@node Introduction to TLS @chapter Introduction to @acronym{TLS} and @acronym{DTLS} @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 @xcite{RFC5246}. The protocol provides confidentiality, and authentication layers over any reliable transport layer. The description, above, refers to @acronym{TLS} 1.0 but applies to all other TLS versions as the differences between the protocols are not major. The @acronym{DTLS} protocol, or ``Datagram @acronym{TLS}'' @xcite{RFC4347} is a protocol with identical goals as @acronym{TLS}, but can operate under unreliable transport layers such as @acronym{UDP}. The discussions below apply to this protocol as well, except when noted otherwise. @menu * TLS layers:: * The transport layer:: * The TLS record protocol:: * The TLS Alert Protocol:: * The TLS Handshake Protocol:: * TLS Extensions:: * How to use TLS in application protocols:: * 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 @ref{fig-tls-layers}. @float Figure,fig-tls-layers @image{gnutls-layers,12cm} @caption{The TLS protocol layers.} @end float @node The transport layer @section The Transport Layer @cindex transport protocol @cindex transport layer @acronym{TLS} is not limited to any transport layer and can be used above any transport layer, as long as it is a reliable one. @acronym{DTLS} can be used over reliable and unreliable transport layers. @acronym{GnuTLS} supports TCP and UDP layers transparently using the Berkeley sockets API. However, any transport layer can be used by providing callbacks for @acronym{GnuTLS} to access the transport layer (for details see @ref{Setting up the transport layer}). @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 record layer functions can be called at any time after the handshake process is finished, when there is need to receive or send data. In @acronym{DTLS} however, due to re-transmission timers used in the handshake out-of-order handshake data might be received for some time (maximum 60 seconds) after the handshake process is finished. The functions to access the record protocol are limited to send and receive functions, which might, given the importance of this protocol in @acronym{TLS}, seem awkward. 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:: * On Record Padding:: @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} or stream algorithms like @code{ARCFOUR_128}. Ciphers are encryption algorithms that use a single, secret, key to encrypt and decrypt data. Block algorithms in CBC mode 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. The supported in @acronym{GnuTLS} ciphers and MAC algorithms are shown in @ref{tab:ciphers} and @ref{tab:macs}. @float Table,tab:ciphers @multitable @columnfractions .20 .70 @headitem Algorithm @tab Description @item AES_@-CBC @tab 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. @item AES_@-GCM @tab This is the AES algorithm in the authenticated encryption GCM mode. This mode combines message authentication and encryption and can be extremely fast on CPUs that support hardware acceleration. @item AES_@-CCM @tab This is the AES algorithm in the authenticated encryption CCM mode. This mode combines message authentication and encryption and is often used by systems without AES or GCM acceleration support. @item AES_@-CCM_@-8 @tab This is the AES algorithm in the authenticated encryption CCM mode with a truncated to 64-bit authentication tag. This mode is for communication with restricted systems. @item CAMELLIA_@-CBC @tab This is an 128-bit block cipher developed by Mitsubishi and NTT. It is one of the approved ciphers of the European NESSIE and Japanese CRYPTREC projects. @item CHACHA20_@-POLY1305 @tab CHACHA20-POLY1305 is an authenticated encryption algorithm based on CHACHA20 cipher and POLY1305 MAC. CHACHA20 is a refinement of SALSA20 algorithm, an approved cipher by the European ESTREAM project. POLY1305 is Wegman-Carter, one-time authenticator. The combination provides a fast stream cipher suitable for systems where a hardware AES accelerator is not available. @item 3DES_@-CBC @tab This 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 @tab ARCFOUR-128 is a compatible algorithm with RSA's RC4 algorithm, which is considered to be a trade secret. It is a fast cipher but considered weak today, and thus it is not enabled by default. @end multitable @caption{Supported ciphers in TLS.} @end float @float Table,tab:macs @multitable @columnfractions .20 .70 @headitem Algorithm @tab Description @item MAC_@-MD5 @tab This is an HMAC based on MD5 a cryptographic hash algorithm designed by Ron Rivest. Outputs 128 bits of data. @item MAC_@-SHA1 @tab An HMAC based on the SHA1 cryptographic hash algorithm designed by NSA. Outputs 160 bits of data. @item MAC_@-SHA256 @tab An HMAC based on SHA2-256. Outputs 256 bits of data. @item MAC_@-SHA384 @tab An HMAC based on SHA2-384. Outputs 384 bits of data. @item MAC_@-AEAD @tab This indicates that an authenticated encryption algorithm, such as GCM, is in use. @end multitable @caption{Supported MAC algorithms in TLS.} @end float @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. The included algorithms perform really good when text, or other compressible data are to be transferred, 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. It should be noted however that compression increases latency. The record layer compression in @acronym{GnuTLS} is implemented based on @xcite{RFC3749}. The supported algorithms are shown below. @showenumdesc{gnutls_compression_method_t,Supported compression algorithms} Note that compression enables attacks such as traffic analysis, or even plaintext recovery under certain circumstances. To avoid some of these attacks GnuTLS allows each record to be compressed independently (i.e., stateless compression), by using the "%STATELESS_COMPRESSION" priority string, in order to be used in cases where the attacker controlled data are pt in separate records. @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 this reason we suggest to always negotiate the highest supported TLS version with the peer@footnote{If this is not possible then please consult @ref{Interoperability}.}. For a detailed discussion of the issues see the archives of the TLS Working Group mailing list and @xcite{CBCATT}. @node On Record Padding @subsection On record padding @cindex record padding @cindex bad_record_mac The TLS protocol allows for extra padding of records in CBC ciphers, to prevent statistical analysis based on the length of exchanged messages (see @xcite{RFC5246} section 6.2.3.2). GnuTLS appears to be one of few implementations that take advantage of this feature: the user can provide some plaintext data with a range of lengths she wishes to hide, and GnuTLS adds extra padding to make sure the attacker cannot tell the real plaintext length is in a range smaller than the user-provided one. Use @funcref{gnutls_record_send_range} to send length-hidden messages and @funcref{gnutls_record_can_use_length_hiding} to check whether the current session supports length hiding. Using the standard @funcref{gnutls_record_send} will only add minimal padding. The TLS implementation in the Symbian operating system, frequently used by Nokia and Sony-Ericsson mobile phones, cannot handle non-minimal record padding. What happens when one of these clients handshake with a GnuTLS server is that the client will fail to compute the correct MAC for the record. The client sends a TLS alert (@code{bad_record_mac}) and disconnects. Typically this will result in error messages such as 'A TLS fatal alert has been received', 'Bad record MAC', or both, on the GnuTLS server side. If compatibility with such devices is a concern, not sending length-hidden messages solves the problem by using minimal padding. If you implement an application that has a configuration file, we recommend that you make it possible for users or administrators to specify a GnuTLS protocol priority string, which is used by your application via @funcref{gnutls_priority_set}. To allow the best flexibility, make it possible to have a different priority string for different incoming IP addresses. @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 (e.g. @code{GNUTLS_@-A_@-CLOSE_@-NOTIFY}), and others refer to the application protocol solely (e.g. @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 re-negotiations using the current session ID. All alert messages are summarized in the table below. 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. @include alerts.texi @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. The main handshake function is @funcref{gnutls_handshake}. In the next paragraphs we elaborate on the handshake protocol, i.e., the ciphersuite negotiation. @menu * TLS Cipher Suites:: TLS session parameters. * Authentication:: TLS authentication. * Client Authentication:: Requesting a certificate from the client. * Resuming Sessions:: Reusing previously established keys. @end menu @node TLS Cipher Suites @subsection TLS ciphersuites The handshake protocol of @acronym{TLS} negotiates cipher suites of a special form illustrated by the @code{TLS_DHE_RSA_WITH_3DES_CBC_SHA} cipher suite name. A typical cipher suite contains 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. All the supported ciphersuites are listed in @ref{ciphersuites}. @node Authentication @subsection Authentication The key exchange algorithms of the @acronym{TLS} protocol offer authentication, which is a prerequisite for a secure connection. The available authentication methods in @acronym{GnuTLS} follow. @itemize @item Certificate authentication: Authenticated key exchange using public key infrastructure and certificates (X.509 or OpenPGP). @item @acronym{SRP} authentication: Authenticated key exchange using a password. @item @acronym{PSK} authentication: Authenticated key exchange using a pre-shared key. @item Anonymous authentication: Key exchange without peer authentication. @end itemize @node Client Authentication @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 @funcref{gnutls_certificate_server_set_request} function. We elaborate in @ref{Certificate credentials}. @node Resuming Sessions @subsection Resuming sessions @anchor{resume} @cindex resuming sessions @cindex session resumption The TLS handshake process performs expensive calculations and a busy server might easily be put under load. To reduce the load, session resumption may be used. This 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 re-using the previously established keys, meaning the server needs to store the state of established connections (unless session tickets are used -- @ref{Session tickets}). Session resumption is an integral part of @acronym{GnuTLS}, and @ref{Session resumption}, @ref{ex-resume-client} illustrate typical uses of it. @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 discussed in the subsections that follow. @menu * Maximum fragment length negotiation:: * Server name indication:: * Session tickets:: * HeartBeat:: * Safe renegotiation:: * OCSP status request:: * SRTP:: * Application Layer Protocol Negotiation (ALPN):: * Extensions and Supplemental Data:: @end menu @node Maximum fragment length negotiation @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. The functions shown below can be used to control this extension. @showfuncB{gnutls_record_get_max_size,gnutls_record_set_max_size} @node Server name indication @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 @funcref{gnutls_server_name_set} and @funcref{gnutls_server_name_get} can be used to enable this extension, or to retrieve the name sent by a client. @showfuncB{gnutls_server_name_set,gnutls_server_name_get} @node Session tickets @subsection Session tickets @cindex TLS extensions @cindex session tickets @cindex tickets To resume a TLS session, the server normally stores session parameters. This complicates deployment, and can be avoided by delegating the storage to the client. Because session parameters are sensitive they are encrypted and authenticated with a key only known to the server and then sent to the client. The Session Tickets extension is described in RFC 5077 @xcite{TLSTKT}. A disadvantage of session tickets is that they eliminate the effects of forward secrecy when a server uses the same key for long time. That is, the secrecy of all sessions on a server using tickets depends on the ticket key being kept secret. For that reason server keys should be rotated and discarded regularly. Since version 3.1.3 GnuTLS clients transparently support session tickets, unless forward secrecy is explicitly requested (with the PFS priority string). @node HeartBeat @subsection HeartBeat @cindex TLS extensions @cindex heartbeat This is a TLS extension that allows to ping and receive confirmation from the peer, and is described in @xcite{RFC6520}. The extension is disabled by default and @funcref{gnutls_heartbeat_enable} can be used to enable it. A policy may be negotiated to only allow sending heartbeat messages or sending and receiving. The current session policy can be checked with @funcref{gnutls_heartbeat_allowed}. The requests coming from the peer result to @code{GNUTLS_@-E_@-HEARTBEAT_@-PING_@-RECEIVED} being returned from the receive function. Ping requests to peer can be send via @funcref{gnutls_heartbeat_ping}. @showfuncB{gnutls_heartbeat_allowed,gnutls_heartbeat_enable} @showfuncD{gnutls_heartbeat_ping,gnutls_heartbeat_pong,gnutls_heartbeat_set_timeouts,gnutls_heartbeat_get_timeout} @node Safe renegotiation @subsection Safe renegotiation @cindex renegotiation @cindex safe renegotiation TLS gives the option to two communicating parties to renegotiate and update their security parameters. One useful example of this feature was for a client to initially connect using anonymous negotiation to a server, and the renegotiate using some authenticated ciphersuite. This occurred to avoid having the client sending its credentials in the clear. However this renegotiation, as initially designed would not ensure that the party one is renegotiating is the same as the one in the initial negotiation. For example one server could forward all renegotiation traffic to an other server who will see this traffic as an initial negotiation attempt. This might be seen as a valid design decision, but it seems it was not widely known or understood, thus today some application protocols use the TLS renegotiation feature in a manner that enables a malicious server to insert content of his choice in the beginning of a TLS session. The most prominent vulnerability was with HTTPS. There servers request a renegotiation to enforce an anonymous user to use a certificate in order to access certain parts of a web site. The attack works by having the attacker simulate a client and connect to a server, with server-only authentication, and send some data intended to cause harm. The server will then require renegotiation from him in order to perform the request. When the proper client attempts to contact the server, the attacker hijacks that connection and forwards traffic to the initial server that requested renegotiation. The attacker will not be able to read the data exchanged between the client and the server. However, the server will (incorrectly) assume that the initial request sent by the attacker was sent by the now authenticated client. The result is a prefix plain-text injection attack. The above is just one example. Other vulnerabilities exists that do not rely on the TLS renegotiation to change the client's authenticated status (either TLS or application layer). While fixing these application protocols and implementations would be one natural reaction, an extension to TLS has been designed that cryptographically binds together any renegotiated handshakes with the initial negotiation. When the extension is used, the attack is detected and the session can be terminated. The extension is specified in @xcite{RFC5746}. GnuTLS supports the safe renegotiation extension. The default behavior is as follows. Clients will attempt to negotiate the safe renegotiation extension when talking to servers. Servers will accept the extension when presented by clients. Clients and servers will permit an initial handshake to complete even when the other side does not support the safe renegotiation extension. Clients and servers will refuse renegotiation attempts when the extension has not been negotiated. Note that permitting clients to connect to servers when the safe renegotiation extension is not enabled, is open up for attacks. Changing this default behavior would prevent interoperability against the majority of deployed servers out there. We will reconsider this default behavior in the future when more servers have been upgraded. Note that it is easy to configure clients to always require the safe renegotiation extension from servers. To modify the default behavior, we have introduced some new priority strings (see @ref{Priority Strings}). The @code{%UNSAFE_RENEGOTIATION} priority string permits (re-)handshakes even when the safe renegotiation extension was not negotiated. The default behavior is @code{%PARTIAL_RENEGOTIATION} that will prevent renegotiation with clients and servers not supporting the extension. This is secure for servers but leaves clients vulnerable to some attacks, but this is a trade-off between security and compatibility with old servers. The @code{%SAFE_RENEGOTIATION} priority string makes clients and servers require the extension for every handshake. The latter is the most secure option for clients, at the cost of not being able to connect to legacy servers. Servers will also deny clients that do not support the extension from connecting. It is possible to disable use of the extension completely, in both clients and servers, by using the @code{%DISABLE_SAFE_RENEGOTIATION} priority string however we strongly recommend you to only do this for debugging and test purposes. The default values if the flags above are not specified are: @table @code @item Server: %PARTIAL_RENEGOTIATION @item Client: %PARTIAL_RENEGOTIATION @end table For applications we have introduced a new API related to safe renegotiation. The @funcref{gnutls_safe_renegotiation_status} function is used to check if the extension has been negotiated on a session, and can be used both by clients and servers. @node OCSP status request @subsection OCSP status request @cindex OCSP status request @cindex Certificate status request The Online Certificate Status Protocol (OCSP) is a protocol that allows the client to verify the server certificate for revocation without messing with certificate revocation lists. Its drawback is that it requires the client to connect to the server's CA OCSP server and request the status of the certificate. This extension however, enables a TLS server to include its CA OCSP server response in the handshake. That is an HTTPS server may periodically run @code{ocsptool} (see @ref{ocsptool Invocation}) to obtain its certificate revocation status and serve it to the clients. That way a client avoids an additional connection to the OCSP server. @showfuncD{gnutls_certificate_set_ocsp_status_request_function,gnutls_certificate_set_ocsp_status_request_file,gnutls_ocsp_status_request_enable_client,gnutls_ocsp_status_request_is_checked} A server is required to provide the OCSP server's response using the @funcref{gnutls_certificate_set_ocsp_status_request_file}. The response may be obtained periodically using the following command. @example ocsptool --ask --load-cert server_cert.pem --load-issuer the_issuer.pem --load-signer the_issuer.pem --outfile ocsp.response @end example Since version 3.1.3 GnuTLS clients transparently support the certificate status request. @node SRTP @subsection SRTP @cindex SRTP @cindex Secure RTP The TLS protocol was extended in @xcite{RFC5764} to provide keying material to the Secure RTP (SRTP) protocol. The SRTP protocol provides an encapsulation of encrypted data that is optimized for voice data. With the SRTP TLS extension two peers can negotiate keys using TLS or DTLS and obtain keying material for use with SRTP. The available SRTP profiles are listed below. @showenumdesc{gnutls_srtp_profile_t,Supported SRTP profiles} To enable use the following functions. @showfuncB{gnutls_srtp_set_profile,gnutls_srtp_set_profile_direct} To obtain the negotiated keys use the function below. @showfuncdesc{gnutls_srtp_get_keys} Other helper functions are listed below. @showfuncC{gnutls_srtp_get_selected_profile,gnutls_srtp_get_profile_name,gnutls_srtp_get_profile_id} @node Application Layer Protocol Negotiation (ALPN) @subsection Application Layer Protocol Negotiation (ALPN) @cindex ALPN @cindex Application Layer Protocol Negotiation The TLS protocol was extended in @code{RFC7301} to provide the application layer a method of negotiating the application protocol version. This allows for negotiation of the application protocol during the TLS handshake, thus reducing round-trips. The application protocol is described by an opaque string. To enable, use the following functions. @showfuncB{gnutls_alpn_set_protocols,gnutls_alpn_get_selected_protocol} Note that these functions are intended to be used with protocols that are registered in the Application Layer Protocol Negotiation IANA registry. While you can use them for other protocols (at the risk of collisions), it is preferable to register them. @node Extensions and Supplemental Data @subsection Extensions and Supplemental Data @cindex Supplemental data It is possible to transfer supplemental data during the TLS handshake, following @xcite{RFC4680}. This is for "custom" protocol modifications for applications which may want to transfer additional data (e.g. additional authentication messages). Such an exchange requires a custom extension to be registered. The provided API for this functionality is low-level and described in @ref{TLS Extension Handling}. @include sec-tls-app.texi @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 uses 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.