-cksum           KeyUsage ::= 19



      -- The following numbers are provisional...
      -- conflicts may exist elsewhere.
      ku-Ticket-cksum                 KeyUsage ::= 25
      ku-ASReq-cksum                  KeyUsage ::= 26
      ku-TGSReq-cksum                 KeyUsage ::= 27
      ku-ASRep-cksum                  KeyUsage ::= 28
      ku-TGSRep-cksum                 KeyUsage ::= 29
      ku-APReq-cksum                  KeyUsage ::= 30
      ku-APRep-cksum                  KeyUsage ::= 31
      ku-KrbCred-cksum                KeyUsage ::= 32
      ku-KrbError-cksum               KeyUsage ::= 33
      ku-KDCRep-cksum                 KeyUsage ::= 34
















Yu                         Expires: Apr 2005                   [Page 75]
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      -- KeyToUse identifies which key is to be used to encrypt or
      -- sign a given value.
      --
      -- Values of KeyToUse are never actually transmitted over the
      -- wire, or even used directly by the implementation in any
      -- way, as key usages are; it exists primarily to identify
      -- which key gets used for what purpose.  Thus, the specific
      -- numeric values associated with this type are irrelevant.
      KeyToUse        ::= ENUMERATED {
          -- unspecified
          key-unspecified,
          -- server long term key
          key-server,
          -- client long term key
          key-client,
          -- key selected by KDC for encryption of a KDC-REP
          key-kdc-rep,
          -- session key from ticket
          key-session,
          -- subsession key negotiated via AP-REQ/AP-REP
          key-subsession,
          ...
      }



      EncryptionKey   ::= SEQUENCE {
          keytype     [0] EType,
          keyvalue    [1] OCTET STRING
      }
























Yu                         Expires: Apr 2005                   [Page 76]
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      -- "Type" specifies which ASN.1 type is encrypted to the
      -- ciphertext in the EncryptedData.  "Keys" specifies a set of
      -- keys of which one key may be used to encrypt the data.
      -- "KeyUsages" specifies a set of key usages, one of which may
      -- be used to encrypt.
      --
      -- None of the parameters is transmitted over the wire.
      EncryptedData {
          Type, KeyToUse:Keys, KeyUsage:KeyUsages
      } ::= SEQUENCE {
          etype       [0] EType,
          kvno        [1] UInt32 OPTIONAL,
          cipher      [2] OCTET STRING (CONSTRAINED BY {
              -- must be encryption of --
              OCTET STRING (CONTAINING Type),
              -- with one of the keys -- KeyToUse:Keys,
              -- with key usage being one of --
              KeyUsage:KeyUsages
          }),
          ...
      }




      CksumType       ::= Int32


      -- The parameters specify which key to use to produce the
      -- signature, as well as which key usage to use.  The
      -- parameters are not actually sent over the wire.
      Checksum {
          KeyToUse:Keys, KeyUsage:KeyUsages
      } ::= SEQUENCE {
          cksumtype   [0] CksumType,
          checksum    [1] OCTET STRING (CONSTRAINED BY {
              -- signed using one of the keys --
              KeyToUse:Keys,
              -- with key usage being one of --
              KeyUsage:KeyUsages
          })
      }












Yu                         Expires: Apr 2005                   [Page 77]
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      -- a Checksum that must contain the checksum
      -- of a particular type
      ChecksumOf {
          Type, KeyToUse:Keys, KeyUsage:KeyUsages
      } ::= Checksum { Keys, KeyUsages } (WITH COMPONENTS {
          ...,
          checksum (CONSTRAINED BY {
              -- must be checksum of --
              OCTET STRING (CONTAINING Type)
          })
      })



      -- parameterized type for wrapping authenticated plaintext
      Signed {
          InnerType, KeyToUse:Keys, KeyUsage:KeyUsages
      } ::= SEQUENCE {
          cksum       [0] ChecksumOf {
              InnerType, Keys, KeyUsages
          } OPTIONAL,
          inner       [1] InnerType,
          ...
      }



      --
      -- *** Tickets
      --



      Ticket          ::= CHOICE {
          rfc1510     [APPLICATION 1] Ticket1510,
          ext         [APPLICATION 4] Signed {
              TicketExt, { key-server }, { ku-Ticket-cksum }
          }
      }



      -- takes a parameter specifying which type gets encrypted
      TicketCommon { EncPart } ::= SEQUENCE {
          tkt-vno     [0] INTEGER (5),
          realm       [1] Realm,
          sname       [2] PrincipalName,
          enc-part    [3] EncryptedData {
              EncPart, { key-server }, { ku-Ticket }
          },
          ...,
          extensions          [4] TicketExtensions OPTIONAL,
          ...
      }



Yu                         Expires: Apr 2005                   [Page 78]
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      Ticket1510 ::= SEQUENCE {
          COMPONENTS OF TicketCommon { EncTicketPart1510 }
      } (WITH COMPONENTS {
          ...,
          -- explicitly force IA5 in strings
          realm (RealmIA5),
          sname (PrincipalNameIA5)
      })



      -- APPLICATION tag goes inside Signed{} as well as outside,
      -- to prevent possible substitution attacks.
      TicketExt ::= [APPLICATION 4] TicketCommon {
          EncTicketPartExt
      } (WITH COMPONENTS {
          ...,
          -- explicitly force UTF-8 in strings
          realm (RealmExt),
          sname (PrincipalNameExt)
      })



      -- Encrypted part of ticket
      EncTicketPart ::= CHOICE {
          rfc1510     [APPLICATION 3] EncTicketPart1510,
          ext         [APPLICATION 5] EncTicketPartExt
      }



      EncTicketPartCommon ::= SEQUENCE {
          flags               [0] TicketFlags,
          key                 [1] EncryptionKey,
          crealm              [2] Realm,
          cname               [3] PrincipalName,
          transited           [4] TransitedEncoding,
          authtime            [5] KerberosTime,
          starttime           [6] KerberosTime OPTIONAL,
          endtime             [7] KerberosTime,
          renew-till          [8] KerberosTime OPTIONAL,
          caddr               [9] HostAddresses OPTIONAL,
          authorization-data  [10] AuthorizationData OPTIONAL,
          ...
      }










Yu                         Expires: Apr 2005                   [Page 79]
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      EncTicketPart1510 ::= SEQUENCE {
          COMPONENTS OF EncTicketPartCommon
      } (WITH COMPONENTS {
          ...,
          -- explicitly force IA5 in strings
          crealm (RealmIA5),
          cname (PrincipalNameIA5)
      })



      EncTicketPartExt ::= EncTicketPartCommon (WITH COMPONENTS {
          ...,
          -- explicitly force UTF-8 in strings
          crealm (RealmExt),
          cname (PrincipalNameExt),
          -- explicitly constrain caddr to be non-empty if present
          caddr (SIZE (1..MAX)),
          -- forbid empty authorization-data encodings
          authorization-data (SIZE (1..MAX))
      })



      --
      -- *** Authorization Data
      --



      ADType          ::= TH-id


      AuthorizationData       ::= SEQUENCE OF SEQUENCE {
          ad-type             [0] ADType,
          ad-data             [1] OCTET STRING
      }



      ad-if-relevant          ADType ::= int32 : 1


      -- Encapsulates another AuthorizationData.
      -- Intended for application servers; receiving application servers
      -- MAY ignore.
      AD-IF-RELEVANT          ::= AuthorizationData












Yu                         Expires: Apr 2005                   [Page 80]
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      -- KDC-issued privilege attributes
      ad-kdcissued            ADType ::= int32 : 4


      AD-KDCIssued            ::= SEQUENCE {
          ad-checksum [0] ChecksumOf {
                              AuthorizationData, { key-session },
                              { ku-ad-KDCIssued-cksum }},
          i-realm     [1] Realm OPTIONAL,
          i-sname     [2] PrincipalName OPTIONAL,
          elements    [3] AuthorizationData
      }



      ad-and-or               ADType ::= int32 : 5


      AD-AND-OR               ::= SEQUENCE {
          condition-count     [0] INTEGER,
          elements            [1] AuthorizationData
      }



      -- KDCs MUST interpret any AuthorizationData wrapped in this.
      ad-mandatory-for-kdc            ADType ::= int32 : 8
      AD-MANDATORY-FOR-KDC            ::= AuthorizationData



      TrType                  ::= TH-id -- must be registered


      -- encoded Transited field
      TransitedEncoding       ::= SEQUENCE {
          tr-type     [0] TrType,
          contents    [1] OCTET STRING
      }



      TEType                  ::= TH-id


      -- ticket extensions: for TicketExt only
      TicketExtensions        ::= SEQUENCE (SIZE (1..MAX)) OF SEQUENCE {
          te-type             [0] TEType,
          te-data             [1] OCTET STRING
      }











Yu                         Expires: Apr 2005                   [Page 81]
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      TicketFlags     ::= KerberosFlags { TicketFlagsBits }


      TicketFlagsBits ::= BIT STRING {
          reserved            (0),
          forwardable         (1),
          forwarded           (2),
          proxiable           (3),
          proxy               (4),
          may-postdate        (5),
          postdated           (6),
          invalid             (7),
          renewable           (8),
          initial             (9),
          pre-authent         (10),
          hw-authent          (11),
          transited-policy-checked (12),
          ok-as-delegate      (13),
          anonymous           (14),
          cksummed-ticket     (15)
      }



      --
      -- *** KDC protocol
      --




























Yu                         Expires: Apr 2005                   [Page 82]
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      AS-REQ  ::= CHOICE {
          rfc1510     [APPLICATION 10] KDC-REQ-1510
          (WITH COMPONENTS {
              ...,
              msg-type (10),
              -- AS-REQ must include client name
              req-body (WITH COMPONENTS { ..., cname PRESENT })
          }),
          ext         [APPLICATION 6]  Signed {
              -- APPLICATION tag goes inside Signed{} as well as
              -- outside, to prevent possible substitution attacks.
              [APPLICATION 6] KDC-REQ-EXT,
              -- not sure this is correct key to use; do we even want
              -- to sign AS-REQ?
              { key-client },
              { ku-ASReq-cksum }
          }
          (WITH COMPONENTS {
              ...,
              msg-type  (6),
              -- AS-REQ must include client name
              req-body (WITH COMPONENTS { ..., cname PRESENT })
          })
      }



      TGS-REQ ::= CHOICE {
          rfc1510     [APPLICATION 12] KDC-REQ-1510
          (WITH COMPONENTS {
              ...,
              msg-type (12),
              -- client name optional in TGS-REQ
              req-body (WITH COMPONENTS { ..., cname ABSENT })
          }),
          ext         [APPLICATION 8]  Signed {
              -- APPLICATION tag goes inside Signed{} as well as
              -- outside, to prevent possible substitution attacks.
              [APPLICATION 8] KDC-REQ-EXT,
              { key-session }, { ku-TGSReq-cksum }
          }
          (WITH COMPONENTS {
              ...,
              msg-type  (8),
              -- client name optional in TGS-REQ
              req-body (WITH COMPONENTS { ..., cname ABSENT })
          })
      }






Yu                         Expires: Apr 2005                   [Page 83]
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      KDC-REQ-COMMON  ::= SEQUENCE {
      -- NOTE: first tag is [1], not [0]
          pvno        [1] INTEGER (5),
          msg-type    [2] INTEGER (10 -- AS-REQ.rfc1510 --
                                   | 12 -- TGS-REQ.rfc1510 --
                                   | 6 -- AS-REQ.ext --
                                   | 8 -- TGS-REQ.ext -- ),
          padata      [3] SEQUENCE OF PA-DATA OPTIONAL
          -- NOTE: not empty
      }



      KDC-REQ-1510    ::= SEQUENCE {
          COMPONENTS OF KDC-REQ-COMMON,
          req-body    [4] KDC-REQ-BODY-1510
      } (WITH COMPONENTS { ..., msg-type (10 | 12) })



      -- APPLICATION tag goes inside Signed{} as well as outside,
      -- to prevent possible substitution attacks.
      KDC-REQ-EXT     ::= SEQUENCE {
          COMPONENTS OF KDC-REQ-COMMON,
          req-body    [4] KDC-REQ-BODY-EXT,
          ...
      } (WITH COMPONENTS {
          ...,
          msg-type (6 | 8),
          padata (SIZE (1..MAX))
      })
























Yu                         Expires: Apr 2005                   [Page 84]
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      KDC-REQ-BODY-COMMON     ::= SEQUENCE {
          kdc-options         [0] KDCOptions,
          cname               [1] PrincipalName OPTIONAL
          -- Used only in AS-REQ --,


          realm               [2] Realm
          -- Server's realm; also client's in AS-REQ --,


          sname               [3] PrincipalName OPTIONAL,
          from                [4] KerberosTime OPTIONAL,
          till                [5] KerberosTime OPTIONAL
          -- was required in rfc1510;
          -- still required for compat versions
          -- of messages --,


          rtime               [6] KerberosTime OPTIONAL,
          nonce               [7] Nonce,
          etype               [8] SEQUENCE OF EType
          -- in preference order --,


          addresses           [9] HostAddresses OPTIONAL,
          enc-authorization-data      [10] EncryptedData {
              AuthorizationData, { key-session | key-subsession },
              { ku-TGSReqAuthData-subkey |
                ku-TGSReqAuthData-sesskey }
          } OPTIONAL,


          additional-tickets  [11] SEQUENCE OF Ticket OPTIONAL
          -- NOTE: not empty --,
          ...
          lang-tags   [5] SEQUENCE (SIZE (1..MAX)) OF
                              LangTag OPTIONAL,
          ...
      }



      KDC-REQ-BODY-1510 ::= SEQUENCE {
          COMPONENTS OF KDC-REQ-BODY-COMMON
      } (WITH COMPONENTS {
          ...,
          cname (PrincipalNameIA5),
          realm (RealmIA5),
          sname (PrincipalNameIA5),
          till PRESENT,
          nonce (UInt32)
      })







Yu                         Expires: Apr 2005                   [Page 85]
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      KDC-REQ-BODY-EXT        ::= KDC-REQ-BODY-COMMON
      (WITH COMPONENTS {
          ...,
          cname (PrincipalNameExt),
          realm (RealmExt),
          sname (PrincipalNameExt),
          addresses (SIZE (1..MAX)),
          enc-authorization-data (EncryptedData {
              AuthorizationData (SIZE (1..MAX)),
              { key-session | key-subsession },
              { ku-TGSReqAuthData-subkey |
                ku-TGSReqAuthData-sesskey }
          }),
          additional-tickets (SIZE (1..MAX))
      })



      KDCOptions      ::= KerberosFlags { KDCOptionsBits }


      KDCOptionsBits  ::= BIT STRING {
          reserved            (0),
          forwardable         (1),
          forwarded           (2),
          proxiable           (3),
          proxy               (4),
          allow-postdate      (5),
          postdated           (6),
          unused7             (7),
          renewable           (8),
          unused9             (9),
          unused10            (10),
          unused11            (11),
          unused12            (12),
          unused13            (13),
          requestanonymous    (14),
          canonicalize        (15),
          disable-transited-check (26),
          renewable-ok        (27),
          enc-tkt-in-skey     (28),
          renew               (30),
          validate            (31)
          -- XXX need "need ticket1" flag?
      }










Yu                         Expires: Apr 2005                   [Page 86]
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      AS-REP          ::= CHOICE {
          rfc1510     [APPLICATION 11] KDC-REP-1510 {
              EncASRepPart1510
          } (WITH COMPONENTS { ..., msg-type (11) }),
          ext         [APPLICATION  7]  Signed {
              [APPLICATION 7] KDC-REP-EXT { EncASRepPartExt },
              { key-reply }, { ku-ASRep-cksum }
          } (WITH COMPONENTS { ..., msg-type (7) })
      }



      TGS-REP         ::= CHOICE {
          rfc1510     [APPLICATION 13] KDC-REP-1510 {
              EncTGSRepPart1510
          } (WITH COMPONENTS { ..., msg-type (13) }),
          ext         [APPLICATION  9]  Signed {
              [APPLICATION 9] KDC-REP-EXT { EncTGSRepPartExt },
              { key-reply }, { ku-TGSRep-cksum }
          } (WITH COMPONENTS { ..., msg-type (9) })
      }

































Yu                         Expires: Apr 2005                   [Page 87]
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      KDC-REP-COMMON { EncPart } ::= SEQUENCE {
          pvno        [0] INTEGER (5),
          msg-type    [1] INTEGER (11 -- AS-REP.rfc1510 -- |
                                   13 -- TGS.rfc1510 -- |
                                   7 -- AS-REP.ext -- |
                                   9 -- TGS-REP.ext -- ),
          padata      [2] SEQUENCE OF PA-DATA OPTIONAL,
          crealm      [3] Realm,
          cname       [4] PrincipalName,
          ticket      [5] Ticket,


          enc-part    [6] EncryptedData {
              EncPart,
              { key-reply },
              -- maybe reach into EncryptedData in AS-REP/TGS-REP
              -- definitions to apply constraints on key usages?
              { ku-EncASRepPart -- if AS-REP -- |
                ku-EncTGSRepPart-subkey -- if TGS-REP and
                                        -- using Authenticator
                                        -- session key -- |
                ku-EncTGSRepPart-sesskey -- if TGS-REP and using
                                         -- subsession key -- }
          },


          ...,
          -- In extensible version, KDC signs original request
          -- to avoid replay attacks against client.
          req-cksum   [7] ChecksumOf { CHOICE {
              as-req          AS-REQ,
              tgs-req         TGS-REQ
          }, { key-reply }, { ku-KDCRep-cksum }} OPTIONAL,
          lang-tag    [8] LangTag OPTIONAL,
          ...
      }



      KDC-REP-1510 { EncPart } ::= SEQUENCE {
          COMPONENTS OF KDC-REP-COMMON { EncPart }
      } (WITH COMPONENTS {
          ...,
          msg-type (11 | 13),
          crealm (RealmIA5),
          cname (PrincipalNameIA5)
      })









Yu                         Expires: Apr 2005                   [Page 88]
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      KDC-REP-EXT { EncPart } ::= KDC-REP-COMMON { EncPart }
      (WITH COMPONENTS {
          ...,
          msg-type (7 | 9),
          crealm (RealmExt),
          cname (PrincipalNameExt)
      })



      EncASRepPart1510        ::= [APPLICATION 25] EncKDCRepPart1510
      EncTGSRepPart1510       ::= [APPLICATION 26] EncKDCRepPart1510


      EncASRepPartExt         ::= [APPLICATION 32] EncKDCRepPartExt
      EncTGSRepPartExt        ::= [APPLICATION 33] EncKDCRepPartExt


      EncKDCRepPartCom        ::= SEQUENCE {
          key                 [0] EncryptionKey,
          last-req            [1] LastReq,
          nonce               [2] Nonce,
          key-expiration      [3] KerberosTime OPTIONAL,
          flags               [4] TicketFlags,
          authtime            [5] KerberosTime,
          starttime           [6] KerberosTime OPTIONAL,
          endtime             [7] KerberosTime,
          renew-till          [8] KerberosTime OPTIONAL,
          srealm              [9] Realm,
          sname               [10] PrincipalName,
          caddr               [11] HostAddresses OPTIONAL,
          ...
      }


      EncKDCRepPart1510       ::= SEQUENCE {
          COMPONENTS OF EncKDCRepPartCom
      } (WITH COMPONENTS {
          ...,
          srealm (RealmIA5),
          sname (PrincipalNameIA5),
          nonce UInt32 })


      EncKdcRepPartExt        ::= EncKDCRepPartCom (WITH COMPONENTS {
          ...,
          srealm (RealmExt),
          sname (PrincipalNameExt)
      })









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      LRType          ::=     TH-id
      LastReq         ::=     SEQUENCE OF SEQUENCE {
          lr-type     [0] LRType,
          lr-value    [1] KerberosTime
      }



      --
      -- *** preauth
      --



      PaDataType      ::= TH-id
      PaDataOID       ::= RELATIVE-OID


      PA-DATA ::= SEQUENCE {
          -- NOTE: first tag is [1], not [0]
          padata-type         [1] PaDataType,
          padata-value        [2] OCTET STRING
      }



      -- AP-REQ authenticating a TGS-REQ
      pa-tgs-req              PaDataType ::= int32 : 1
      PA-TGS-REQ              ::= AP-REQ



      -- Encrypted timestamp preauth
      -- Encryption key used is client's long-term key.
      pa-enc-timestamp        PaDataType ::= int32 : 2


      PA-ENC-TIMESTAMP ::= EncryptedData {
          PA-ENC-TS-ENC, { key-client }, { ku-pa-enc-ts }
      }


      PA-ENC-TS-ENC           ::= SEQUENCE {
              patimestamp     [0] KerberosTime -- client's time --,
              pausec          [1] Microseconds OPTIONAL
      }














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      -- Hints returned in AS-REP or KRB-ERROR to help client
      -- choose a password-derived key, either for preauthentication
      -- or for decryption of the reply.
      pa-etype-info           PaDataType ::= int32 : 11


      ETYPE-INFO              ::= SEQUENCE OF ETYPE-INFO-ENTRY


      ETYPE-INFO-ENTRY        ::= SEQUENCE {
              etype           [0] EType,
              salt            [1] OCTET STRING OPTIONAL
      }



      -- Similar to etype-info, but with parameters provided for
      -- the string-to-key function.
      pa-etype-info2          PaDataType ::= int32 : 19


      ETYPE-INFO2             ::= SEQUENCE (SIZE (1..MAX))
                                      OF ETYPE-INFO-ENTRY


      ETYPE-INFO2-ENTRY       ::= SEQUENCE {
              etype           [0] EType,
              salt            [1] KerberosString OPTIONAL,
              s2kparams       [2] OCTET STRING OPTIONAL
      }



      -- Obsolescent.  Salt for client's long-term key.
      -- Its character encoding is unspecified.
      pa-pw-salt              PaDataType ::= int32 : 3
      -- The "padata-value" does not encode an ASN.1 type.
      -- Instead, "padata-value" must consist of the salt string to
      -- be used by the client, in an unspecified character
      -- encoding.



      -- An extensible AS-REQ may be sent as a padata in a
      -- non-extensible AS-REQ to allow for backwards compatibility.
      pa-as-req               PaDataType ::= int32 : 42 -- provisional
      PA-AS-REQ               ::= AS-REQ (WITH COMPONENTS ext)



      --
      -- *** Session key exchange
      --








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      AP-REQ          ::= CHOICE {
          rfc1510     [APPLICATION 14] AP-REQ-1510,
          ext         [APPLICATION 18] Signed {
              AP-REQ-EXT, { key-session }, { ku-APReq-cksum }
          }
      }



      AP-REQ-COMMON   ::= SEQUENCE {
          pvno                [0] INTEGER (5),
          msg-type            [1] INTEGER (14 | 18),
          ap-options          [2] APOptions,
          ticket              [3] Ticket,
          authenticator       [4] EncryptedData {
              Authenticator,
              { key-session },
              { ku-APReq-authenticator |
                ku-pa-TGSReq-authenticator }
          },
          ...,
          extensions          [5] ApReqExtensions OPTIONAL,
          lang-tag            [6] SEQUENCE (SIZE (1..MAX))
                                      OF LangTag OPTIONAL,
          ...
      }



      AP-REQ-1510 ::= SEQUENCE {
          COMPONENTS OF AP-REQ-COMMON
      } (WITH COMPONENTS {
          ...,
          msg-type (14),
          authenticator (EncryptedData {
              Authenticator (WITH COMPONENTS {
                  ...,
                  crealm (RealmIA5),
                  cname (PrincipalNameIA5),
                  seqnum (UInt32)
              }), { key-session }, { ku-APReq-authenticator }})
      })













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      AP-REQ-EXT      ::= AP-REQ-COMMON
      (WITH COMPONENTS {
          ...,
          msg-type (18),
          -- The following constraints on Authenticator assume that
          -- we want to restrict the use of AP-REQ-EXT with TicketExt
          -- only, since that is the only way we can enforce UTF-8.
          authenticator (EncryptedData {
              Authenticator (WITH COMPONENTS {
                  ...,
                  crealm (RealmExt),
                  cname (PrincipalNameExt),
                  authorization-data (SIZE (1..MAX))
              }), { key-session }, { ku-APReq-authenticator }})
      })



      ApReqExtType    ::= TH-id


      ApReqExtensions ::= SEQUENCE (SIZE (1..MAX)) OF SEQUENCE {
          apReqExt-Type       [0] ApReqExtType,
          apReqExt-Data       [1] OCTET STRING
      }



      APOptions       ::= KerberosFlags { APOptionsBits }


      APOptionsBits ::= BIT STRING {
          reserved            (0),
          use-session-key     (1),
          mutual-required     (2)
      }



      -- plaintext of authenticator
      Authenticator   ::= [APPLICATION 2] SEQUENCE  {
          authenticator-vno   [0] INTEGER (5),
          crealm              [1] Realm,
          cname               [2] PrincipalName,
          cksum               [3] Checksum {{ key-session },
              { ku-Authenticator-cksum |
                ku-pa-TGSReq-cksum }} OPTIONAL,
          cusec               [4] Microseconds,
          ctime               [5] KerberosTime,
          subkey              [6] EncryptionKey OPTIONAL,
          seq-number          [7] SeqNum OPTIONAL,
          authorization-data  [8] AuthorizationData OPTIONAL
      }





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      AP-REP          ::= CHOICE {
          rfc1510     [APPLICATION 15] AP-REP-1510,
          ext         [APPLICATION 19] Signed {
              AP-REP-EXT,
              { key-session | key-subsession }, { ku-APRep-cksum }}
      }



      AP-REP-COMMON { EncPart }       ::= SEQUENCE {
          pvno        [0] INTEGER (5),
          msg-type    [1] INTEGER (15 | 19),
          enc-part    [2] EncryptedData {
              EncPart,
              { key-session | key-subsession }, { ku-EncAPRepPart }},
          ...,
          extensions          [3] ApRepExtensions OPTIONAL,
          ...
      }



      AP-REP-1510     ::= SEQUENCE {
          COMPONENTS OF AP-REP-COMMON { EncAPRepPart1510 }
      } (WITH COMPONENTS {
          ...,
          msg-type (15)
      })



      AP-REP-EXT      ::= [APPLICATION 19] AP-REP-COMMON {
          EncAPRepPartExt
      } (WITH COMPONENTS { ..., msg-type (19) })



      ApRepExtType    ::= TH-id


      ApRepExtensions ::= SEQUENCE (SIZE (1..MAX)) OF SEQUENCE {
          apRepExt-Type       [0] ApRepExtType,
          apRepExt-Data       [1] OCTET STRING
      }



      EncAPRepPart    ::= CHOICE {
          rfc1510     [APPLICATION 27] EncAPRepPart1510,
          ext         [APPLICATION 31] EncAPRepPartExt
      }








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      EncAPRepPart1510        ::= SEQUENCE {
          COMPONENTS OF ENCAPRepPartCom
      } (WITH COMPONENTS {
          ...,
          seq-number (UInt32),
          authorization-data ABSENT
      })



      EncAPRepPartExt         ::= EncAPRepPartCom



      EncAPRepPartCom          ::= SEQUENCE {
          ctime               [0] KerberosTime,
          cusec               [1] Microseconds,
          subkey              [2] EncryptionKey OPTIONAL,
          seq-number          [3] SeqNum OPTIONAL,
          ...,
          authorization-data  [4] AuthorizationData OPTIONAL,
          ...
      }



      --
      -- *** Application messages
      --



      -- Do we chew up another tag for KRB-SAFE-EXT?  That would
      -- allow us to  make safe-body optional, allowing for a GSS-MIC
      -- sort of message.
      KRB-SAFE        ::= [APPLICATION 20] SEQUENCE {
          pvno        [0] INTEGER (5),
          msg-type    [1] INTEGER (20),
          safe-body   [2] KRB-SAFE-BODY,
          cksum       [3] ChecksumOf {
              KRB-SAFE-BODY,
              { key-session | key-subsession }, { ku-KrbSafe-cksum }},
          ...         -- ASN.1 extensions must be excluded
                      -- when sending to rfc1510 implementations
      }












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      KRB-SAFE-BODY   ::= SEQUENCE {
          user-data   [0] OCTET STRING,
          timestamp   [1] KerberosTime OPTIONAL,
          usec        [2] Microseconds OPTIONAL,
          seq-number  [3] SeqNum OPTIONAL,
          s-address   [4] HostAddress,
          r-address   [5] HostAddress OPTIONAL,
          ...         -- ASN.1 extensions must be excluded
                      -- when sending to rfc1510 implementations
      }



      KRB-PRIV        ::= [APPLICATION 21] SEQUENCE {
          pvno        [0] INTEGER (5),
          msg-type    [1] INTEGER (21),
          enc-part    [3] EncryptedData {
              EncKrbPrivPart,
              { key-session | key-subsession }, { ku-EncKrbPrivPart }},
          ...
      }



      EncKrbPrivPart  ::= [APPLICATION 28] SEQUENCE {
          user-data   [0] OCTET STRING,
          timestamp   [1] KerberosTime OPTIONAL,
          usec        [2] Microseconds OPTIONAL,
          seq-number  [3] SeqNum OPTIONAL,
          s-address   [4] HostAddress -- sender's addr --,
          r-address   [5] HostAddress OPTIONAL -- recip's addr --,
          ...         -- ASN.1 extensions must be excluded
                      -- when sending to rfc1510 implementations
      }



      KRB-CRED        ::= CHOICE {
          rfc1510     [APPLICATION 22] KRB-CRED-1510,
          ext         [APPLICATION 24] Signed {
              KRB-CRED-EXT,
              { key-session | key-subsession }, { ku-KrbCred-cksum }}
      }













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      KRB-CRED-COMMON ::= SEQUENCE {
          pvno        [0] INTEGER (5),
          msg-type    [1] INTEGER (22 | 24),
          tickets     [2] SEQUENCE OF Ticket,
          enc-part    [3] EncryptedData {
              EncKrbCredPart,
              { key-session | key-subsession }, { ku-EncKrbCredPart }},
          ...
      }



      KRB-CRED-1510 ::= SEQUENCE {
          COMPONENTS OF KRB-CRED-COMMON
      } (WITH COMPONENTS { ..., msg-type (22) })



      KRB-CRED-EXT    ::= [APPLICATION 24] KRB-CRED-COMMON
          (WITH COMPONENTS { ..., msg-type (24) })



      EncKrbCredPart  ::= [APPLICATION 29] SEQUENCE {
          ticket-info [0] SEQUENCE OF KrbCredInfo,
          nonce       [1] Nonce OPTIONAL,
          timestamp   [2] KerberosTime OPTIONAL,
          usec        [3] Microseconds OPTIONAL,
          s-address   [4] HostAddress OPTIONAL,
          r-address   [5] HostAddress OPTIONAL
      }



      KrbCredInfo     ::= SEQUENCE {
          key         [0] EncryptionKey,
          prealm      [1] Realm OPTIONAL,
          pname       [2] PrincipalName OPTIONAL,
          flags       [3] TicketFlags OPTIONAL,
          authtime    [4] KerberosTime OPTIONAL,
          starttime   [5] KerberosTime OPTIONAL,
          endtime     [6] KerberosTime OPTIONAL,
          renew-till  [7] KerberosTime OPTIONAL,
          srealm      [8] Realm OPTIONAL,
          sname       [9] PrincipalName OPTIONAL,
          caddr       [10] HostAddresses OPTIONAL
      }










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      TGT-REQ         ::= [APPLICATION 16] SEQUENCE {
          pvno            [0] INTEGER (5),
          msg-type        [1] INTEGER (16),
          sname           [2] PrincipalName OPTIONAL,
          srealm          [3] Realm OPTIONAL,
          ...
      }



      --
      -- *** Error messages
      --



      ErrCode ::= Int32


      KRB-ERROR       ::= CHOICE {
          rfc1510     [APPLICATION 30] KRB-ERROR-1510,
          ext         [APPLICATION 23] Signed {
              KRB-ERROR-EXT, { ku-KrbError-cksum } }
      }



      KRB-ERROR-COMMON ::= SEQUENCE {
          pvno        [0] INTEGER (5),
          msg-type    [1] INTEGER (30 | 23),
          ctime       [2] KerberosTime OPTIONAL,
          cusec       [3] Microseconds OPTIONAL,
          stime       [4] KerberosTime,
          susec       [5] Microseconds,
          error-code  [6] ErrCode,
          crealm      [7] Realm OPTIONAL,
          cname       [8] PrincipalName OPTIONAL,
          realm       [9] Realm -- Correct realm --,
          sname       [10] PrincipalName -- Correct name --,
          e-text      [11] KerberosString OPTIONAL,
          e-data      [12] OCTET STRING OPTIONAL,
          ...,
          typed-data          [13] TYPED-DATA OPTIONAL,
          nonce               [14] Nonce OPTIONAL,
          lang-tag            [15] LangTag OPTIONAL,
          ...
      }










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      KRB-ERROR-1510 ::= SEQUENCE {
          COMPONENTS OF KRB-ERROR-COMMON
      } (WITH COMPONENTS {
          ...,
          msg-type (30)
      })



      KRB-ERROR-EXT ::= [APPLICATION 23] KRB-ERROR-COMMON
          (WITH COMPONENTS { ..., msg-type (23) })



      METHOD-DATA     ::= SEQUENCE OF PA-DATA



      TDType ::= TH-id


      TYPED-DATA      ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
          data-type   [0] TDType,
          data-value  [1] OCTET STRING OPTIONAL
      }
































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      --
      -- *** Error codes
      --


      -- No error
      kdc-err-none                          ErrCode ::= 0
      -- Client's entry in database has expired
      kdc-err-name-exp                      ErrCode ::= 1
      -- Server's entry in database has expired
      kdc-err-service-exp                   ErrCode ::= 2
      -- Requested protocol version number not supported
      kdc-err-bad-pvno                      ErrCode ::= 3
      -- Client's key encrypted in old master key
      kdc-err-c-old-mast-kvno               ErrCode ::= 4
      -- Server's key encrypted in old master key
      kdc-err-s-old-mast-kvno               ErrCode ::= 5
      -- Client not found in Kerberos database
      kdc-err-c-principal-unknown           ErrCode ::= 6
      -- Server not found in Kerberos database
      kdc-err-s-principal-unknown           ErrCode ::= 7
      -- Multiple principal entries in database
      kdc-err-principal-not-unique          ErrCode ::= 8
      -- The client or server has a null key
      kdc-err-null-key                      ErrCode ::= 9
      -- Ticket not eligible for postdating
      kdc-err-cannot-postdate               ErrCode ::= 10
      -- Requested start time is later than end time
      kdc-err-never-valid                   ErrCode ::= 11
      -- KDC policy rejects request
      kdc-err-policy                        ErrCode ::= 12
      -- KDC cannot accommodate requested option
      kdc-err-badoption                     ErrCode ::= 13
      -- KDC has no support for encryption type
      kdc-err-etype-nosupp                  ErrCode ::= 14
      -- KDC has no support for checksum type
      kdc-err-sumtype-nosupp                ErrCode ::= 15
      -- KDC has no support for padata type
      kdc-err-padata-type-nosupp            ErrCode ::= 16
      -- KDC has no support for transited type
      kdc-err-trtype-nosupp                 ErrCode ::= 17
      -- Clients credentials have been revoked
      kdc-err-client-revoked                ErrCode ::= 18
      -- Credentials for server have been revoked
      kdc-err-service-revoked               ErrCode ::= 19
      -- TGT has been revoked
      kdc-err-tgt-revoked                   ErrCode ::= 20







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      -- Client not yet valid - try again later
      kdc-err-client-notyet                 ErrCode ::= 21
      -- Server not yet valid - try again later
      kdc-err-service-notyet                ErrCode ::= 22
      -- Password has expired - change password to reset
      kdc-err-key-expired                   ErrCode ::= 23
      -- Pre-authentication information was invalid
      kdc-err-preauth-failed                ErrCode ::= 24
      -- Additional pre-authenticationrequired
      kdc-err-preauth-required              ErrCode ::= 25
      -- Requested server and ticket don't match
      kdc-err-server-nomatch                ErrCode ::= 26
      -- Server principal valid for user2user only
      kdc-err-must-use-user2user            ErrCode ::= 27
      -- KDC Policy rejects transited path
      kdc-err-path-not-accpeted             ErrCode ::= 28
      -- A service is not available
      kdc-err-svc-unavailable               ErrCode ::= 29
      -- Integrity check on decrypted field failed
      krb-ap-err-bad-integrity              ErrCode ::= 31
      -- Ticket expired
      krb-ap-err-tkt-expired                ErrCode ::= 32
      -- Ticket not yet valid
      krb-ap-err-tkt-nyv                    ErrCode ::= 33
      -- Request is a replay
      krb-ap-err-repeat                     ErrCode ::= 34
      -- The ticket isn't for us
      krb-ap-err-not-us                     ErrCode ::= 35
      -- Ticket and authenticator don't match
      krb-ap-err-badmatch                   ErrCode ::= 36
      -- Clock skew too great
      krb-ap-err-skew                       ErrCode ::= 37
      -- Incorrect net address
      krb-ap-err-badaddr                    ErrCode ::= 38
      -- Protocol version mismatch
      krb-ap-err-badversion                 ErrCode ::= 39
      -- Invalid msg type
      krb-ap-err-msg-type                   ErrCode ::= 40















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      -- Message stream modified
      krb-ap-err-modified                   ErrCode ::= 41
      -- Message out of order
      krb-ap-err-badorder                   ErrCode ::= 42
      -- Specified version of key is not available
      krb-ap-err-badkeyver                  ErrCode ::= 44
      -- Service key not available
      krb-ap-err-nokey                      ErrCode ::= 45
      -- Mutual authentication failed
      krb-ap-err-mut-fail                   ErrCode ::= 46
      -- Incorrect message direction
      krb-ap-err-baddirection               ErrCode ::= 47
      -- Alternative authentication method required
      krb-ap-err-method                     ErrCode ::= 48
      -- Incorrect sequence number in message
      krb-ap-err-badseq                     ErrCode ::= 49
      -- Inappropriate type of checksum in message
      krb-ap-err-inapp-cksum                ErrCode ::= 50
      -- Policy rejects transited path
      krb-ap-path-not-accepted              ErrCode ::= 51
      -- Response too big for UDP, retry with TCP
      krb-err-response-too-big              ErrCode ::= 52
      -- Generic error (description in e-text)
      krb-err-generic                       ErrCode ::= 60





























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      -- Field is too long for this implementation
      krb-err-field-toolong                 ErrCode ::= 61
      -- Reserved for PKINIT
      kdc-error-client-not-trusted          ErrCode ::= 62
      -- Reserved for PKINIT
      kdc-error-kdc-not-trusted             ErrCode ::= 63
      -- Reserved for PKINIT
      kdc-error-invalid-sig                 ErrCode ::= 64
      -- Reserved for PKINIT
      kdc-err-key-too-weak                  ErrCode ::= 65
      -- Reserved for PKINIT
      kdc-err-certificate-mismatch          ErrCode ::= 66
      -- No TGT available to validate USER-TO-USER
      krb-ap-err-no-tgt                     ErrCode ::= 67
      -- USER-TO-USER TGT issued different KDC
      kdc-err-wrong-realm                   ErrCode ::= 68
      -- Ticket must be for USER-TO-USER
      krb-ap-err-user-to-user-required      ErrCode ::= 69
      -- Reserved for PKINIT
      kdc-err-cant-verify-certificate       ErrCode ::= 70
      -- Reserved for PKINIT
      kdc-err-invalid-certificate           ErrCode ::= 71
      -- Reserved for PKINIT
      kdc-err-revoked-certificate           ErrCode ::= 72
      -- Reserved for PKINIT
      kdc-err-revocation-status-unknown     ErrCode ::= 73
      -- Reserved for PKINIT
      kdc-err-revocation-status-unavailable ErrCode ::= 74



      END



B.  Kerberos and Character Encodings (Informative)


   [adapted from KCLAR 5.2.1]


   The original specification of the Kerberos protocol in RFC 1510 uses
   GeneralString in numerous places for human-readable string data.
   Historical implementations of Kerberos cannot utilize the full power
   of GeneralString.  This ASN.1 type requires the use of designation
   and invocation escape sequences as specified in ISO 2022 | ECMA-35
   [ISO2022] to switch character sets, and the default character set
   that is designated as G0 is the ISO 646 | ECMA-6 [ISO646]
   International Reference Version (IRV) (aka U.S. ASCII), which mostly
   works.


   ISO 2022 | ECMA-35 defines four character-set code elements (G0..G3)
   and two Control-function code elements (C0..C1).  DER previously
   [X690-1997] prohibited the designation of character sets as any but
   the G0 and C0 sets.  This had the side effect of prohibiting the use


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   of (ISO Latin) character-sets such as ISO 8859-1 [ISO8859-1] or any
   other character-sets that utilize a 96-character set, since it is
   prohibited by ISO 2022 | ECMA-35 to designate them as the G0 code
   element.  Recent revisions to the ASN.1 standards resolve this
   contradiction.


   In practice, many implementations treat RFC 1510 GeneralStrings as if
   they were 8-bit strings of whichever character set the implementation
   defaults to, without regard for correct usage of character-set
   designation escape sequences.  The default character set is often
   determined by the current user's operating system dependent locale.
   At least one major implementation places unescaped UTF-8 encoded
   Unicode characters in the GeneralString.  This failure to conform to
   the GeneralString specifications results in interoperability issues
   when conflicting character encodings are utilized by the Kerberos
   clients, services, and KDC.


   This unfortunate situation is the result of improper documentation of
   the restrictions of the ASN.1 GeneralString type in prior Kerberos
   specifications.


   [the following should probably be rewritten and moved into the
   principal name section]


   For compatibility, implementations MAY choose to accept GeneralString
   values that contain characters other than those permitted by
   IA5String, but they should be aware that character set designation
   codes will likely be absent, and that the encoding should probably be
   treated as locale-specific in almost every way.  Implementations MAY
   also choose to emit GeneralString values that are beyond those
   permitted by IA5String, but should be aware that doing so is
   extraordinarily risky from an interoperability perspective.


   Some existing implementations use GeneralString to encode unescaped
   locale-specific characters.  This is a violation of the ASN.1
   standard.  Most of these implementations encode US-ASCII in the left-
   hand half, so as long the implementation transmits only US-ASCII, the
   ASN.1 standard is not violated in this regard.  As soon as such an
   implementation encodes unescaped locale-specific characters with the
   high bit set, it violates the ASN.1 standard.


   Other implementations have been known to use GeneralString to contain
   a UTF-8 encoding.  This also violates the ASN.1 standard, since UTF-8
   is a different encoding, not a 94 or 96 character "G" set as defined
   by ISO 2022.  It is believed that these implementations do not even
   use the ISO 2022 escape sequence to change the character encoding.
   Even if implementations were to announce the change of encoding by
   using that escape sequence, the ASN.1 standard prohibits the use of
   any escape sequences other than those used to designate/invoke "G" or
   "C" sets allowed by GeneralString.



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C.  Kerberos History (Informative)


   [Adapted from KCLAR "BACKGROUND"]


   The Kerberos model is based in part on Needham and Schroeder's
   trusted third-party authentication protocol [NS78] and on
   modifications suggested by Denning and Sacco [DS81].  The original
   design and implementation of Kerberos Versions 1 through 4 was the
   work of two former Project Athena staff members, Steve Miller of
   Digital Equipment Corporation and Clifford Neuman (now at the
   Information Sciences Institute of the University of Southern
   California), along with Jerome Saltzer, Technical Director of Project
   Athena, and Jeffrey Schiller, MIT Campus Network Manager.  Many other
   members of Project Athena have also contributed to the work on
   Kerberos.


   Version 5 of the Kerberos protocol (described in this document) has
   evolved from Version 4 based on new requirements and desires for
   features not available in Version 4.  The design of Version 5 of the
   Kerberos protocol was led by Clifford Neuman and John Kohl with much
   input from the community.  The development of the MIT reference
   implementation was led at MIT by John Kohl and Theodore Ts'o, with
   help and contributed code from many others.  Since RFC1510 was
   issued, extensions and revisions to the protocol have been proposed
   by many individuals.  Some of these proposals are reflected in this
   document.  Where such changes involved significant effort, the
   document cites the contribution of the proposer.


   Reference implementations of both version 4 and version 5 of Kerberos
   are publicly available and commercial implementations have been
   developed and are widely used.  Details on the differences between
   Kerberos Versions 4 and 5 can be found in [KNT94].


D.  Notational Differences from [KCLAR]


   [ possible point for discussion ]


   [KCLAR] uses notational conventions slightly different from this
   document.  As a derivative of RFC 1510, the text of [KCLAR] uses C-
   language style identifier names for defined values.  In ASN.1
   notation, identifiers referencing defined values must begin with a
   lowercase letter and contain hyphen (-) characters rather than
   underscore (_) characters, while identifiers referencing types begin
   with an uppercase letter.  [KCLAR] and RFC 1510 use all-uppercase
   identifiers with underscores to identify defined values.  This has
   the potential to create confusion, but neither document defines
   values using actual ASN.1 value-assignment notation.


   It is debatable whether it is advantageous to write all identifier
   names (regardless of their ASN.1 token type) in all-uppercase letters
   for the purpose of emphasis in running text.  The alternative is to


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   use double-quote characters (") when ambiguity is possible.


Normative References


   [ISO646]
        "7-bit coded character set", ISO/IEC 646:1991 | ECMA-6:1991.


   [ISO2022]
        "Information technology -- Character code structure and
        extension techniques", ISO/IEC 2022:1994 | ECMA-35:1994.


   [KCRYPTO]
        K. Raeburn, "Encryption and Checksum Specifications for Kerberos
        5", draft-ietf-krb-wg-crypto-07.txt, work in progress.


   [RFC2119]
        S. Bradner, RFC2119: "Key words for use in RFC's to Indicate
        Requirement Levels", March 1997.


   [RFC3660]
        H. Alvestrand, "Tags for the Identification of Languages",
        RFC 3660, January 2001.


   [SASLPREP]
        Kurt D. Zeilenga, "SASLprep: Stringprep profile for user names
        and passwords", draft-ietf-sasl-saslprep-10.txt, work in
        progress.


   [X680]
        "Information technology -- Abstract Syntax Notation One (ASN.1):
        Specification of basic notation", ITU-T Recommendation X.680
        (2002) | ISO/IEC 8824-1:2002.


   [X682]
        "Information technology -- Abstract Syntax Notation One (ASN.1):
        Constraint specification", ITU-T Recommendation X.682 (2002) |
        ISO/IEC 8824-3:2002.


   [X683]
        "Information technology -- Abstract Syntax Notation One (ASN.1):
        Parameterization of ASN.1 specifications", ITU-T Recommendation
        X.683 (2002) | ISO/IEC 8824-4:2002.


   [X690]
        "Information technology -- ASN.1 encoding Rules: Specification
        of Basic Encoding Rules (BER), Canonical Encoding Rules (CER)
        and Distinguished Encoding Rules (DER)", ITU-T Recommendation
        X.690 (2002) | ISO/IEC 8825-1:2002.





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Informative References


   [DS81]
        Dorothy E. Denning and Giovanni Maria Sacco, "Time-stamps in Key
        Distribution Protocols," Communications of the ACM, Vol. 24(8),
        pp. 533-536 (August 1981).


   [Dub00]
        Olivier Dubuisson, "ASN.1 - Communication between Heterogeneous
        Systems", Elsevier-Morgan Kaufmann, 2000.
        <http://www.oss.com/asn1/dubuisson.html>


   [ISO8859-1]
        "Information technology -- 8-bit single-byte coded graphic
        character sets -- Part 1: Latin alphabet No. 1", ISO/IEC 8859-
        1:1998.


   [KCLAR]
        Clifford Neuman, Tom Yu, Sam Hartman, Ken Raeburn, "The Kerberos
        Network Authentication Service (V5)", draft-ietf-krb-wg-
        kerberos-clarifications-07.txt, work in progress.


   [KNT94]
        John T. Kohl, B. Clifford Neuman, and Theodore Y. Ts'o, "The
        Evolution of the Kerberos Authentication System".  In
        Distributed Open Systems, pages 78-94.  IEEE Computer Society
        Press, 1994.


   [Lar96]
        John Larmouth, "Understanding OSI", International Thomson
        Computer Press, 1996.
        <http://www.isi.salford.ac.uk/books/osi.html>


   [Lar99]
        John Larmouth, "ASN.1 Complete",  Elsevier-Morgan Kaufmann,
        1999.  <http://www.oss.com/asn1/larmouth.html>


   [NS78]
        Roger M. Needham and Michael D. Schroeder, "Using Encryption for
        Authentication in Large Networks of Computers", Communications
        of the ACM, Vol. 21(12), pp. 993-999 (December, 1978).


   [RFC1510]
        J. Kohl and B. C. Neuman, "The Kerberos Network Authentication
        Service (v5)", RFC1510, September 1993, Status: Proposed
        Standard.


   [RFC1964]
        J. Linn, "The Kerberos Version 5 GSS-API Mechanism", RFC 1964,
        June 1996, Status: Proposed Standard.



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   [X690-2002]
        "Information technology -- ASN.1 encoding rules: Specification
        of Basic Encoding Rules (BER), Canonical Encoding Rules (CER)
        and Distinguished Encoding Rules (DER)", ITU-T Recommendation
        X.690 (2002) | ISO/IEC 8825-1:2002.


Author's Address


   Tom Yu
   77 Massachusetts Ave
   Cambridge, MA 02139
   USA
   tlyu@mit.edu


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