\input texinfo @c -*-texinfo-*- @comment $Id$ @comment %**start of header @setfilename gnutls.info @include version.texi @settitle GNU TLS @value{VERSION} @c don't indent the paragraphs. @paragraphindent 0 @c Unify some of the indices. @syncodeindex tp fn @syncodeindex pg cp @comment %**end of header @finalout @copying This manual is last updated @value{UPDATED} for version @value{VERSION} of GNU TLS. Copyright (C) 2001, 2002, 2003, 2004, 2005, 2006, 2007 Free Software Foundation, Inc. @quotation Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. @end quotation @end copying @dircategory GNU Libraries @direntry * libgnutls: (gnutls). A Transport Layer Security Library. @end direntry @dircategory Network Applications @direntry * GnuTLS: (gnutls). Package for Transport Layer Security. * certtool: (gnutls)Invoking certtool. Manipulate certificates and keys. * srptool: (gnutls)Invoking srptool. Simple SRP password tool. * gnutls-serv: (gnutls)Invoking gnutls-serv. GNU TLS test server. * gnutls-cli: (gnutls)Invoking gnutls-cli. GNU TLS test client. * gnutls-cli-debug: (gnutls)Invoking gnutls-cli-debug. GNU TLS debug client. @end direntry @titlepage @title GNU TLS @subtitle Transport Layer Security Library for the GNU system @subtitle for version @value{VERSION}, @value{UPDATED} @sp 7 @image{gnutls-logo,6cm,6cm} @author Nikos Mavroyanopoulos @author Simon Josefsson (@email{bug-gnutls@@gnu.org}) @page @vskip 0pt plus 1filll @insertcopying @end titlepage @macro xcite{ref} [\ref\] (@pxref{Bibliography}) @end macro @contents @ifnottex @node Top @top GNU TLS @insertcopying @end ifnottex @menu * Preface:: * The Library:: * Introduction to TLS:: * Authentication methods:: * More on certificate authentication:: * How to use TLS in application protocols:: * How to use GnuTLS in applications:: * Included programs:: * Function reference:: * All the supported ciphersuites in GnuTLS:: * Guile Bindings:: * Internal architecture of GnuTLS:: * Copying Information:: * Concept Index:: * Function and Data Index:: @c * @mybibnode{}:: * Bibliography:: @end menu @node Preface @chapter Preface This document tries to demonstrate and explain the @acronym{GnuTLS} library API. A brief introduction to the protocols and the technology involved, is also included so that an application programmer can better understand the @acronym{GnuTLS} purpose and actual offerings. Even if @acronym{GnuTLS} is a typical library software, it operates over several security and cryptographic protocols, which require the programmer to make careful and correct usage of them, otherwise he risks to offer just a false sense of security. Security and the network security terms are very general terms even for computer software thus cannot be easily restricted to a single cryptographic library. For that reason, do not consider a program secure just because it uses @acronym{GnuTLS}; there are several ways to compromise a program or a communication line and @acronym{GnuTLS} only helps with some of them. Although this document tries to be self contained, basic network programming and PKI knowlegde is assumed in most of it. A good introduction to networking can be found in @xcite{STEVENS} and for Public Key Infrastructure in @xcite{GUTPKI}. @anchor{Availability} Updated versions of the @acronym{GnuTLS} software and this document will be available from @url{http://www.gnutls.org/} and @url{http://www.gnu.org/software/gnutls/}. @menu * Getting help:: * Commercial Support:: * Downloading and Installing:: * Bug Reports:: * Contributing:: @end menu @node Getting help @section Getting Help A mailing list where users may help each other exists, and you can reach it by sending e-mail to @email{help-gnutls@@gnu.org}. Archives of the mailing list discussions, and an interface to manage subscriptions, is available through the World Wide Web at @url{http://lists.gnu.org/mailman/listinfo/help-gnutls}. A mailing list for developers are also available, see @url{http://www.gnu.org/software/gnutls/lists.html}. Bug reports should be sent to @email{bug-gnutls@@gnu.org}, see @xref{Bug Reports}. @node Commercial Support @section Commercial Support Commercial support is available for users of GnuTLS. The kind of support that can be purchased may include: @itemize @item Implement new features. Such as a new TLS extension. @item Port GnuTLS to new platforms. This could include porting to an embedded platforms that may need memory or size optimization. @item Integrating TLS as a security environment in your existing project. @item System design of components related to TLS. @end itemize If you are interested, please write to: @verbatim Simon Josefsson Datakonsult Hagagatan 24 113 47 Stockholm Sweden E-mail: simon@josefsson.org @end verbatim If your company provide support related to GnuTLS and would like to be mentioned here, contact the author (@pxref{Bug Reports}). @node Downloading and Installing @section Downloading and Installing @cindex Installation @cindex Download GnuTLS is available for download from the following URL: @url{http://www.gnutls.org/download.html} The latest version is stored in a file, e.g., @samp{gnutls-@value{VERSION}.tar.gz} where the @samp{@value{VERSION}} value is the highest version number in the directory. GnuTLS uses a Linux-like development cycle: even minor version numbers indicate a stable release and a odd minor version number indicates a development release. For example, GnuTLS 1.6.3 denote a stable release since 6 is even, and GnuTLS 1.7.11 denote a development release since 7 is odd. GnuTLS depends on Libgcrypt, and you will need to install Libgcrypt before installing GnuTLS. Libgcrypt is available from @url{ftp://ftp.gnupg.org/gcrypt/libgcrypt}. Libgcrypt needs another library, libgpg-error, and you need to install libgpg-error before installing Libgcrypt. Libgpg-error is available from @url{ftp://ftp.gnupg.org/gcrypt/libgpg-error}. Don't forget to verify the cryptographic signature after downloading source code packages. The package is then extracted, configured and built like many other packages that use Autoconf. For detailed information on configuring and building it, refer to the @file{INSTALL} file that is part of the distribution archive. Typically you invoke @code{./configure} and then @code{make check install}. There are a number of compile-time parameters, as discussed below. The compression libraries (libz and lzo) are optional dependencies. You can get libz from @url{http://www.zlib.net/}. You can get lzo from @url{http://www.oberhumer.com/opensource/lzo/}. If you do not have lzo installed, GnuTLS will enable an internal copy, called minilzo. Use parameter @code{--without-lzo} to disable LZO completely. Use parameter @code{--with-included-lzo} to unconditionally use the internal minilzo copy. The X.509 part of GnuTLS needs ASN.1 functionality, from a library called libtasn1. A copy of libtasn1 is included in GnuTLS. If you want to install it separately (e.g., to make it possibly to use libtasn1 in other programs), you can get it from @url{http://www.gnu.org/software/gnutls/download.html}. The OpenPGP part of GnuTLS-extra needs OpenCDK for parsing OpenPGP packets. A copy of OpenCDK is included in GnuTLS. If you want to install it separately (e.g., to make it possibly to use libtasn1 in other programs), you can get it from @url{http://www.gnu.org/software/gnutls/download.html}. Use parameter @code{--with-included-opencdk} to unconditionally use the internal copy of OpenCDK. Use parameter @code{--disable-openpgp-authentication} to disable the OpenPGP functionality in GnuTLS. Regarding the Guile bindings, there are additional installation considerations, see @xref{Guile Preparations}. A few @code{configure} options may be relevant, summarized in the table. @table @code @item --disable-srp-authentication @itemx --disable-psk-authentication @itemx --disable-anon-authentication @itemx --disable-extra-pki @itemx --disable-openpgp-authentication @itemx --disable-openssl-compatibility Disable or enable particular features. Generally not recommended. @end table For the complete list, refer to the output from @code{configure --help}. @node Bug Reports @section Bug Reports @cindex Reporting Bugs If you think you have found a bug in GnuTLS, please investigate it and report it. @itemize @bullet @item Please make sure that the bug is really in GnuTLS, and preferably also check that it hasn't already been fixed in the latest version. @item You have to send us a test case that makes it possible for us to reproduce the bug. @item You also have to explain what is wrong; if you get a crash, or if the results printed are not good and in that case, in what way. Make sure that the bug report includes all information you would need to fix this kind of bug for someone else. @end itemize Please make an effort to produce a self-contained report, with something definite that can be tested or debugged. Vague queries or piecemeal messages are difficult to act on and don't help the development effort. If your bug report is good, we will do our best to help you to get a corrected version of the software; if the bug report is poor, we won't do anything about it (apart from asking you to send better bug reports). If you think something in this manual is unclear, or downright incorrect, or if the language needs to be improved, please also send a note. Send your bug report to: @center @samp{bug-gnutls@@gnu.org} @node Contributing @section Contributing @cindex Contributing @cindex Hacking If you want to submit a patch for inclusion -- from solve a typo you discovered, up to adding support for a new feature -- you should submit it as a bug report (@pxref{Bug Reports}). There are some things that you can do to increase the chances for it to be included in the official package. Unless your patch is very small (say, under 10 lines) we require that you assign the copyright of your work to the Free Software Foundation. This is to protect the freedom of the project. If you have not already signed papers, we will send you the necessary information when you submit your contribution. For contributions that doesn't consist of actual programming code, the only guidelines are common sense. Use it. For code contributions, a number of style guides will help you: @itemize @bullet @item Coding Style. Follow the GNU Standards document (@pxref{top, GNU Coding Standards,, standards}). If you normally code using another coding standard, there is no problem, but you should use @samp{indent} to reformat the code (@pxref{top, GNU Indent,, indent}) before submitting your work. @item Use the unified diff format @samp{diff -u}. @item Return errors. No reason whatsoever should abort the execution of the library. Even memory allocation errors, e.g. when malloc return NULL, should work although result in an error code. @item Design with thread safety in mind. Don't use global variables. Don't even write to per-handle global variables unless the documented behaviour of the function you write is to write to the per-handle global variable. @item Avoid using the C math library. It causes problems for embedded implementations, and in most situations it is very easy to avoid using it. @item Document your functions. Use comments before each function headers, that, if properly formatted, are extracted into Texinfo manuals and GTK-DOC web pages. @item Supply a ChangeLog and NEWS entries, where appropriate. @end itemize @node The Library @chapter The Library In brief @acronym{GnuTLS} can be described as a library which offers an API to access secure communication protocols. These protocols provide privacy over insecure lines, and were designed to prevent eavesdropping, tampering, or message forgery. Technically @acronym{GnuTLS} is a portable ANSI C based library which implements the TLS 1.1 and SSL 3.0 protocols (@xref{Introduction to TLS}, for a more detailed description of the protocols), accompanied with the required framework for authentication and public key infrastructure. The library is available under the GNU Lesser GPL license@footnote{A copy of the license is included in the distribution}. Important features of the @acronym{GnuTLS} library include: @itemize @item Support for TLS 1.0, TLS 1.1, and SSL 3.0 protocols. @item Support for both @acronym{X.509} and @acronym{OpenPGP} certificates. @item Support for handling and verification of certificates. @item Support for @acronym{SRP} for TLS authentication. @item Support for @acronym{PSK} for TLS authentication. @item Support for TLS Extension mechanism. @item Support for TLS Compression Methods. @end itemize Additionally @acronym{GnuTLS} provides a limited emulation API for the widely used OpenSSL@footnote{@url{http://www.openssl.org/}} library, to ease integration with existing applications. @acronym{GnuTLS} consists of three independent parts, namely the ``TLS protocol part'', the ``Certificate part'', and the ``Crypto backend'' part. The `TLS protocol part' is the actual protocol implementation, and is entirely implemented within the @acronym{GnuTLS} library. The `Certificate part' consists of the certificate parsing, and verification functions which is partially implemented in the @acronym{GnuTLS} library. The @acronym{Libtasn1}@footnote{@url{ftp://ftp.gnupg.org/gcrypt/alpha/gnutls/libtasn1/}}, a library which offers @acronym{ASN.1} parsing capabilities, is used for the @acronym{X.509} certificate parsing functions, and @acronym{Opencdk}@footnote{@url{ftp://ftp.gnupg.org/gcrypt/alpha/gnutls/opencdk/}} is used for the @acronym{OpenPGP} key support in @acronym{GnuTLS}. The ``Crypto backend'' is provided by the @acronym{Libgcrypt}@footnote{@url{ftp://ftp.gnupg.org/gcrypt/alpha/libgcrypt/}} library. In order to ease integration in embedded systems, parts of the @acronym{GnuTLS} library can be disabled at compile time. That way a small library, with the required features, can be generated. @menu * General Idea:: * Error handling:: * Memory handling:: * Callback functions:: @end menu @node General Idea @section General Idea A brief description of how @acronym{GnuTLS} works internally is shown at the figure below. This section may be easier to understand after having seen the examples (@pxref{examples}). @image{gnutls-internals,12cm,8cm} As shown in the figure, there is a read-only global state that is initialized once by the global initialization function. This global structure, among others, contains the memory allocation functions used, and some structures needed for the @acronym{ASN.1} parser. This structure is never modified by any @acronym{GnuTLS} function, except for the deinitialization function which frees all memory allocated in the global structure and is called after the program has permanently finished using @acronym{GnuTLS}. The credentials structure is used by some authentication methods, such as certificate authentication (@pxref{Certificate Authentication}). A credentials structure may contain certificates, private keys, temporary parameters for diffie hellman or RSA key exchange, and other stuff that may be shared between several TLS sessions. This structure should be initialized using the appropriate initialization functions. For example an application which uses certificate authentication would probably initialize the credentials, using the appropriate functions, and put its trusted certificates in this structure. The next step is to associate the credentials structure with each @acronym{TLS} session. A @acronym{GnuTLS} session contains all the required stuff for a session to handle one secure connection. This session calls directly to the transport layer functions, in order to communicate with the peer. Every session has a unique session ID shared with the peer. Since TLS sessions can be resumed, servers would probably need a database backend to hold the session's parameters. Every @acronym{GnuTLS} session after a successful handshake calls the appropriate backend function (@xref{resume}, for information on initialization) to store the newly negotiated session. The session database is examined by the server just after having received the client hello@footnote{The first message in a @acronym{TLS} handshake}, and if the session ID sent by the client, matches a stored session, the stored session will be retrieved, and the new session will be a resumed one, and will share the same session ID with the previous one. @node Error handling @section Error Handling In @acronym{GnuTLS} most functions return an integer type as a result. In almost all cases a zero or a positive number means success, and a negative number indicates failure, or a situation that some action has to be taken. Thus negative error codes may be fatal or not. Fatal errors terminate the connection immediately and further sends and receives will be disallowed. An example of a fatal error code is @code{GNUTLS_E_DECRYPTION_FAILED}. Non-fatal errors may warn about something, i.e., a warning alert was received, or indicate the some action has to be taken. This is the case with the error code @code{GNUTLS_E_REHANDSHAKE} returned by @ref{gnutls_record_recv}. This error code indicates that the server requests a re-handshake. The client may ignore this request, or may reply with an alert. You can test if an error code is a fatal one by using the @ref{gnutls_error_is_fatal}. If any non fatal errors, that require an action, are to be returned by a function, these error codes will be documented in the function's reference. @xref{Error Codes}, for all the error codes. @node Memory handling @section Memory Handling @acronym{GnuTLS} internally handles heap allocated objects differently, depending on the sensitivity of the data they contain. However for performance reasons, the default memory functions do not overwrite sensitive data from memory, nor protect such objects from being written to the swap. In order to change the default behavior the @ref{gnutls_global_set_mem_functions} function is available which can be used to set other memory handlers than the defaults. The @acronym{Libgcrypt} library on which @acronym{GnuTLS} depends, has such secure memory allocation functions available. These should be used in cases where even the system's swap memory is not considered secure. See the documentation of @acronym{Libgcrypt} for more information. @node Callback functions @section Callback Functions @cindex Callback functions There are several cases where @acronym{GnuTLS} may need some out of band input from your program. This is now implemented using some callback functions, which your program is expected to register. An example of this type of functions are the push and pull callbacks which are used to specify the functions that will retrieve and send data to the transport layer. @itemize @item @ref{gnutls_transport_set_push_function} @item @ref{gnutls_transport_set_pull_function} @end itemize Other callback functions such as the one set by @ref{gnutls_srp_set_server_credentials_function}, may require more complicated input, including data to be allocated. These callbacks should allocate and free memory using the functions shown below. @itemize @item @ref{gnutls_malloc} @item @ref{gnutls_free} @end itemize @node Introduction to TLS @chapter Introduction to @acronym{TLS} @acronym{TLS} stands for ``Transport Layer Security'' and is the successor of SSL, the Secure Sockets Layer protocol @xcite{SSL3} designed by Netscape. @acronym{TLS} is an Internet protocol, defined by @acronym{IETF}@footnote{IETF, or Internet Engineering Task Force, is a large open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet. It is open to any interested individual.}, described in @acronym{RFC} 2246 and also in @xcite{RESCOLA}. The protocol provides confidentiality, and authentication layers over any reliable transport layer. The description, below, refers to @acronym{TLS} 1.0 but also applies to @acronym{TLS} 1.1 @xcite{RFC4346} and @acronym{SSL} 3.0, since the differences of these protocols are minor. Older protocols such as @acronym{SSL} 2.0 are not discussed nor implemented in @acronym{GnuTLS} since they are not considered secure today. @menu * TLS layers:: * The transport layer:: * The TLS record protocol:: * The TLS Alert Protocol:: * The TLS Handshake Protocol:: * TLS Extensions:: * Selecting cryptographic key sizes:: * On SSL 2 and older protocols:: @end menu @node TLS layers @section TLS Layers @cindex TLS Layers @acronym{TLS} is a layered protocol, and consists of the Record Protocol, the Handshake Protocol and the Alert Protocol. The Record Protocol is to serve all other protocols and is above the transport layer. The Record protocol offers symmetric encryption, data authenticity, and optionally compression. The Alert protocol offers some signaling to the other protocols. It can help informing the peer for the cause of failures and other error conditions. @xref{The Alert Protocol}, for more information. The alert protocol is above the record protocol. The Handshake protocol is responsible for the security parameters' negotiation, the initial key exchange and authentication. @xref{The Handshake Protocol}, for more information about the handshake protocol. The protocol layering in TLS is shown in the figure below. @image{gnutls-layers,12cm,8cm} @node The transport layer @section The Transport Layer @cindex Transport protocol @acronym{TLS} is not limited to one transport layer, it can be used above any transport layer, as long as it is a reliable one. A set of functions is provided and their purpose is to load to @acronym{GnuTLS} the required callbacks to access the transport layer. @itemize @item @ref{gnutls_transport_set_push_function} @item @ref{gnutls_transport_set_pull_function} @item @ref{gnutls_transport_set_ptr} @item @ref{gnutls_transport_set_lowat} @item @ref{gnutls_transport_set_errno} @end itemize These functions accept a callback function as a parameter. The callback functions should return the number of bytes written, or -1 on error and should set @code{errno} appropriately. In some environments, setting @code{errno} is unreliable, for example Windows have several errno variables in different CRTs, or it may be that errno is not a thread-local variable. If this is a concern to you, call @code{gnutls_transport_set_errno} with the intended errno value instead of setting @code{errno} directly. @acronym{GnuTLS} currently only interprets the EINTR and EAGAIN errno values and returns the corresponding @acronym{GnuTLS} error codes @code{GNUTLS_E_INTERRUPTED} and @code{GNUTLS_E_AGAIN}. These values are usually returned by interrupted system calls, or when non blocking IO is used. All @acronym{GnuTLS} functions can be resumed (called again), if any of these error codes is returned. The error codes above refer to the system call, not the @acronym{GnuTLS} function, since signals do not interrupt @acronym{GnuTLS}' functions. For non blocking sockets or other custom made pull/push functions the @ref{gnutls_transport_set_lowat} must be called, with a zero low water mark value. By default, if the transport functions are not set, @acronym{GnuTLS} will use the Berkeley Sockets functions. In this case @acronym{GnuTLS} will use some hacks in order for @code{select} to work, thus making it easy to add @acronym{TLS} support to existing TCP/IP servers. @node The TLS record protocol @section The TLS Record Protocol @cindex Record protocol The Record protocol is the secure communications provider. Its purpose is to encrypt, authenticate and ---optionally--- compress packets. The following functions are available: @table @asis @item @ref{gnutls_record_send}: To send a record packet (with application data). @item @ref{gnutls_record_recv}: To receive a record packet (with application data). @item @ref{gnutls_record_get_direction}: To get the direction of the last interrupted function call. @end table As you may have already noticed, the functions which access the Record protocol, are quite limited, given the importance of this protocol in @acronym{TLS}. This is because the Record protocol's parameters are all set by the Handshake protocol. The Record protocol initially starts with NULL parameters, which means no encryption, and no MAC is used. Encryption and authentication begin just after the handshake protocol has finished. @menu * Encryption algorithms used in the record layer:: * Compression algorithms used in the record layer:: * Weaknesses and countermeasures:: @end menu @node Encryption algorithms used in the record layer @subsection Encryption Algorithms Used in the Record Layer @cindex Symmetric encryption algorithms Confidentiality in the record layer is achieved by using symmetric block encryption algorithms like @code{3DES}, @code{AES}@footnote{AES, or Advanced Encryption Standard, is actually the RIJNDAEL algorithm. This is the algorithm that replaced DES.}, or stream algorithms like @code{ARCFOUR_128}@footnote{@code{ARCFOUR_128} is a compatible algorithm with RSA's RC4 algorithm, which is considered to be a trade secret.}. Ciphers are encryption algorithms that use a single, secret, key to encrypt and decrypt data. Block algorithms in TLS also provide protection against statistical analysis of the data. Thus, if you're using the @acronym{TLS} protocol, a random number of blocks will be appended to data, to prevent eavesdroppers from guessing the actual data size. Supported cipher algorithms: @table @code @item 3DES_CBC @code{3DES_CBC} is the DES block cipher algorithm used with triple encryption (EDE). Has 64 bits block size and is used in CBC mode. @item ARCFOUR_128 ARCFOUR is a fast stream cipher. @item ARCFOUR_40 This is the ARCFOUR cipher that is fed with a 40 bit key, which is considered weak. @item AES_CBC AES or RIJNDAEL is the block cipher algorithm that replaces the old DES algorithm. Has 128 bits block size and is used in CBC mode. This is not officially supported in TLS. @end table Supported MAC algorithms: @table @code @item MAC_MD5 MD5 is a cryptographic hash algorithm designed by Ron Rivest. Outputs 128 bits of data. @item MAC_SHA SHA is a cryptographic hash algorithm designed by NSA. Outputs 160 bits of data. @end table @node Compression algorithms used in the record layer @subsection Compression Algorithms Used in the Record Layer @cindex Compression algorithms The TLS record layer also supports compression. The algorithms implemented in @acronym{GnuTLS} can be found in the table below. All the algorithms except for DEFLATE which is referenced in @xcite{RFC3749}, should be considered as @acronym{GnuTLS}' extensions@footnote{You should use @ref{gnutls_handshake_set_private_extensions} to enable private extensions.}, and should be advertised only when the peer is known to have a compliant client, to avoid interoperability problems. The included algorithms perform really good when text, or other compressible data are to be transfered, but offer nothing on already compressed data, such as compressed images, zipped archives etc. These compression algorithms, may be useful in high bandwidth TLS tunnels, and in cases where network usage has to be minimized. As a drawback, compression increases latency. The record layer compression in @acronym{GnuTLS} is implemented based on the proposal @xcite{RFC3749}. The supported compression algorithms are: @table @code @item DEFLATE Zlib compression, using the deflate algorithm. @item LZO LZO is a very fast compression algorithm. This algorithm is only available if the @acronym{GnuTLS-extra} library has been initialized and the private extensions are enabled. @end table @node Weaknesses and countermeasures @subsection Weaknesses and Countermeasures Some weaknesses that may affect the security of the Record layer have been found in @acronym{TLS} 1.0 protocol. These weaknesses can be exploited by active attackers, and exploit the facts that @enumerate @item @acronym{TLS} has separate alerts for ``decryption_failed'' and ``bad_record_mac'' @item The decryption failure reason can be detected by timing the response time. @item The IV for CBC encrypted packets is the last block of the previous encrypted packet. @end enumerate Those weaknesses were solved in @acronym{TLS} 1.1 @xcite{RFC4346} which is implemented in @acronym{GnuTLS}. For a detailed discussion see the archives of the TLS Working Group mailing list and the paper @xcite{CBCATT}. @node The TLS Alert Protocol @section The TLS Alert Protocol @anchor{The Alert Protocol} @cindex Alert protocol The Alert protocol is there to allow signals to be sent between peers. These signals are mostly used to inform the peer about the cause of a protocol failure. Some of these signals are used internally by the protocol and the application protocol does not have to cope with them (see @code{GNUTLS_A_CLOSE_NOTIFY}), and others refer to the application protocol solely (see @code{GNUTLS_A_USER_CANCELLED}). An alert signal includes a level indication which may be either fatal or warning. Fatal alerts always terminate the current connection, and prevent future renegotiations using the current session ID. The alert messages are protected by the record protocol, thus the information that is included does not leak. You must take extreme care for the alert information not to leak to a possible attacker, via public log files etc. @table @asis @item @ref{gnutls_alert_send}: To send an alert signal. @item @ref{gnutls_error_to_alert}: To map a gnutls error number to an alert signal. @item @ref{gnutls_alert_get}: Returns the last received alert. @item @ref{gnutls_alert_get_name}: Returns the name, in a character array, of the given alert. @end table @node The TLS Handshake Protocol @section The TLS Handshake Protocol @anchor{The Handshake Protocol} @cindex Handshake protocol The Handshake protocol is responsible for the ciphersuite negotiation, the initial key exchange, and the authentication of the two peers. This is fully controlled by the application layer, thus your program has to set up the required parameters. Available functions to control the handshake protocol include: @table @asis @item @ref{gnutls_cipher_set_priority}: To set the priority of bulk cipher algorithms. @item @ref{gnutls_mac_set_priority}: To set the priority of MAC algorithms. @item @ref{gnutls_kx_set_priority}: To set the priority of key exchange algorithms. @item @ref{gnutls_compression_set_priority}: To set the priority of compression methods. @item @ref{gnutls_certificate_type_set_priority}: To set the priority of certificate types (e.g., @acronym{OpenPGP}, @acronym{X.509}). @item @ref{gnutls_protocol_set_priority}: To set the priority of protocol versions (e.g., @acronym{SSL} 3.0, @acronym{TLS} 1.0). @item @ref{gnutls_set_default_priority}: To set some defaults in the current session. That way you don't have to call each priority function, independently, but you have to live with the defaults. @item @ref{gnutls_credentials_set}: To set the appropriate credentials structures. @item @ref{gnutls_certificate_server_set_request}: To set whether client certificate is required or not. @item @ref{gnutls_handshake}: To initiate the handshake. @end table Other functions include the @ref{gnutls_protocol_convert_priority}, @ref{gnutls_cipher_convert_priority}, etc., that allow converting priorities given in text format to the internal integer format. @subsection TLS Cipher Suites The Handshake Protocol of @acronym{TLS} negotiates cipher suites of the form @code{TLS_DHE_RSA_WITH_3DES_CBC_SHA}. The usual cipher suites contain these parameters: @itemize @item The key exchange algorithm. @code{DHE_RSA} in the example. @item The Symmetric encryption algorithm and mode @code{3DES_CBC} in this example. @item The MAC@footnote{MAC stands for Message Authentication Code. It can be described as a keyed hash algorithm. See RFC2104.} algorithm used for authentication. @code{MAC_SHA} is used in the above example. @end itemize The cipher suite negotiated in the handshake protocol will affect the Record Protocol, by enabling encryption and data authentication. Note that you should not over rely on @acronym{TLS} to negotiate the strongest available cipher suite. Do not enable ciphers and algorithms that you consider weak. The priority functions, dicussed above, allow the application layer to enable and set priorities on the individual ciphers. It may imply that all combinations of ciphersuites are allowed, but this is not true. For several reasons, not discussed here, some combinations were not defined in the @acronym{TLS} protocol. The supported ciphersuites are shown in @ref{ciphersuites}. @subsection Client Authentication @cindex Client Certificate authentication In the case of ciphersuites that use certificate authentication, the authentication of the client is optional in @acronym{TLS}. A server may request a certificate from the client --- using the @ref{gnutls_certificate_server_set_request} function. If a certificate is to be requested from the client during the handshake, the server will send a certificate request message that contains a list of acceptable certificate signers. In @acronym{GnuTLS} the certificate signers list is constructed using the trusted Certificate Authorities by the server. That is the ones set using @itemize @item @ref{gnutls_certificate_set_x509_trust_file} @item @ref{gnutls_certificate_set_x509_trust_mem} @end itemize Sending of the names of the CAs can be controlled using @ref{gnutls_certificate_send_x509_rdn_sequence}. The client, then, may send a certificate, signed by one of the server's acceptable signers. @subsection Resuming Sessions @anchor{resume} @cindex Resuming sessions The @ref{gnutls_handshake} function, is expensive since a lot of calculations are performed. In order to support many fast connections to the same server a client may use session resuming. @strong{Session resuming} is a feature of the @acronym{TLS} protocol which allows a client to connect to a server, after a successful handshake, without the expensive calculations. This is achieved by using the previously established keys. @acronym{GnuTLS} supports this feature, and the example (@pxref{ex:resume-client}) illustrates a typical use of it. Keep in mind that sessions are expired after some time, for security reasons, thus it may be normal for a server not to resume a session even if you requested that. Also note that you must enable, using the priority functions, at least the algorithms used in the last session. @subsection Resuming Internals The resuming capability, mostly in the server side, is one of the problems of a thread-safe TLS implementations. The problem is that all threads must share information in order to be able to resume sessions. The gnutls approach is, in case of a client, to leave all the burden of resuming to the client. I.e., copy and keep the necessary parameters. See the functions: @itemize @item @ref{gnutls_session_get_data} @item @ref{gnutls_session_get_id} @item @ref{gnutls_session_set_data} @end itemize The server side is different. A server has to specify some callback functions which store, retrieve and delete session data. These can be registered with: @itemize @item @ref{gnutls_db_set_remove_function} @item @ref{gnutls_db_set_store_function} @item @ref{gnutls_db_set_retrieve_function} @item @ref{gnutls_db_set_ptr} @end itemize It might also be useful to be able to check for expired sessions in order to remove them, and save space. The function @ref{gnutls_db_check_entry} is provided for that reason. @node TLS Extensions @section TLS Extensions @cindex TLS Extensions A number of extensions to the @acronym{TLS} protocol have been proposed mainly in @xcite{TLSEXT}. The extensions supported in @acronym{GnuTLS} are: @itemize @item Maximum fragment length negotiation @item Server name indication @end itemize and they will be discussed in the subsections that follow. @subsection Maximum Fragment Length Negotiation @cindex TLS Extensions @cindex Maximum fragment length This extension allows a @acronym{TLS} implementation to negotiate a smaller value for record packet maximum length. This extension may be useful to clients with constrained capabilities. See the @ref{gnutls_record_set_max_size} and the @ref{gnutls_record_get_max_size} functions. @subsection Server Name Indication @anchor{serverind} @cindex TLS Extensions @cindex Server name indication A common problem in @acronym{HTTPS} servers is the fact that the @acronym{TLS} protocol is not aware of the hostname that a client connects to, when the handshake procedure begins. For that reason the @acronym{TLS} server has no way to know which certificate to send. This extension solves that problem within the @acronym{TLS} protocol, and allows a client to send the HTTP hostname before the handshake begins within the first handshake packet. The functions @ref{gnutls_server_name_set} and @ref{gnutls_server_name_get} can be used to enable this extension, or to retrieve the name sent by a client. @node Selecting cryptographic key sizes @section Selecting Cryptographic Key Sizes @cindex key sizes In TLS, since a lot of algorithms are involved, it is not easy to set a consistent security level. For this reason this section will present some correspondance between key sizes of symmetric algorithms and public key algorithms based on the most conservative values of @xcite{SELKEY}. Those can be used to generate certificates with appropriate key sizes as well as parameters for Diffie Hellman and SRP authentication. @multitable @columnfractions .15 .20 .20 .20 @item Year @tab Symmetric key size @tab RSA key size, DH and SRP prime size @tab ECC key size @item 1982 @tab 56 @tab 417 @tab 105 @item 1988 @tab 61 @tab 566 @tab 114 @item 2002 @tab 72 @tab 1028 @tab 139 @item 2015 @tab 82 @tab 1613 @tab 173 @item 2028 @tab 92 @tab 2362 @tab 210 @item 2040 @tab 101 @tab 3214 @tab 244 @item 2050 @tab 109 @tab 4047 @tab 272 @end multitable The first column provides an estimation of the year until these parameters are considered safe and the rest of the columns list the parameters for the various algorithms. Note however that the values suggested here are nothing more than an educated guess that is valid today. There are no guarrantees that an algorithm will remain unbreakable or that these values will remain constant in time. There could be scientific breakthroughs that cannot be predicted or total failure of the current public key systems by quantum computers. On the other hand though the cryptosystems used in TLS are selected in a conservative way and such catastrophic breakthroughs or failures are believed to be unlikely. @node On SSL 2 and older protocols @section On SSL 2 and Older Protocols @cindex SSL 2 One of the initial decisions in the @acronym{GnuTLS} development was to implement the known security protocols for the transport layer. Initially @acronym{TLS} 1.0 was implemented since it was the latest at that time, and was considered to be the most advanced in security properties. Later the @acronym{SSL} 3.0 protocol was implemented since it is still the only protocol supported by several servers and there are no serious security vulnerabilities known. One question that may arise is why we didn't implement @acronym{SSL} 2.0 in the library. There are several reasons, most important being that it has serious security flaws, unacceptable for a modern security library. Other than that, this protocol is barely used by anyone these days since it has been deprecated since 1996. The security problems in @acronym{SSL} 2.0 include: @itemize @item Message integrity compromised. The @acronym{SSLv2} message authentication uses the MD5 function, and is insecure. @item Man-in-the-middle attack. There is no protection of the handshake in @acronym{SSLv2}, which permits a man-in-the-middle attack. @item Truncation attack. @acronym{SSLv2} relies on TCP FIN to close the session, so the attacker can forge a TCP FIN, and the peer cannot tell if it was a legitimate end of data or not. @item Weak message integrity for export ciphers. The cryptographic keys in @acronym{SSLv2} are used for both message authentication and encryption, so if weak encryption schemes are negotiated (say 40-bit keys) the message authentication code use the same weak key, which isn't necessary. @end itemize @cindex PCT Other protocols such as Microsoft's @acronym{PCT} 1 and @acronym{PCT} 2 were not implemented because they were also abandoned and deprecated by @acronym{SSL} 3.0 and later @acronym{TLS} 1.0. @node Authentication methods @chapter Authentication Methods The @acronym{TLS} protocol provides confidentiality and encryption, but also offers authentication, which is a prerequisite for a secure connection. The available authentication methods in @acronym{GnuTLS} are: @itemize @item Certificate authentication @item Anonymous authentication @item @acronym{SRP} authentication @item @acronym{PSK} authentication @end itemize @menu * Certificate authentication:: * Anonymous authentication:: * Authentication using SRP:: * Authentication using PSK:: * Authentication and credentials:: * Parameters stored in credentials:: @end menu @node Certificate authentication @section Certificate Authentication @subsection Authentication Using @acronym{X.509} Certificates @cindex @acronym{X.509} certificates @acronym{X.509} certificates contain the public parameters, of a public key algorithm, and an authority's signature, which proves the authenticity of the parameters. @xref{The X.509 trust model}, for more information on @acronym{X.509} protocols. @subsection Authentication Using @acronym{OpenPGP} Keys @cindex @acronym{OpenPGP} Keys @acronym{OpenPGP} keys also contain public parameters of a public key algorithm, and signatures from several other parties. Depending on whether a signer is trusted the key is considered trusted or not. @acronym{GnuTLS}'s @acronym{OpenPGP} authentication implementation is based on the @xcite{TLSPGP} proposal. @xref{The OpenPGP trust model}, for more information about the @acronym{OpenPGP} trust model. For a more detailed introduction to @acronym{OpenPGP} and @acronym{GnuPG} see @xcite{GPGH}. @subsection Using Certificate Authentication In @acronym{GnuTLS} both the @acronym{OpenPGP} and @acronym{X.509} certificates are part of the certificate authentication and thus are handled using a common API. When using certificates the server is required to have at least one certificate and private key pair. A client may or may not have such a pair. The certificate and key pair should be loaded, before any @acronym{TLS} session is initialized, in a certificate credentials structure. This should be done by using @ref{gnutls_certificate_set_x509_key_file} or @ref{gnutls_certificate_set_openpgp_key_file} depending on the certificate type. In the @acronym{X.509} case, the functions will also accept and use a certificate list that leads to a trusted authority. The certificate list must be ordered in such way that every certificate certifies the one before it. The trusted authority's certificate need not to be included, since the peer should possess it already. As an alternative, a callback may be used so the server or the client specify the certificate and the key at the handshake time. That callback can be set using the functions: @itemize @item @ref{gnutls_certificate_server_set_retrieve_function} @item @ref{gnutls_certificate_client_set_retrieve_function} @end itemize Certificate verification is possible by loading the trusted authorities into the credentials structure by using @ref{gnutls_certificate_set_x509_trust_file} or @ref{gnutls_certificate_set_openpgp_keyring_file} for openpgp keys. Note however that the peer's certificate is not automatically verified, you should call @ref{gnutls_certificate_verify_peers2}, after a successful handshake, to verify the signatures of the certificate. An alternative way, which reports a more detailed verification output, is to use @ref{gnutls_certificate_get_peers} to obtain the raw certificate of the peer and verify it using the functions discussed in @ref{The X.509 trust model}. In a handshake, the negotiated cipher suite depends on the certificate's parameters, so not all key exchange methods will be available with some certificates. @acronym{GnuTLS} will disable ciphersuites that are not compatible with the key, or the enabled authentication methods. For example keys marked as sign-only, will not be able to access the plain RSA ciphersuites, but only the @code{DHE_RSA} ones. It is recommended not to use RSA keys for both signing and encryption. If possible use the same key for the @code{DHE_RSA} and @code{RSA_EXPORT} ciphersuites, which use signing, and a different key for the plain RSA ciphersuites, which use encryption. All the key exchange methods shown below are available in certificate authentication. Note that the DHE key exchange methods are generally slower@footnote{It really depends on the group used. Primes with lesser bits are always faster, but also easier to break. Values less than 768 should not be used today} than plain RSA and require Diffie Hellman parameters to be generated and associated with a credentials structure, by the server. The @code{RSA-EXPORT} method also requires 512 bit RSA parameters, that should also be generated and associated with the credentials structure. See the functions: @itemize @item @ref{gnutls_dh_params_generate2} @item @ref{gnutls_certificate_set_dh_params} @item @ref{gnutls_rsa_params_generate2} @item @ref{gnutls_certificate_set_rsa_export_params} @end itemize Sometimes in order to avoid bottlenecks in programs it is usefull to store and read parameters from formats that can be generated by external programs such as @code{certtool}. This is possible with @acronym{GnuTLS} by using the following functions: @itemize @item @ref{gnutls_dh_params_import_pkcs3} @item @ref{gnutls_rsa_params_import_pkcs1} @item @ref{gnutls_dh_params_export_pkcs3} @item @ref{gnutls_rsa_params_export_pkcs1} @end itemize Key exchange algorithms for @acronym{OpenPGP} and @acronym{X.509} certificates: @table @code @item RSA: The RSA algorithm is used to encrypt a key and send it to the peer. The certificate must allow the key to be used for encryption. @item RSA_EXPORT: The RSA algorithm is used to encrypt a key and send it to the peer. In the EXPORT algorithm, the server signs temporary RSA parameters of 512 bits --- which are considered weak --- and sends them to the client. @item DHE_RSA: The RSA algorithm is used to sign Ephemeral Diffie Hellman parameters which are sent to the peer. The key in the certificate must allow the key to be used for signing. Note that key exchange algorithms which use Ephemeral Diffie Hellman parameters, offer perfect forward secrecy. That means that even if the private key used for signing is compromised, it cannot be used to reveal past session data. @item DHE_DSS: The DSS algorithm is used to sign Ephemeral Diffie Hellman parameters which are sent to the peer. The certificate must contain DSA parameters to use this key exchange algorithm. DSS stands for Digital Signature Standard. @end table @node Anonymous authentication @section Anonymous Authentication @cindex Anonymous authentication The anonymous key exchange performs encryption but there is no indication of the identity of the peer. This kind of authentication is vulnerable to a man in the middle attack, but this protocol can be used even if there is no prior communication and trusted parties with the peer, or when full anonymity is required. Unless really required, do not use anonymous authentication. Available key exchange methods are shown below. Note that the key exchange methods for anonymous authentication require Diffie Hellman parameters to be generated by the server and associated with an anonymous credentials structure. Supported anonymous key exchange algorithms: @table @code @item ANON_DH: This algorithm exchanges Diffie Hellman parameters. @end table @node Authentication using SRP @section Authentication using @acronym{SRP} @cindex @acronym{SRP} authentication Authentication via the Secure Remote Password protocol, @acronym{SRP}@footnote{@acronym{SRP} is described in @xcite{RFC2945}}, is supported. The @acronym{SRP} key exchange is an extension to the @acronym{TLS} protocol, and it is a password based authentication (unlike @acronym{X.509} or @acronym{OpenPGP} that use certificates). The two peers can be identified using a single password, or there can be combinations where the client is authenticated using @acronym{SRP} and the server using a certificate. The advantage of @acronym{SRP} authentication, over other proposed secure password authentication schemes, is that @acronym{SRP} does not require the server to hold the user's password. This kind of protection is similar to the one used traditionally in the @emph{UNIX} @file{/etc/passwd} file, where the contents of this file did not cause harm to the system security if they were revealed. The @acronym{SRP} needs instead of the plain password something called a verifier, which is calculated using the user's password, and if stolen cannot be used to impersonate the user. Check @xcite{TOMSRP} for a detailed description of the @acronym{SRP} protocol and the Stanford @acronym{SRP} libraries, which includes a PAM module that synchronizes the system's users passwords with the @acronym{SRP} password files. That way @acronym{SRP} authentication could be used for all the system's users. The implementation in @acronym{GnuTLS} is based on paper @xcite{TLSSRP}. The supported @acronym{SRP} key exchange methods are: @table @code @item SRP: Authentication using the @acronym{SRP} protocol. @item SRP_DSS: Client authentication using the @acronym{SRP} protocol. Server is authenticated using a certificate with DSA parameters. @item SRP_RSA: Client authentication using the @acronym{SRP} protocol. Server is authenticated using a certificate with RSA parameters. @end table If clients supporting @acronym{SRP} know the username and password before the connection, should initialize the client credentials and call the function @ref{gnutls_srp_set_client_credentials}. Alternatively they could specify a callback function by using the function @ref{gnutls_srp_set_client_credentials_function}. This has the advantage that allows probing the server for @acronym{SRP} support. In that case the callback function will be called twice per handshake. The first time is before the ciphersuite is negotiated, and if the callback returns a negative error code, the callback will be called again if @acronym{SRP} has been negotiated. This uses a special @acronym{TLS}-@acronym{SRP} handshake idiom in order to avoid, in interactive applications, to ask the user for @acronym{SRP} password and username if the server does not negotiate an @acronym{SRP} ciphersuite. In server side the default behaviour of @acronym{GnuTLS} is to read the usernames and @acronym{SRP} verifiers from password files. These password files are the ones used by the @emph{Stanford srp libraries} and can be specified using the @ref{gnutls_srp_set_server_credentials_file}. If a different password file format is to be used, then the function @ref{gnutls_srp_set_server_credentials_function}, should be called, in order to set an appropriate callback. Some helper functions such as @itemize @item @ref{gnutls_srp_verifier} @item @ref{gnutls_srp_base64_encode} @item @ref{gnutls_srp_base64_decode} @end itemize are included in @acronym{GnuTLS}, and can be used to generate and maintain @acronym{SRP} verifiers and password files. A program to manipulate the required parameters for @acronym{SRP} authentication is also included. @xref{srptool}, for more information. @node Authentication using PSK @section Authentication using @acronym{PSK} @cindex @acronym{PSK} authentication Authentication using Pre-shared keys is a method to authenticate using usernames and binary keys. This protocol avoids making use of public key infrastructure and expensive calculations, thus it is suitable for constraint clients. The implementation in @acronym{GnuTLS} is based on paper @xcite{TLSPSK}. The supported @acronym{PSK} key exchange methods are: @table @code @item PSK: Authentication using the @acronym{PSK} protocol. @item DHE-PSK: Authentication using the @acronym{PSK} protocol and Diffie Hellman key exchange. This method offers perfect forward secrecy. @end table Clients supporting @acronym{PSK} should supply the username and key before the connection to the client credentials by calling the function @ref{gnutls_psk_set_client_credentials}. Alternatively they could specify a callback function by using the function @ref{gnutls_psk_set_client_credentials_function}. This has the advantage that the callback will be called only if @acronym{PSK} has been negotiated. In server side the default behaviour of @acronym{GnuTLS} is to read the usernames and @acronym{PSK} keys from a password file. The password file should contain usernames and keys in hexadecimal format. The name of the password file can be stored to the credentials structure by calling @ref{gnutls_psk_set_server_credentials_file}. If a different password file format is to be used, then the function @ref{gnutls_psk_set_server_credentials_function}, should be used instead. Some helper functions such as: @itemize @item @ref{gnutls_hex_encode} @item @ref{gnutls_hex_decode} @end itemize are included in @acronym{GnuTLS}, and may be used to generate and maintain @acronym{PSK} keys. @node Authentication and credentials @section Authentication and Credentials In @acronym{GnuTLS} every key exchange method is associated with a credentials type. So in order to enable to enable a specific method, the corresponding credentials type should be initialized and set using @ref{gnutls_credentials_set}. A mapping is shown below. Key exchange algorithms and the corresponding credential types: @multitable @columnfractions .3 .3 .3 @headitem Key exchange @tab Client credentials @tab Server credentials @item @code{KX_RSA} @item @code{KX_DHE_RSA} @item @code{KX_DHE_DSS} @item @code{KX_RSA_EXPORT} @tab @code{CRD_CERTIFICATE} @tab @code{CRD_CERTIFICATE} @item @code{KX_SRP_RSA} @tab @code{CRD_SRP} @tab @code{CRD_SRP} @item @code{KX_SRP_DSS} @tab @tab @code{CRD_CERTIFICATE} @item @code{KX_SRP} @tab @code{CRD_SRP} @tab @code{CRD_SRP} @item @code{KX_ANON_DH} @tab @code{CRD_ANON} @tab @code{CRD_ANON} @item @code{KX_PSK} @tab @code{CRD_PSK} @tab @code{CRD_PSK} @end multitable @node Parameters stored in credentials @section Parameters Stored in Credentials Several parameters such as the ones used for Diffie-Hellman authentication are stored within the credentials structures, so all sessions can access them. Those parameters are stored in structures such as @code{gnutls_dh_params_t} and @code{gnutls_rsa_params_t}, and functions like @ref{gnutls_certificate_set_dh_params} and @ref{gnutls_certificate_set_rsa_export_params} can be used to associate those parameters with the given credentials structure. Since those parameters need to be renewed from time to time and a global structure such as the credentials, may not be easy to modify since it is accessible by all sessions, an alternative interface is available using a callback function. This can be set using the @ref{gnutls_certificate_set_params_function}. An example is shown below. @example #include gnutls_rsa_params_t rsa_params; gnutls_dh_params_t dh_params; /* This function will be called once a session requests DH * or RSA parameters. The parameters returned (if any) will * be used for the first handshake only. */ static int get_params( gnutls_session_t session, gnutls_params_type_t type, gnutls_params_st *st) @{ if (type == GNUTLS_PARAMS_RSA_EXPORT) st->params.rsa_export = rsa_params; else if (type == GNUTLS_PARAMS_DH) st->params.dh = dh_params; else return -1; st->type = type; /* do not deinitialize those parameters. */ st->deinit = 0; return 0; @} int main() @{ gnutls_certificate_credentials_t cert_cred; initialize_params(); /* ... */ gnutls_certificate_set_params_function( cert_cred, get_params); @} @end example @node More on certificate authentication @chapter More on Certificate Authentication @anchor{Certificate Authentication} @cindex Certificate authentication @menu * The X.509 trust model:: * The OpenPGP trust model:: * Digital signatures:: @end menu @node The X.509 trust model @section The @acronym{X.509} Trust Model @cindex @acronym{X.509} certificates The @acronym{X.509} protocols rely on a hierarchical trust model. In this trust model Certification Authorities (CAs) are used to certify entities. Usually more than one certification authorities exist, and certification authorities may certify other authorities to issue certificates as well, following a hierarchical model. @image{gnutls-x509,7cm,9.5cm} One needs to trust one or more CAs for his secure communications. In that case only the certificates issued by the trusted authorities are acceptable. See the figure above for a typical example. The API for handling @acronym{X.509} certificates is described at section @ref{sec:x509api}. Some examples are listed below. @menu * X.509 certificates:: * Verifying X.509 certificate paths:: * PKCS #10 certificate requests:: * PKCS #12 structures:: @end menu @node X.509 certificates @subsection @acronym{X.509} Certificates An @acronym{X.509} certificate usually contains information about the certificate holder, the signer, a unique serial number, expiration dates and some other fields @xcite{RFC3280} as shown in the table below. @table @code @item version: The field that indicates the version of the certificate. @item serialNumber: This field holds a unique serial number per certificate. @item issuer: Holds the issuer's distinguished name. @item validity: The activation and expiration dates. @item subject: The subject's distinguished name of the certificate. @item extensions: The extensions are fields only present in version 3 certificates. @end table The certificate's @emph{subject or issuer name} is not just a single string. It is a Distinguished name and in the @acronym{ASN.1} notation is a sequence of several object IDs with their corresponding values. Some of available OIDs to be used in an @acronym{X.509} distinguished name are defined in @file{gnutls/x509.h}. The @emph{Version} field in a certificate has values either 1 or 3 for version 3 certificates. Version 1 certificates do not support the extensions field so it is not possible to distinguish a CA from a person, thus their usage should be avoided. The @emph{validity} dates are there to indicate the date that the specific certificate was activated and the date the certificate's key would be considered invalid. Certificate @emph{extensions} are there to include information about the certificate's subject that did not fit in the typical certificate fields. Those may be e-mail addresses, flags that indicate whether the belongs to a CA etc. All the supported @acronym{X.509} version 3 extensions are shown in the table below. @table @code @item subject key id (2.5.29.14): An identifier of the key of the subject. @item authority key id (2.5.29.35): An identifier of the authority's key used to sign the certificate. @item subject alternative name (2.5.29.17): Alternative names to subject's distinguished name. @item key usage (2.5.29.15): Constraints the key's usage of the certificate. @item extended key usage (2.5.29.37): Constraints the purpose of the certificate. @item basic constraints (2.5.29.19): Indicates whether this is a CA certificate or not, and specify the maximum path lengths of certificate chains. @item CRL distribution points (2.5.29.31): This extension is set by the CA, in order to inform about the issued CRLs. @item Proxy Certification Information (1.3.6.1.5.5.7.1.14): Proxy Certificates includes this extension that contains the OID of the proxy policy language used, and can specify limits on the maximum lengths of proxy chains. Proxy Certificates are specified in @xcite{RFC3820}. @end table In @acronym{GnuTLS} the @acronym{X.509} certificate structures are handled using the @code{gnutls_x509_crt_t} type and the corresponding private keys with the @code{gnutls_x509_privkey_t} type. All the available functions for @acronym{X.509} certificate handling have their prototypes in @file{gnutls/x509.h}. An example program to demonstrate the @acronym{X.509} parsing capabilities can be found at section @ref{ex:x509-info}. @node Verifying X.509 certificate paths @subsection Verifying @acronym{X.509} Certificate Paths @cindex Verifying certificate paths Verifying certificate paths is important in @acronym{X.509} authentication. For this purpose the function @ref{gnutls_x509_crt_verify} is provided. The output of this function is the bitwise OR of the elements of the @code{gnutls_certificate_status_t} enumeration. A detailed description of these elements can be found in figure below. The function @ref{gnutls_certificate_verify_peers2} is equivalent to the previous one, and will verify the peer's certificate in a TLS session. @table @code @item CERT_INVALID: The certificate is not signed by one of the known authorities, or the signature is invalid. @item CERT_REVOKED: The certificate has been revoked by its CA. @item CERT_SIGNER_NOT_FOUND: The certificate's issuer is not known. This is the case when the issuer is not in the trusted certificates list. @item GNUTLS_CERT_SIGNER_NOT_CA: The certificate's signer was not a CA. This may happen if this was a version 1 certificate, which is common with some CAs, or a version 3 certificate without the basic constrains extension. @anchor{GNUTLS_CERT_INSECURE_ALGORITHM} @item GNUTLS_CERT_INSECURE_ALGORITHM: The certificate was signed using an insecure algorithm such as MD2 or MD5. These algorithms have been broken and should not be trusted. @end table There is also to possibility to pass some input to the verification functions in the form of flags. For @ref{gnutls_x509_crt_verify} the flags are passed straightforward, but @ref{gnutls_certificate_verify_peers2} depends on the flags set by calling @ref{gnutls_certificate_set_verify_flags}. All the available flags are part of the enumeration @ref{gnutls_certificate_verify_flags} and are explained in the table below. @anchor{gnutls_certificate_verify_flags} @tindex gnutls_certificate_verify_flags @table @code @item GNUTLS_VERIFY_DISABLE_CA_SIGN: If set a signer does not have to be a certificate authority. This flag should normaly be disabled, unless you know what this means. @item GNUTLS_VERIFY_ALLOW_X509_V1_CA_CRT: Allow only trusted CA certificates that have version 1. This is safer than GNUTLS_VERIFY_ALLOW_ANY_X509_V1_CA_CRT, and should be used instead. That way only signers in your trusted list will be allowed to have certificates of version 1. @item GNUTLS_VERIFY_ALLOW_ANY_X509_V1_CA_CRT: Allow CA certificates that have version 1 (both root and intermediate). This is dangerous since those haven't the basicConstraints extension. Must be used in combination with GNUTLS_VERIFY_ALLOW_X509_V1_CA_CRT. @item GNUTLS_VERIFY_DO_NOT_ALLOW_SAME: If a certificate is not signed by anyone trusted but exists in the trusted CA list do not treat it as trusted. @item GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD2: Allow certificates to be signed using the old MD2 algorithm. @item GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5: Allow certificates to be signed using the broken MD5 algorithm. @end table Although the verification of a certificate path indicates that the certificate is signed by trusted authority, does not reveal anything about the peer's identity. It is required to verify if the certificate's owner is the one you expect. For more information consult @xcite{RFC2818} and section @ref{ex:verify} for an example. @node PKCS #10 certificate requests @subsection @acronym{PKCS} #10 Certificate Requests @cindex Certificate requests @cindex @acronym{PKCS} #10 A certificate request is a structure, which contain information about an applicant of a certificate service. It usually contains a private key, a distinguished name and secondary data such as a challenge password. @acronym{GnuTLS} supports the requests defined in @acronym{PKCS} #10 @xcite{RFC2986}. Other certificate request's format such as PKIX's @xcite{RFC4211} are not currently supported. In @acronym{GnuTLS} the @acronym{PKCS} #10 structures are handled using the @code{gnutls_x509_crq_t} type. An example of a certificate request generation can be found at section @ref{ex:crq}. @node PKCS #12 structures @subsection @acronym{PKCS} #12 Structures @cindex @acronym{PKCS} #12 A @acronym{PKCS} #12 structure @xcite{PKCS12} usually contains a user's private keys and certificates. It is commonly used in browsers to export and import the user's identities. In @acronym{GnuTLS} the @acronym{PKCS} #12 structures are handled using the @code{gnutls_pkcs12_t} type. This is an abstract type that may hold several @code{gnutls_pkcs12_bag_t} types. The Bag types are the holders of the actual data, which may be certificates, private keys or encrypted data. An Bag of type encrypted should be decrypted in order for its data to be accessed. An example of a @acronym{PKCS} #12 structure generation can be found at section @ref{ex:pkcs12}. @node The OpenPGP trust model @section The @acronym{OpenPGP} Trust Model @cindex @acronym{OpenPGP} Keys The @acronym{OpenPGP} key authentication relies on a distributed trust model, called the ``web of trust''. The ``web of trust'' uses a decentralized system of trusted introducers, which are the same as a CA. @acronym{OpenPGP} allows anyone to sign anyone's else public key. When Alice signs Bob's key, she is introducing Bob's key to anyone who trusts Alice. If someone trusts Alice to introduce keys, then Alice is a trusted introducer in the mind of that observer. @image{gnutls-pgp,11cm,9cm} For example: If David trusts Alice to be an introducer, and Alice signed Bob's key, Dave also trusts Bob's key to be the real one. There are some key points that are important in that model. In the example Alice has to sign Bob's key, only if she is sure that the key belongs to Bob. Otherwise she may also make Dave falsely believe that this is Bob's key. Dave has also the responsibility to know who to trust. This model is similar to real life relations. Just see how Charlie behaves in the previous example. Although he has signed Bob's key - because he knows, somehow, that it belongs to Bob - he does not trust Bob to be an introducer. Charlie decided to trust only Kevin, for some reason. A reason could be that Bob is lazy enough, and signs other people's keys without being sure that they belong to the actual owner. @subsection @acronym{OpenPGP} Keys In @acronym{GnuTLS} the @acronym{OpenPGP} key structures @xcite{RFC2440} are handled using the @code{gnutls_openpgp_crt_t} type and the corresponding private keys with the @code{gnutls_openpgp_privkey_t} type. All the prototypes for the key handling functions can be found at @file{gnutls/openpgp.h}. @subsection Verifying an @acronym{OpenPGP} Key The verification functions of @acronym{OpenPGP} keys, included in @acronym{GnuTLS}, are simple ones, and do not use the features of the ``web of trust''. For that reason, if the verification needs are complex, the assistance of external tools like @acronym{GnuPG} and GPGME (@url{http://www.gnupg.org/related_software/gpgme/}) is recommended. There is one verification function in @acronym{GnuTLS}, the @ref{gnutls_openpgp_crt_verify_ring}. This checks an @acronym{OpenPGP} key against a given set of public keys (keyring) and returns the key status. The key verification status is the same as in @acronym{X.509} certificates, although the meaning and interpretation are different. For example an @acronym{OpenPGP} key may be valid, if the self signature is ok, even if no signers were found. The meaning of verification status is shown in the figure below. @table @code @item CERT_INVALID: A signature on the key is invalid. That means that the key was modified by somebody, or corrupted during transport. @item CERT_REVOKED: The key has been revoked by its owner. @item CERT_SIGNER_NOT_FOUND: The key was not signed by a known signer. @item GNUTLS_CERT_INSECURE_ALGORITHM: The certificate was signed using an insecure algorithm such as MD2 or MD5. These algorithms have been broken and should not be trusted. @end table @node Digital signatures @section Digital Signatures @cindex Digital signatures @include signatures.texi @node How to use TLS in application protocols @chapter How To Use @acronym{TLS} in Application Protocols This chapter is intended to provide some hints on how to use the @acronym{TLS} over simple custom made application protocols. The discussion below mainly refers to the @emph{TCP/IP} transport layer but may be extended to other ones too. @menu * Separate ports:: * Upward negotiation:: @end menu @node Separate ports @section Separate Ports Traditionally @acronym{SSL} was used in application protocols by assigning a new port number for the secure services. That way two separate ports were assigned, one for the non secure sessions, and one for the secured ones. This has the benefit that if a user requests a secure session then the client will try to connect to the secure port and fail otherwise. The only possible attack with this method is a denial of service one. The most famous example of this method is the famous ``HTTP over TLS'' or @acronym{HTTPS} protocol @xcite{RFC2818}. Despite its wide use, this method is not as good as it seems. This approach starts the @acronym{TLS} Handshake procedure just after the client connects on the ---so called--- secure port. That way the @acronym{TLS} protocol does not know anything about the client, and popular methods like the host advertising in HTTP do not work@footnote{See also the Server Name Indication extension on @ref{serverind}.}. There is no way for the client to say ``I connected to YYY server'' before the Handshake starts, so the server cannot possibly know which certificate to use. Other than that it requires two separate ports to run a single service, which is unnecessary complication. Due to the fact that there is a limitation on the available privileged ports, this approach was soon obsoleted. @node Upward negotiation @section Upward Negotiation Other application protocols@footnote{See LDAP, IMAP etc.} use a different approach to enable the secure layer. They use something called the ``TLS upgrade'' method. This method is quite tricky but it is more flexible. The idea is to extend the application protocol to have a ``STARTTLS'' request, whose purpose it to start the TLS protocols just after the client requests it. This is a really neat idea and does not require an extra port. This method is used by almost all modern protocols and there is even the @xcite{RFC2817} paper which proposes extensions to HTTP to support it. The tricky part, in this method, is that the ``STARTTLS'' request is sent in the clear, thus is vulnerable to modifications. A typical attack is to modify the messages in a way that the client is fooled and thinks that the server does not have the ``STARTTLS'' capability. See a typical conversation of a hypothetical protocol: @quotation (client connects to the server) CLIENT: HELLO I'M MR. XXX SERVER: NICE TO MEET YOU XXX CLIENT: PLEASE START TLS SERVER: OK *** TLS STARTS CLIENT: HERE ARE SOME CONFIDENTIAL DATA @end quotation And see an example of a conversation where someone is acting in between: @quotation (client connects to the server) CLIENT: HELLO I'M MR. XXX SERVER: NICE TO MEET YOU XXX CLIENT: PLEASE START TLS (here someone inserts this message) SERVER: SORRY I DON'T HAVE THIS CAPABILITY CLIENT: HERE ARE SOME CONFIDENTIAL DATA @end quotation As you can see above the client was fooled, and was dummy enough to send the confidential data in the clear. How to avoid the above attack? As you may have already thought this one is easy to avoid. The client has to ask the user before it connects whether the user requests @acronym{TLS} or not. If the user answered that he certainly wants the secure layer the last conversation should be: @quotation (client connects to the server) CLIENT: HELLO I'M MR. XXX SERVER: NICE TO MEET YOU XXX CLIENT: PLEASE START TLS (here someone inserts this message) SERVER: SORRY I DON'T HAVE THIS CAPABILITY CLIENT: BYE (the client notifies the user that the secure connection was not possible) @end quotation This method, if implemented properly, is far better than the traditional method, and the security properties remain the same, since only denial of service is possible. The benefit is that the server may request additional data before the @acronym{TLS} Handshake protocol starts, in order to send the correct certificate, use the correct password file@footnote{in @acronym{SRP} authentication}, or anything else! @node How to use GnuTLS in applications @chapter How To Use @acronym{GnuTLS} in Applications @anchor{examples} @cindex Example programs @menu * Preparation:: * Multi-threaded applications:: * Client examples:: * Server examples:: * Miscellaneous examples:: * Compatibility with the OpenSSL library:: * Opaque PRF Input TLS Extension:: @end menu @node Preparation @section Preparation To use @acronym{GnuTLS}, you have to perform some changes to your sources and your build system. The necessary changes are explained in the following subsections. @menu * Headers:: * Version check:: * Building the source:: @end menu @node Headers @subsection Headers All the data types and functions of the @acronym{GnuTLS} library are defined in the header file @file{gnutls/gnutls.h}. This must be included in all programs that make use of the @acronym{GnuTLS} library. The extra functionality of the @acronym{GnuTLS-extra} library is available by including the header file @file{gnutls/extra.h} in your programs. @node Version check @subsection Version Check It is often desirable to check that the version of `gnutls' used is indeed one which fits all requirements. Even with binary compatibility new features may have been introduced but due to problem with the dynamic linker an old version is actually used. So you may want to check that the version is okay right after program startup. See the function @ref{gnutls_check_version}. @node Building the source @subsection Building the Source If you want to compile a source file including the `gnutls/gnutls.h' header file, you must make sure that the compiler can find it in the directory hierarchy. This is accomplished by adding the path to the directory in which the header file is located to the compilers include file search path (via the -I option). However, the path to the include file is determined at the time the source is configured. To solve this problem, @acronym{GnuTLS} ships with two small helper programs @command{libgnutls-config} and @command{libgnutls-extra-config} that knows about the path to the include file and other configuration options. The options that need to be added to the compiler invocation at compile time are output by the @code{--cflags} option to @command{libgnutls-config}. The following example shows how it can be used at the command line: @example gcc -c foo.c `libgnutls-config --cflags` @end example Adding the output of @command{libgnutls-config --cflags} to the compilers command line will ensure that the compiler can find the @acronym{GnuTLS} header file. A similar problem occurs when linking the program with the library. Again, the compiler has to find the library files. For this to work, the path to the library files has to be added to the library search path (via the -L option). For this, the option @code{--libs} to @command{libgnutls-config} can be used. For convenience, this option also outputs all other options that are required to link the program with the @acronym{GnuTLS} libararies. The example shows how to link `foo.o' with the @acronym{GnuTLS} libraries to a program @emph{foo}. @example gcc -o foo foo.o `libgnutls-config --libs` @end example Of course you can also combine both examples to a single command by specifying both options to `libgnutls-config': @example gcc -o foo foo.c `libgnutls-config --cflags --libs` @end example @node Multi-threaded applications @section Multi-Threaded Applications Although the @acronym{GnuTLS} library is thread safe by design, some parts of the crypto backend, such as the random generator, are not. Since @emph{libgcrypt 1.1.92} there was an automatic detection of the thread library used by the application, so most applications wouldn't need to do any changes to ensure thread-safety. Due to the unportability of the automatic thread detection, this was removed from later releases of @emph{libgcrypt}, so applications have now to register callback functions to ensure proper locking in sensitive parts of @emph{libgcrypt}. There are helper macros to help you properly initialize the libraries. Examples are shown below. @itemize @item POSIX threads @example #include #include #include #include GCRY_THREAD_OPTION_PTHREAD_IMPL; int main() @{ /* The order matters. */ gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pthread); gnutls_global_init(); @} @end example @item GNU PTH threads @example #include #include #include #include GCRY_THREAD_OPTION_PTH_IMPL; int main() @{ gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pth); gnutls_global_init(); @} @end example @item Other thread packages @example /* The gcry_thread_cbs structure must have been * initialized. */ static struct gcry_thread_cbs gcry_threads_other = @{ ... @}; int main() @{ gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_other); @} @end example @end itemize @node Client examples @section Client Examples This section contains examples of @acronym{TLS} and @acronym{SSL} clients, using @acronym{GnuTLS}. Note that these examples contain little or no error checking. Some of the examples require functions implemented by another example. @menu * Simple client example with anonymous authentication:: * Simple client example with X.509 certificate support:: * Obtaining session information:: * Verifying peer's certificate:: * Using a callback to select the certificate to use:: * Client with Resume capability example:: * Simple client example with SRP authentication:: * Simple client example with TLS/IA support:: * Helper function for TCP connections:: @end menu @node Simple client example with anonymous authentication @subsection Simple Client Example with Anonymous Authentication The simplest client using TLS is the one that doesn't do any authentication. This means no external certificates or passwords are needed to set up the connection. As could be expected, the connection is vulnerable to man-in-the-middle (active or redirection) attacks. However, the data is integrity and privacy protected. @verbatiminclude examples/ex-client1.c @node Simple client example with X.509 certificate support @subsection Simple Client Example with @acronym{X.509} Certificate Support Let's assume now that we want to create a TCP client which communicates with servers that use @acronym{X.509} or @acronym{OpenPGP} certificate authentication. The following client is a very simple @acronym{TLS} client, it does not support session resuming, not even certificate verification. The TCP functions defined in this example are used in most of the other examples below, without redefining them. @verbatiminclude examples/ex-client2.c @node Obtaining session information @subsection Obtaining Session Information Most of the times it is desirable to know the security properties of the current established session. This includes the underlying ciphers and the protocols involved. That is the purpose of the following function. Note that this function will print meaningful values only if called after a successful @ref{gnutls_handshake}. @verbatiminclude examples/ex-session-info.c @node Verifying peer's certificate @subsection Verifying Peer's Certificate @anchor{ex:verify} A @acronym{TLS} session is not secure just after the handshake procedure has finished. It must be considered secure, only after the peer's certificate and identity have been verified. That is, you have to verify the signature in peer's certificate, the hostname in the certificate, and expiration dates. Just after this step you should treat the connection as being a secure one. @verbatiminclude examples/ex-rfc2818.c An other example is listed below which provides a more detailed verification output. @verbatiminclude examples/ex-verify.c @node Using a callback to select the certificate to use @subsection Using a Callback to Select the Certificate to Use There are cases where a client holds several certificate and key pairs, and may not want to load all of them in the credentials structure. The following example demonstrates the use of the certificate selection callback. @verbatiminclude examples/ex-cert-select.c @node Client with Resume capability example @subsection Client with Resume Capability Example @anchor{ex:resume-client} This is a modification of the simple client example. Here we demonstrate the use of session resumption. The client tries to connect once using @acronym{TLS}, close the connection and then try to establish a new connection using the previously negotiated data. @verbatiminclude examples/ex-client-resume.c @node Simple client example with SRP authentication @subsection Simple Client Example with @acronym{SRP} Authentication The following client is a very simple @acronym{SRP} @acronym{TLS} client which connects to a server and authenticates using a @emph{username} and a @emph{password}. The server may authenticate itself using a certificate, and in that case it has to be verified. @verbatiminclude examples/ex-client-srp.c @node Simple client example with TLS/IA support @subsection Simple Client Example with @acronym{TLS/IA} Support The following client is a simple client which uses the @acronym{TLS/IA} extension to authenticate with the server. @verbatiminclude examples/ex-client-tlsia.c @node Helper function for TCP connections @subsection Helper Function for TCP Connections This helper function abstracts away TCP connection handling from the other examples. It is required to build some examples. @verbatiminclude examples/tcp.c @node Server examples @section Server Examples This section contains examples of @acronym{TLS} and @acronym{SSL} servers, using @acronym{GnuTLS}. @menu * Echo Server with X.509 authentication:: * Echo Server with X.509 authentication II:: * Echo Server with OpenPGP authentication:: * Echo Server with SRP authentication:: * Echo Server with anonymous authentication:: @end menu @node Echo Server with X.509 authentication @subsection Echo Server with @acronym{X.509} Authentication This example is a very simple echo server which supports @acronym{X.509} authentication, using the RSA ciphersuites. @verbatiminclude examples/ex-serv1.c @node Echo Server with X.509 authentication II @subsection Echo Server with @acronym{X.509} Authentication II The following example is a server which supports @acronym{X.509} authentication. This server supports the export-grade cipher suites, the DHE ciphersuites and session resuming. @verbatiminclude examples/ex-serv-export.c @node Echo Server with OpenPGP authentication @subsection Echo Server with @acronym{OpenPGP} Authentication @cindex @acronym{OpenPGP} Server The following example is an echo server which supports @acronym{@acronym{OpenPGP}} key authentication. You can easily combine this functionality ---that is have a server that supports both @acronym{X.509} and @acronym{OpenPGP} certificates--- but we separated them to keep these examples as simple as possible. @verbatiminclude examples/ex-serv-pgp.c @node Echo Server with SRP authentication @subsection Echo Server with @acronym{SRP} Authentication This is a server which supports @acronym{SRP} authentication. It is also possible to combine this functionality with a certificate server. Here it is separate for simplicity. @verbatiminclude examples/ex-serv-srp.c @node Echo Server with anonymous authentication @subsection Echo Server with Anonymous Authentication This example server support anonymous authentication, and could be used to serve the example client for anonymous authentication. @verbatiminclude examples/ex-serv-anon.c @node Miscellaneous examples @section Miscellaneous Examples @menu * Checking for an alert:: * X.509 certificate parsing example:: * Certificate request generation:: * PKCS #12 structure generation:: @end menu @node Checking for an alert @subsection Checking for an Alert This is a function that checks if an alert has been received in the current session. @verbatiminclude examples/ex-alert.c @node X.509 certificate parsing example @subsection @acronym{X.509} Certificate Parsing Example @anchor{ex:x509-info} To demonstrate the @acronym{X.509} parsing capabilities an example program is listed below. That program reads the peer's certificate, and prints information about it. @verbatiminclude examples/ex-x509-info.c @node Certificate request generation @subsection Certificate Request Generation @anchor{ex:crq} The following example is about generating a certificate request, and a private key. A certificate request can be later be processed by a CA, which should return a signed certificate. @verbatiminclude examples/ex-crq.c @node PKCS #12 structure generation @subsection @acronym{PKCS} #12 Structure Generation @anchor{ex:pkcs12} The following example is about generating a @acronym{PKCS} #12 structure. @verbatiminclude examples/ex-pkcs12.c @node Compatibility with the OpenSSL library @section Compatibility with the OpenSSL Library @cindex OpenSSL To ease @acronym{GnuTLS}' integration with existing applications, a compatibility layer with the widely used OpenSSL library is included in the @code{gnutls-openssl} library. This compatibility layer is not complete and it is not intended to completely reimplement the OpenSSL API with @acronym{GnuTLS}. It only provides source-level compatibility. There is currently no attempt to make it binary-compatible with OpenSSL. The prototypes for the compatibility functions are in the @file{gnutls/openssl.h} header file. Current limitations imposed by the compatibility layer include: @itemize @item Error handling is not thread safe. @end itemize @node Opaque PRF Input TLS Extension @section Opaque PRF Input TLS Extension @cindex Opaque PRF Input GnuTLS supports the Opaque PRF Input TLS extension (@code{draft-rescorla-tls-opaque-prf-input-00.txt}). The API consists of one API for use in the client, @ref{gnutls_oprfi_enable_client}, and one API for use in the server, @ref{gnutls_oprfi_enable_server}. You must invoke both functions before calling @ref{gnutls_handshake}. The server utilizes a callback function into the application. The callback can look at the random string provided by the client, and also set the server string. The string lengths must be equal according to the protocol. @node Included programs @chapter Included Programs Included with @acronym{GnuTLS} are also a few command line tools that let you use the library for common tasks without writing an application. The applications are discussed in this chapter. @menu * Invoking srptool:: * Invoking gnutls-cli:: * Invoking gnutls-cli-debug:: * Invoking gnutls-serv:: * Invoking certtool:: @end menu @node Invoking srptool @section Invoking srptool @anchor{srptool} @cindex srptool The @file{srptool} is a very simple program that emulates the programs in the @emph{Stanford SRP libraries}. It is intended for use in places where you don't expect @acronym{SRP} authentication to be the used for system users. Traditionally @emph{libsrp} used two files. One called 'tpasswd' which holds usernames and verifiers, and 'tpasswd.conf' which holds generators and primes. How to use srptool: @itemize @item To create tpasswd.conf which holds the g and n values for @acronym{SRP} protocol (generator and a large prime), run: @example $ srptool --create-conf /etc/tpasswd.conf @end example @item This command will create /etc/tpasswd and will add user 'test' (you will also be prompted for a password). Verifiers are stored by default in the way libsrp expects. @example $ srptool --passwd /etc/tpasswd \ --passwd-conf /etc/tpasswd.conf -u test @end example @item This command will check against a password. If the password matches the one in /etc/tpasswd you will get an ok. @example $ srptool --passwd /etc/tpasswd \ --passwd-conf /etc/tpasswd.conf --verify -u test @end example @end itemize @node Invoking gnutls-cli @section Invoking gnutls-cli @cindex gnutls-cli Simple client program to set up a TLS connection to some other computer. It sets up a TLS connection and forwards data from the standard input to the secured socket and vice versa. @verbatim GNU TLS test client Usage: gnutls-cli [options] hostname -d, --debug integer Enable debugging -r, --resume Connect, establish a session. Connect again and resume this session. -s, --starttls Connect, establish a plain session and start TLS when EOF or a SIGALRM is received. --crlf Send CR LF instead of LF. --x509fmtder Use DER format for certificates to read from. -f, --fingerprint Send the openpgp fingerprint, instead of the key. --disable-extensions Disable all the TLS extensions. --print-cert Print the certificate in PEM format. -p, --port integer The port to connect to. --recordsize integer The maximum record size to advertize. -V, --verbose More verbose output. --ciphers cipher1 cipher2... Ciphers to enable. --protocols protocol1 protocol2... Protocols to enable. --comp comp1 comp2... Compression methods to enable. --macs mac1 mac2... MACs to enable. --kx kx1 kx2... Key exchange methods to enable. --ctypes certType1 certType2... Certificate types to enable. --x509cafile FILE Certificate file to use. --x509crlfile FILE CRL file to use. --pgpkeyfile FILE PGP Key file to use. --pgpkeyring FILE PGP Key ring file to use. --pgptrustdb FILE PGP trustdb file to use. --pgpcertfile FILE PGP Public Key (certificate) file to use. --x509keyfile FILE X.509 key file to use. --x509certfile FILE X.509 Certificate file to use. --srpusername NAME SRP username to use. --srppasswd PASSWD SRP password to use. --insecure Don't abort program if server certificate can't be validated. -l, --list Print a list of the supported algorithms and modes. -h, --help prints this help -v, --version prints the program's version number --copyright prints the program's license @end verbatim @node Invoking gnutls-cli-debug @section Invoking gnutls-cli-debug @cindex gnutls-cli-debug This program was created to assist in debugging @acronym{GnuTLS}, but it might be useful to extract a @acronym{TLS} server's capabilities. It's purpose is to connect onto a @acronym{TLS} server, perform some tests and print the server's capabilities. If called with the `-v' parameter a more checks will be performed. An example output is: @smallexample crystal:/cvs/gnutls/src$ ./gnutls-cli-debug localhost -p 5556 Resolving 'localhost'... Connecting to '127.0.0.1:5556'... Checking for TLS 1.1 support... yes Checking fallback from TLS 1.1 to... N/A Checking for TLS 1.0 support... yes Checking for SSL 3.0 support... yes Checking for version rollback bug in RSA PMS... no Checking for version rollback bug in Client Hello... no Checking whether we need to disable TLS 1.0... N/A Checking whether the server ignores the RSA PMS version... no Checking whether the server can accept Hello Extensions... yes Checking whether the server can accept cipher suites not in SSL 3.0 spec... yes Checking whether the server can accept a bogus TLS record version in the client hello... yes Checking for certificate information... N/A Checking for trusted CAs... N/A Checking whether the server understands TLS closure alerts... yes Checking whether the server supports session resumption... yes Checking for export-grade ciphersuite support... no Checking RSA-export ciphersuite info... N/A Checking for anonymous authentication support... no Checking anonymous Diffie Hellman group info... N/A Checking for ephemeral Diffie Hellman support... no Checking ephemeral Diffie Hellman group info... N/A Checking for AES cipher support (TLS extension)... yes Checking for 3DES cipher support... yes Checking for ARCFOUR 128 cipher support... yes Checking for ARCFOUR 40 cipher support... no Checking for MD5 MAC support... yes Checking for SHA1 MAC support... yes Checking for ZLIB compression support (TLS extension)... yes Checking for LZO compression support (GnuTLS extension)... yes Checking for max record size (TLS extension)... yes Checking for SRP authentication support (TLS extension)... yes Checking for OpenPGP authentication support (TLS extension)... no @end smallexample @node Invoking gnutls-serv @section Invoking gnutls-serv @cindex gnutls-serv Simple server program that listens to incoming TLS connections. @verbatim GNU TLS test server Usage: gnutls-serv [options] -d, --debug integer Enable debugging -g, --generate Generate Diffie Hellman Parameters. -p, --port integer The port to connect to. -q, --quiet Suppress some messages. --nodb Does not use the resume database. --http Act as an HTTP Server. --echo Act as an Echo Server. --dhparams FILE DH params file to use. --x509fmtder Use DER format for certificates --x509cafile FILE Certificate file to use. --x509crlfile FILE CRL file to use. --pgpkeyring FILE PGP Key ring file to use. --pgptrustdb FILE PGP trustdb file to use. --pgpkeyfile FILE PGP Key file to use. --pgpcertfile FILE PGP Public Key (certificate) file to use. --x509keyfile FILE X.509 key file to use. --x509certfile FILE X.509 Certificate file to use. --x509dsakeyfile FILE Alternative X.509 key file to use. --x509dsacertfile FILE Alternative X.509 certificate file to use. --srppasswd FILE SRP password file to use. --srppasswdconf FILE SRP password conf file to use. --ciphers cipher1 cipher2... Ciphers to enable. --protocols protocol1 protocol2... Protocols to enable. --comp comp1 comp2... Compression methods to enable. --macs mac1 mac2... MACs to enable. --kx kx1 kx2... Key exchange methods to enable. --ctypes certType1 certType2... Certificate types to enable. -l, --list Print a list of the supported algorithms and modes. -h, --help prints this help -v, --version prints the program's version number --copyright prints the program's license @end verbatim @subsection Setting Up a Test HTTPS Server @cindex HTTPS server @cindex debug server Running your own TLS server based on GnuTLS can be useful when debugging clients and/or GnuTLS itself. This section describes how to use @code{gnutls-serv} as a simple HTTPS server. The most basic server can be started as: @example gnutls-serv --http @end example It will only support anonymous ciphersuites, which many TLS clients refuse to use. The next step is to add support for X.509. First we generate a CA: @example certtool --generate-privkey > x509-ca-key.pem echo 'cn = GnuTLS test CA' > ca.tmpl echo 'ca' >> ca.tmpl echo 'cert_signing_key' >> ca.tmpl certtool --generate-self-signed --load-privkey x509-ca-key.pem \ --template ca.tmpl --outfile x509-ca.pem ... @end example Then generate a server certificate. Remember to change the dns_name value to the name of your server host, or skip that command to avoid the field. @example certtool --generate-privkey > x509-server-key.pem echo 'organization = GnuTLS test server' > server.tmpl echo 'cn = test.gnutls.org' >> server.tmpl echo 'tls_www_server' >> server.tmpl echo 'encryption_key' >> server.tmpl echo 'signing_key' >> server.tmpl echo 'dns_name = test.gnutls.org' >> server.tmpl certtool --generate-certificate --load-privkey x509-server-key.pem \ --load-ca-certificate x509-ca.pem --load-ca-privkey x509-ca-key.pem \ --template server.tmpl --outfile x509-server.pem ... @end example For use in the client, you may want to generate a client certificate as well. @example certtool --generate-privkey > x509-client-key.pem echo 'cn = GnuTLS test client' > client.tmpl echo 'tls_www_client' >> client.tmpl echo 'encryption_key' >> client.tmpl echo 'signing_key' >> client.tmpl certtool --generate-certificate --load-privkey x509-client-key.pem \ --load-ca-certificate x509-ca.pem --load-ca-privkey x509-ca-key.pem \ --template client.tmpl --outfile x509-client.pem ... @end example To be able to import the client key/certificate into some applications, you will need to convert them into a PKCS#12 structure. This also encrypts the security sensitive key with a password. @example certtool --to-p12 --load-privkey x509-client-key.pem --load-certificate x509-client.pem --outder --outfile x509-client.p12 @end example For icing, we'll create a proxy certificate for the client too. @example certtool --generate-privkey > x509-proxy-key.pem echo 'cn = GnuTLS test client proxy' > proxy.tmpl certtool --generate-proxy --load-privkey x509-proxy-key.pem \ --load-ca-certificate x509-client.pem --load-ca-privkey x509-client-key.pem \ --load-certificate x509-client.pem --template proxy.tmpl \ --outfile x509-proxy.pem ... @end example Then start the server again: @example gnutls-serv --http \ --x509cafile x509-ca.pem \ --x509keyfile x509-server-key.pem \ --x509certfile x509-server.pem @end example Try connecting to the server using your web browser. Note that the server listens to port 5556 by default. While you are at it, to allow connections using DSA, you can also create a DSA key and certificate for the server. These credentials will be used in the final example below. @example certtool --generate-privkey --dsa > x509-server-key-dsa.pem certtool --generate-certificate --load-privkey x509-server-key-dsa.pem \ --load-ca-certificate x509-ca.pem --load-ca-privkey x509-ca-key.pem \ --template server.tmpl --outfile x509-server-dsa.pem ... @end example The next step is to create OpenPGP credentials for the server. @example gpg --gen-key ...enter whatever details you want, use 'test.gnutls.org' as name... @end example Make a note of the OpenPGP key identifier of the newly generated key, here it was @code{5D1D14D8}. You will need to export the key for GnuTLS to be able to use it. @example gpg -a --export 5D1D14D8 > openpgp-server.txt gpg --export 5D1D14D8 > openpgp-server.bin gpg --export-secret-keys 5D1D14D8 > openpgp-server-key.bin gpg -a --export-secret-keys 5D1D14D8 > openpgp-server-key.txt @end example Let's start the server with support for OpenPGP credentials: @example gnutls-serv --http \ --pgpkeyfile openpgp-server-key.txt \ --pgpcertfile openpgp-server.txt @end example The next step is to add support for SRP authentication. @example srptool --create-conf srp-tpasswd.conf srptool --passwd-conf srp-tpasswd.conf --username jas --passwd srp-passwd.txt Enter password: [TYPE "foo"] @end example Start the server with SRP support: @example gnutls-serv --http \ --srppasswdconf srp-tpasswd.conf \ --srppasswd srp-passwd.txt @end example Let's also add support for PSK. @example $ psktool --passwd psk-passwd.txt @end example Start the server with PSK support: @example gnutls-serv --http \ --pskpasswd psk-passwd.txt @end example Finally, we start the server with all the earlier parameters and you get this command: @example gnutls-serv --http \ --x509cafile x509-ca.pem \ --x509keyfile x509-server-key.pem \ --x509certfile x509-server.pem \ --x509dsakeyfile x509-server-key-dsa.pem \ --x509dsacertfile x509-server-dsa.pem \ --pgpkeyfile openpgp-server-key.txt \ --pgpcertfile openpgp-server.txt \ --srppasswdconf srp-tpasswd.conf \ --srppasswd srp-passwd.txt \ --pskpasswd psk-passwd.txt @end example @node Invoking certtool @section Invoking certtool @cindex certtool This is a program to generate @acronym{X.509} certificates, certificate requests, CRLs and private keys. @verbatim Certtool help Usage: certtool [options] -s, --generate-self-signed Generate a self-signed certificate. -c, --generate-certificate Generate a signed certificate. --generate-proxy Generate a proxy certificate. --generate-crl Generate a CRL. -u, --update-certificate Update a signed certificate. -p, --generate-privkey Generate a private key. -q, --generate-request Generate a PKCS #10 certificate request. -e, --verify-chain Verify a PEM encoded certificate chain. The last certificate in the chain must be a self signed one. --verify-crl Verify a CRL. --generate-dh-params Generate PKCS #3 encoded Diffie Hellman parameters. --get-dh-params Get the included PKCS #3 encoded Diffie Hellman parameters. --load-privkey FILE Private key file to use. --load-request FILE Certificate request file to use. --load-certificate FILE Certificate file to use. --load-ca-privkey FILE Certificate authority's private key file to use. --load-ca-certificate FILE Certificate authority's certificate file to use. --password PASSWORD Password to use. -i, --certificate-info Print information on a certificate. -l, --crl-info Print information on a CRL. --p12-info Print information on a PKCS #12 structure. --p7-info Print information on a PKCS #7 structure. --smime-to-p7 Convert S/MIME to PKCS #7 structure. -k, --key-info Print information on a private key. --fix-key Regenerate the parameters in a private key. --to-p12 Generate a PKCS #12 structure. -8, --pkcs8 Use PKCS #8 format for private keys. --dsa Use DSA keys. --hash STR Hash algorithm to use for signing (MD5,SHA1,RMD160). --export-ciphers Use weak encryption algorithms. --inder Use DER format for input certificates and private keys. --outder Use DER format for output certificates and private keys. --bits BITS specify the number of bits for key generation. --outfile FILE Output file. --infile FILE Input file. --template FILE Template file to use for non interactive operation. -d, --debug LEVEL specify the debug level. Default is 1. -h, --help shows this help text -v, --version shows the program's version --copyright shows the program's license @end verbatim The program can be used interactively or non interactively by specifying the @code{--template} command line option. See below for an example of a template file. How to use certtool interactively: @itemize @item To generate parameters for Diffie Hellman key exchange, use the command: @example $ certtool --generate-dh-params --outfile dh.pem @end example @item To generate parameters for the RSA-EXPORT key exchange, use the command: @example $ certtool --generate-privkey --bits 512 --outfile rsa.pem @end example @end itemize @itemize @item To create a self signed certificate, use the command: @example $ certtool --generate-privkey --outfile ca-key.pem $ certtool --generate-self-signed --load-privkey ca-key.pem \ --outfile ca-cert.pem @end example Note that a self-signed certificate usually belongs to a certificate authority, that signs other certificates. @item To create a private key, run: @example $ certtool --generate-privkey --outfile key.pem @end example @item To create a certificate request, run: @example $ certtool --generate-request --load-privkey key.pem \ --outfile request.pem @end example @item To generate a certificate using the previous request, use the command: @example $ certtool --generate-certificate --load-request request.pem \ --outfile cert.pem \ --load-ca-certificate ca-cert.pem --load-ca-privkey ca-key.pem @end example @item To view the certificate information, use: @example $ certtool --certificate-info --infile cert.pem @end example @item To generate a @acronym{PKCS} #12 structure using the previous key and certificate, use the command: @example $ certtool --load-certificate cert.pem --load-privkey key.pem \ --to-p12 --outder --outfile key.p12 @end example @item Proxy certificate can be used to delegate your credential to a temporary, typically short-lived, certificate. To create one from the previously created certificate, first create a temporary key and then generate a proxy certificate for it, using the commands: @example $ certtool --generate-privkey > proxy-key.pem $ certtool --generate-proxy --load-ca-privkey key.pem \ --load-privkey proxy-key.pem --load-certificate cert.pem \ --outfile proxy-cert.pem @end example @end itemize Certtool's template file format: @itemize @item Firstly create a file named 'cert.cfg' that contains the information about the certificate. An example file is listed below. @item Then execute: @example $ certtool --generate-certificate cert.pem --load-privkey key.pem \ --template cert.cfg \ --load-ca-certificate ca-cert.pem --load-ca-privkey ca-key.pem @end example @end itemize An example certtool template file: @example # X.509 Certificate options # # DN options # The organization of the subject. organization = "Koko inc." # The organizational unit of the subject. unit = "sleeping dept." # The locality of the subject. # locality = # The state of the certificate owner. state = "Attiki" # The country of the subject. Two letter code. country = GR # The common name of the certificate owner. cn = "Cindy Lauper" # A user id of the certificate owner. #uid = "clauper" # If the supported DN OIDs are not adequate you can set # any OID here. # For example set the X.520 Title and the X.520 Pseudonym # by using OID and string pairs. #dn_oid = "2.5.4.12" "Dr." "2.5.4.65" "jackal" # This is deprecated and should not be used in new # certificates. # pkcs9_email = "none@@none.org" # The serial number of the certificate serial = 007 # In how many days, counting from today, this certificate will expire. expiration_days = 700 # X.509 v3 extensions # A dnsname in case of a WWW server. #dns_name = "www.none.org" # An IP address in case of a server. #ip_address = "192.168.1.1" # An email in case of a person email = "none@@none.org" # An URL that has CRLs (certificate revocation lists) # available. Needed in CA certificates. #crl_dist_points = "http://www.getcrl.crl/getcrl/" # Whether this is a CA certificate or not #ca # Whether this certificate will be used for a TLS client #tls_www_client # Whether this certificate will be used for a TLS server #tls_www_server # Whether this certificate will be used to sign data (needed # in TLS DHE ciphersuites). signing_key # Whether this certificate will be used to encrypt data (needed # in TLS RSA ciphersuites). Note that it is prefered to use different # keys for encryption and signing. #encryption_key # Whether this key will be used to sign other certificates. #cert_signing_key # Whether this key will be used to sign CRLs. #crl_signing_key # Whether this key will be used to sign code. #code_signing_key # Whether this key will be used to sign OCSP data. #ocsp_signing_key # Whether this key will be used for time stamping. #time_stamping_key @end example @node Function reference @chapter Function Reference @cindex Function reference @menu * Core functions:: * X.509 certificate functions:: * GnuTLS-extra functions:: * OpenPGP functions:: * TLS Inner Application (TLS/IA) functions:: * Error codes and descriptions:: @end menu @node Core functions @section Core Functions The prototypes for the following functions lie in @file{gnutls/gnutls.h}. @include gnutls-api.texi @node X.509 certificate functions @section @acronym{X.509} Certificate Functions @anchor{sec:x509api} @cindex @acronym{X.509} Functions The following functions are to be used for @acronym{X.509} certificate handling. Their prototypes lie in @file{gnutls/x509.h}. @include x509-api.texi @node GnuTLS-extra functions @section @acronym{GnuTLS-extra} Functions @cindex @acronym{GnuTLS-extra} functions These functions are only available in the GPL version of the library called @code{gnutls-extra}. The prototypes for this library lie in @file{gnutls/extra.h}. @include gnutls-extra-api.texi @node OpenPGP functions @section @acronym{OpenPGP} Functions @cindex @acronym{OpenPGP} functions @anchor{sec:openpgpapi} The following functions are to be used for @acronym{OpenPGP} certificate handling. Their prototypes lie in @file{gnutls/openpgp.h}. You need to link with @file{libgnutls-extra} to be able to use these functions (@pxref{GnuTLS-extra functions}). @include pgp-api.texi @node TLS Inner Application (TLS/IA) functions @section @acronym{TLS} Inner Application (@acronym{TLS/IA}) Functions @cindex @acronym{TLS} Inner Application (@acronym{TLS/IA}) functions @cindex Inner Application (@acronym{TLS/IA}) functions The following functions are used for @acronym{TLS} Inner Application (@acronym{TLS/IA}). Their prototypes lie in @file{gnutls/extra.h}. You need to link with @file{libgnutls-extra} to be able to use these functions (@pxref{GnuTLS-extra functions}). The typical control flow in an TLS/IA client (that would not require an Application Phase for resumed sessions) would be similar to the following: @example int client_avp (gnuls_session_t *session, void *ptr, const char *last, size_t lastlen, char **new, size_t *newlen) @{ ... @} ... int main () @{ gnutls_ia_client_credentials_t iacred; ... gnutls_init (&session, GNUTLS_CLIENT); ... /* Enable TLS/IA. */ gnutls_ia_allocate_client_credentials(&iacred); gnutls_ia_set_client_avp_function(iacred, client_avp); gnutls_credentials_set (session, GNUTLS_CRD_IA, iacred); ... ret = gnutls_handshake (session); // Error handling... ... if (gnutls_ia_handshake_p (session)) @{ ret = gnutls_ia_handshake (session); // Error handling... ... @end example See below for detailed descriptions of all the functions used above. The function @code{client_avp} would have to be implemented by your application. The function is responsible for handling the AVP data. See @code{gnutls_ia_set_client_avp_function} below for more information on how that function should be implemented. The control flow in a typical server is similar to the above, use @code{gnutls_ia_server_credentials_t} instead of @code{gnutls_ia_client_credentials_t}, and replace the call to the client functions with the corresponding server functions. @include ia-api.texi @node Error codes and descriptions @section Error Codes and Descriptions @anchor{Error Codes} @cindex Error codes The error codes used throughout the library are described below. The return code @code{GNUTLS_E_SUCCESS} indicate successful operation, and is guaranteed to have the value 0, so you can use it in logical expressions. @include error_codes.texi @node All the supported ciphersuites in GnuTLS @chapter All the Supported Ciphersuites in @acronym{GnuTLS} @anchor{ciphersuites} @cindex Ciphersuites @multitable @columnfractions .45 .20 .35 @item @code{TLS_RSA_NULL_MD5} @tab 0x00 0x01 @tab RFC 2246 @item @code{TLS_ANON_DH_3DES_EDE_CBC_SHA} @tab 0x00 0x1B @tab RFC 2246 @item @code{TLS_ANON_DH_ARCFOUR_MD5} @tab 0x00 0x18 @tab RFC 2246 @item @code{TLS_ANON_DH_AES_128_CBC_SHA} @tab 0x00 0x34 @tab RFC 2246 @item @code{TLS_ANON_DH_AES_256_CBC_SHA} @tab 0x00 0x3A @tab RFC 2246 @item @code{TLS_RSA_ARCFOUR_SHA} @tab 0x00 0x05 @tab RFC 2246 @item @code{TLS_RSA_ARCFOUR_MD5} @tab 0x00 0x04 @tab RFC 2246 @item @code{TLS_RSA_3DES_EDE_CBC_SHA} @tab 0x00 0x0A @tab RFC 2246 @item @code{TLS_RSA_EXPORT_ARCFOUR_40_MD5} @tab 0x00 0x03 @tab RFC 2246 @item @code{TLS_DHE_DSS_3DES_EDE_CBC_SHA} @tab 0x00 0x13 @tab RFC 2246 @item @code{TLS_DHE_RSA_3DES_EDE_CBC_SHA} @tab 0x00 0x16 @tab RFC 2246 @item @code{TLS_RSA_AES_128_CBC_SHA} @tab 0x00 0x2F @tab RFC 3268 @item @code{TLS_RSA_AES_256_CBC_SHA} @tab 0x00 0x35 @tab RFC 3268 @item @code{TLS_DHE_DSS_AES_256_CBC_SHA} @tab 0x00 0x38 @tab RFC 3268 @item @code{TLS_DHE_DSS_AES_128_CBC_SHA} @tab 0x00 0x32 @tab RFC 3268 @item @code{TLS_DHE_RSA_AES_256_CBC_SHA} @tab 0x00 0x39 @tab RFC 3268 @item @code{TLS_DHE_RSA_AES_128_CBC_SHA} @tab 0x00 0x33 @tab RFC 3268 @item @code{TLS_SRP_SHA_3DES_EDE_CBC_SHA} @tab 0x00 0x50 @tab draft-ietf-tls-srp @item @code{TLS_SRP_SHA_AES_128_CBC_SHA} @tab 0x00 0x53 @tab draft-ietf-tls-srp @item @code{TLS_SRP_SHA_AES_256_CBC_SHA} @tab 0x00 0x56 @tab draft-ietf-tls-srp @item @code{TLS_SRP_SHA_RSA_3DES_EDE_CBC_SHA} @tab 0x00 0x51 @tab draft-ietf-tls-srp @item @code{TLS_SRP_SHA_DSS_3DES_EDE_CBC_SHA} @tab 0x00 0x52 @tab draft-ietf-tls-srp @item @code{TLS_SRP_SHA_RSA_AES_128_CBC_SHA} @tab 0x00 0x54 @tab draft-ietf-tls-srp @item @code{TLS_SRP_SHA_DSS_AES_128_CBC_SHA} @tab 0x00 0x55 @tab draft-ietf-tls-srp @item @code{TLS_SRP_SHA_RSA_AES_256_CBC_SHA} @tab 0x00 0x57 @tab draft-ietf-tls-srp @item @code{TLS_SRP_SHA_DSS_AES_256_CBC_SHA} @tab 0x00 0x58 @tab draft-ietf-tls-srp @item @code{TLS_DHE_DSS_ARCFOUR_SHA} @tab 0x00 0x66 @tab draft-ietf-tls-56-bit-ciphersuites @item @code{TLS_PSK_ARCFOUR_SHA} @tab 0x00 0x8A @tab draft-ietf-tls-psk @item @code{TLS_PSK_3DES_EDE_CBC_SHA} @tab 0x00 0x8B @tab draft-ietf-tls-psk @item @code{TLS_PSK_AES_128_CBC_SHA} @tab 0x00 0x8C @tab draft-ietf-tls-psk @item @code{TLS_PSK_AES_256_CBC_SHA} @tab 0x00 0x8D @tab draft-ietf-tls-psk @end multitable @c @c Guile Bindings @c @include guile.texi @node Internal architecture of GnuTLS @chapter Internal Architecture of GnuTLS @cindex Internal architecture This chapter is to give a brief description of the way @acronym{GnuTLS} works. The focus is to give an idea to potential developers and those who want to know what happens inside the black box. @section The TLS Protocol The main needs for the TLS protocol to be used are shown in the image below. @image{gnutls-client-server-use-case,9cm} This is being accomplished by the following object diagram. Note that since @acronym{GnuTLS} is being developed in C object are just structures with attributes. The operations listed are functions that require the first parameter to be that object. @image{gnutls-objects,15cm} @section TLS Handshake Protocol The @acronym{GnuTLS} handshake protocol is implemented as a state machine that waits for input or returns immediately when the non-blocking transport layer functions are used. The main idea is shown in the following figure. @image{gnutls-handshake-state,9cm} Also the way the input is processed varies per ciphersuite. Several implementations of the internal handlers are available and @ref{gnutls_handshake} only multiplexes the input to the appropriate handler. For example a @acronym{PSK} ciphersuite has a different implementation of the @code{process_client_key_exchange} than a certificate ciphersuite. @image{gnutls-handshake-sequence,12cm} @section TLS Authentication Methods In @acronym{GnuTLS} authentication methods can be implemented quite easily. Since the required changes to add a new authentication method affect only the handshake protocol, a simple interface is used. An authentication method needs only to implement the functions as seen in the figure below. @image{gnutls-mod_auth_st,12cm} The functions that need to be implemented are the ones responsible for interpreting the handshake protocol messages. It is common for such functions to read data from one or more @code{credentials_t} structures@footnote{such as the @code{gnutls_certificate_credentials_t} structures} and write data, such as certificates, usernames etc. to @code{auth_info_t} structures. Simple examples of existing authentication methods can be seen in @code{auth_psk.c} for PSK ciphersuites and @code{auth_srp.c} for SRP ciphersuites. After implementing these functions the structure holding its pointers has to be registered in @code{gnutls_algorithms.c} in the @code{_gnutls_kx_algorithms} structure. @section TLS Extension Handling As with authentication methods, the TLS extensions handlers can be implemented using the following interface. @image{gnutls-extensions_st,12cm} Here there are two functions, one for receiving the extension data and one for sending. These functions have to check internally whether they operate in client or server side. A simple example of an extension handler can be seen in @code{ext_srp.c} After implementing these functions, together with the extension number they handle, they have to be registered in @code{gnutls_extensions.c} in the @code{_gnutls_extensions} structure. @subsection Adding a New TLS Extension Adding support for a new TLS extension is done from time to time, and the process to do so is not difficult. Here are the steps you need to follow if you wish to do this yourself. For sake of discussion, let's consider adding support for the hypothetical TLS extension @code{foobar}. @enumerate @item Modify @code{configure.in} to add @code{--enable-foobar} or @code{--disable-foobar}. Which to chose depends on whether you intend to make the extension be enabled by default. Look at existing checks (i.e., SRP, authz) for how to model the code. For example: @example AC_MSG_CHECKING([whether to disable foobar support]) AC_ARG_ENABLE(foobar, AS_HELP_STRING([--disable-foobar], [disable foobar support]), ac_enable_foobar=no) if test x$ac_enable_foobar != xno; then AC_MSG_RESULT(no) AC_DEFINE(ENABLE_FOOBAR, 1, [enable foobar]) else ac_full=0 AC_MSG_RESULT(yes) fi AM_CONDITIONAL(ENABLE_FOOBAR, test "$ac_enable_foobar" != "no") @end example @item Add IANA extension value to @code{extensions_t} in @code{gnutls_int.h}. A good name for the value would be GNUTLS_EXTENSION_FOOBAR. Check with @url{http://www.iana.org/assignments/tls-extensiontype-values} for allocated values. For experiments, you could pick a number but remember that some consider it a bad idea to deploy such modified version since it will lead to interoperability problems in the future when the IANA allocates that number to someone else, or when the foobar protocol is allocated another number. @item Add an entry to @code{_gnutls_extensions} in @code{gnutls_extensions.c}. A typical entry would be: @example #if ENABLE_FOOBAR GNUTLS_EXTENSION_ENTRY (GNUTLS_EXTENSION_FOOBAR, _gnutls_foobar_recv_params, _gnutls_foobar_send_params), #endif @end example The GNUTLS_EXTENSION_FOOBAR is the integer value you added to @code{gnutls_int.h} earlier. The two functions are new functions that you will need to implement, most likely you'll need to add an @code{#include "ext_foobar.h"} as well. @item Add new files @code{ext_foobar.c} and @code{ext_foobar.h} that implements the extension. The functions you are responsible to add are those mentioned in the previous step. As a starter, you could add this: @example int _gnutls_foobar_recv_params (gnutls_session_t session, const opaque * data, size_t data_size) @{ return 0; @} int _gnutls_foobar_send_params (gnutls_session_t session, opaque * data, size_t _data_size) @{ return 0; @} @end example The @code{_gnutls_foobar_recv_params} function is responsible for parsing incoming extension data (both in the client and server). The @code{_gnutls_foobar_send_params} function is responsible for sending extension data (both in the client and server). If you receive length fields that doesn't match, return @code{GNUTLS_E_UNEXPECTED_PACKET_LENGTH}. If you receive invalid data, return @code{GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER}. You can use other error codes too. Return 0 on success. The function typically store some information in the @code{session} variable for later usage. If you need to add new fields there, check @code{tls_ext_st} in @code{gnutls_int.h} and compare with existing TLS extension specific variables. Recall that both the client and server both send and receives parameters, and your code most likely will need to do different things depending on which mode it is in. It may be useful to make this distinction explicit in the code. Thus, for example, a better template than above would be: @example int _gnutls_foobar_recv_params (gnutls_session_t session, const opaque * data, size_t data_size) @{ if (session->security_parameters.entity == GNUTLS_CLIENT) return foobar_recv_client (session, data, data_size); else return foobar_recv_server (session, data, data_size); @} int _gnutls_foobar_send_params (gnutls_session_t session, opaque * data, size_t data_size) @{ if (session->security_parameters.entity == GNUTLS_CLIENT) return foobar_send_client (session, data, data_size); else return foobar_send_server (session, data, data_size); @} @end example The functions used would be declared as @code{static} functions, of the appropriate prototype, in the same file. When adding the files, you'll need to add them to @code{Makefile.am} as well, for example: @example if ENABLE_FOOBAR COBJECTS += ext_foobar.c HFILES += ext_foobar.h endif @end example @item Add API functions to enable/disable the extension. Normally the client will have one API to request use of the extension, and setting some extension specific data. The server will have one API to let the library know that it is willing to accept the extension, often this is implemented through a callback but it doesn't have to. The APIs need to be added to @code{includes/gnutls/gnutls.h} or @code{includes/gnutls/extra.h} as appropriate. It is recommended that if you don't have a requirement to use the LGPL license for your extension, that you place your work under the GPL license and thus in the libgnutls-extra library. You can implement the API function in the @code{ext_foobar.c} file, or if that file ends up becoming rather larger, add a @code{gnutls_foobar.c} file. @end enumerate @section Certificate Handling What is provided by the certificate handling functions is summarized in the following diagram. @image{gnutls-certificate-user-use-case,12cm} @node Copying Information @appendix Copying Information @menu * GNU Free Documentation License:: License for copying this manual. * GNU LGPL:: License for copying the core GnuTLS library. * GNU GPL:: License for copying GNUTLS extra and tools. @end menu @node GNU Free Documentation License @appendixsec GNU Free Documentation License @cindex FDL, GNU Free Documentation License @include fdl.texi @node GNU LGPL @appendixsec GNU Lesser General Public License @cindex LGPL, GNU Lesser General Public License @cindex License, GNU LGPL @include lgpl-2.1.texi @node GNU GPL @appendixsec GNU General Public License @cindex GPL, GNU General Public License @cindex License, GNU GPL @include gpl-2.0.texi @node Concept Index @unnumbered Concept Index @printindex cp @node Function and Data Index @unnumbered Function and Data Index @printindex fn @node Bibliography @unnumbered Bibliography @table @asis @item @anchor{SELKEY}[SELKEY] Arjen Lenstra and Eric Verheul, "Selecting Cryptographic Key Sizes", 2003, available from @url{http://www.win.tue.nl/~klenstra/key.pdf}. @item @anchor{CBCATT}[CBCATT] Bodo Moeller, "Security of CBC Ciphersuites in SSL/TLS: Problems and Countermeasures", 2002, available from @url{http://www.openssl.org/~bodo/tls-cbc.txt}. @item @anchor{GPGH}[GPGH] Mike Ashley, "The GNU Privacy Handbook", 2002, available from @url{http://www.gnupg.org/gph/en/manual.pdf}. @item @anchor{GUTPKI}[GUTPKI] Peter Gutmann, "Everything you never wanted to know about PKI but were forced to find out", Available from @url{http://www.cs.auckland.ac.nz/~pgut001/}. @item @anchor{RFC2246}[RFC2246] Tim Dierks and Christopher Allen, "The TLS Protocol Version 1.0", January 1999, Available from @url{http://kaizi.viagenie.qc.ca/ietf/rfc/rfc2246.txt}. @item @anchor{RFC4346}[RFC4346] Tim Dierks and Eric Rescorla, "The TLS Protocol Version 1.1", Match 2006, Available from @url{http://kaizi.viagenie.qc.ca/ietf/rfc/rfc4346.txt}. @item @anchor{RFC2440}[RFC2440] Jon Callas, Lutz Donnerhacke, Hal Finney and Rodney Thayer, "OpenPGP Message Format", November 1998, Available from @url{http://kaizi.viagenie.qc.ca/ietf/rfc/rfc2440.txt}. @item @anchor{RFC4211}[RFC4211] J. Schaad, "Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF)", September 2005, Available from @url{http://kaizi.viagenie.qc.ca/ietf/rfc/rfc4211.txt}. @item @anchor{RFC2817}[RFC2817] Rohit Khare and Scott Lawrence, "Upgrading to TLS Within HTTP/1.1", May 2000, Available from @url{http://kaizi.viagenie.qc.ca/ietf/rfc/rfc2817.txt} @item @anchor{RFC2818}[RFC2818] Eric Rescola, "HTTP Over TLS", May 2000, Available from @url{http://kaizi.viagenie.qc.ca/ietf/rfc/rfc2818.txt}. @item @anchor{RFC2945}[RFC2945] Tom Wu, "The SRP Authentication and Key Exchange System", September 2000, Available from @url{http://kaizi.viagenie.qc.ca/ietf/rfc/rfc2945.txt}. @item @anchor{RFC2986}[RFC2986] Magnus Nystrom and Burt Kaliski, "PKCS 10 v1.7: Certification Request Syntax Specification", November 2000, Available from @url{http://kaizi.viagenie.qc.ca/ietf/rfc/rfc2986.txt}. @item @anchor{RFC3280}[RFC3280] Russell Housley, Tim Polk, Warwick Ford and David Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", April 2002, Available from @url{http://kaizi.viagenie.qc.ca/ietf/rfc/rfc3280.txt}. @item @anchor{RFC3749}[RFC3749] Scott Hollenbeck, "Transport Layer Security Protocol Compression Methods", May 2004, Available from @url{http://kaizi.viagenie.qc.ca/ietf/rfc/rfc3749.txt}. @item @anchor{RFC3820}[RFC3820] Steven Tuecke, Von Welch, Doug Engert, Laura Pearlman, and Mary Thompson, "Internet X.509 Public Key Infrastructure (PKI) Proxy Certificate Profile", June 2004, available from @url{http://www.ietf.org/rfc3820}. @item @anchor{PKCS12}[PKCS12] RSA Laboratories, "PKCS 12 v1.0: Personal Information Exchange Syntax", June 1999, Available from @url{http://www.rsa.com}. @item @anchor{RESCOLA}[RESCOLA] Eric Rescola, "SSL and TLS: Designing and Building Secure Systems", 2001 @item @anchor{SSL3}[SSL3] Alan Freier, Philip Karlton and Paul Kocher, "The SSL Protocol Version 3.0", November 1996, Available from @url{http://wp.netscape.com/eng/ssl3/draft302.txt}. @item @anchor{STEVENS}[STEVENS] Richard Stevens, "UNIX Network Programming, Volume 1", Prentice Hall PTR, January 1998 @item @anchor{TLSEXT}[TLSEXT] Simon Blake-Wilson, Magnus Nystrom, David Hopwood, Jan Mikkelsen and Tim Wright, "Transport Layer Security (TLS) Extensions", June 2003, Available from @url{http://kaizi.viagenie.qc.ca/ietf/rfc/rfc3546.txt}. @item @anchor{TLSPGP}[TLSPGP] Nikos Mavrogiannopoulos, "Using OpenPGP keys for TLS authentication", April 2004, Internet draft, work in progress. Available from @url{http://www.normos.org/ietf/draft/draft-ietf-tls-openpgp-keys-05.txt}. @item @anchor{TLSSRP}[TLSSRP] David Taylor, Trevor Perrin, Tom Wu and Nikos Mavrogiannopoulos, "Using SRP for TLS Authentication", August 2005, Internet draft, work in progress. Available from @url{http://www.normos.org/ietf/draft/draft-ietf-tls-srp-08.txt}. @item @anchor{TLSPSK}[TLSPSK] Pasi Eronen and Hannes Tschofenig, "Pre-shared key Ciphersuites for TLS", December 2005, Available from @url{http://kaizi.viagenie.qc.ca/ietf/rfc/rfc4279.txt}. @item @anchor{TOMSRP}[TOMSRP] Tom Wu, "The Stanford SRP Authentication Project", Available at @url{http://srp.stanford.edu/}. @item @anchor{WEGER}[WEGER] Arjen Lenstra and Xiaoyun Wang and Benne de Weger, "Colliding X.509 Certificates", Cryptology ePrint Archive, Report 2005/067, Available at @url{http://eprint.iacr.org/}. @end table @bye