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SSL_CTX_set_tlsext_ticket_key_cb(3)                                    OpenSSL



NAME
       SSL_CTX_set_tlsext_ticket_key_cb - set a callback for session ticket
       processing

LIBRARY
       libcrypto, -lcrypto

SYNOPSIS
        #include <openssl/tls1.h>

        long SSL_CTX_set_tlsext_ticket_key_cb(SSL_CTX sslctx,
               int (*cb)(SSL *s, unsigned char key_name[16],
                         unsigned char iv[EVP_MAX_IV_LENGTH],
                         EVP_CIPHER_CTX *ctx, HMAC_CTX *hctx, int enc));

DESCRIPTION
       SSL_CTX_set_tlsext_ticket_key_cb() sets a callback fuction cb for
       handling session tickets for the ssl context sslctx. Session tickets,
       defined in RFC5077 provide an enhanced session resumption capability
       where the server implementation is not required to maintain per session
       state. It only applies to TLS and there is no SSLv3 implementation.

       The callback is available when the OpenSSL library was built without
       OPENSSL_NO_TLSEXT being defined.

       The callback function cb will be called for every client instigated TLS
       session when session ticket extension is presented in the TLS hello
       message. It is the responsibility of this function to create or
       retrieve the cryptographic parameters and to maintain their state.

       The OpenSSL library uses your callback function to help implement a
       common TLS  ticket construction state according to RFC5077 Section 4
       such that per session state is unnecessary and a small set of
       cryptographic variables needs to be maintained by the callback function
       implementation.

       In order to reuse a session, a TLS client must send the a session
       ticket extension to the server. The client can only send exactly one
       session ticket.  The server, through the callback function, either
       agrees to reuse the session ticket information or it starts a full TLS
       handshake to create a new session ticket.

       Before the callback function is started ctx and hctx have been
       initialised with EVP_CIPHER_CTX_init and HMAC_CTX_init respectively.

       For new sessions tickets, when the client doesn't present a session
       ticket, or an attempted retreival of the ticket failed, or a renew
       option was indicated, the callback function will be called with enc
       equal to 1. The OpenSSL library expects that the function will set an
       arbitary name, initialize iv, and set the cipher context ctx and the
       hash context hctx.

       The name is 16 characters long and is used as a key identifier.

       The iv length is the length of the IV of the corresponding cipher. The
       maximum IV length is EVP_MAX_IV_LENGTH bytes defined in evp.h.

       The initialization vector iv should be a random value. The cipher
       context ctx should use the initialisation vector iv. The cipher context
       can be set using EVP_EncryptInit_ex. The hmac context can be set using
       HMAC_Init_ex.

       When the client presents a session ticket, the callback function with
       be called with enc set to 0 indicating that the cb function should
       retreive a set of parameters. In this case name and iv have already
       been parsed out of the session ticket. The OpenSSL library expects that
       the name will be used to retrieve a cryptographic parameters and that
       the cryptographic context ctx will be set with the retreived parameters
       and the initialization vector iv. using a function like
       EVP_DecryptInit_ex. The hctx needs to be set using HMAC_Init_ex.

       If the name is still valid but a renewal of the ticket is required the
       callback function should return 2. The library will call the callback
       again with an arguement of enc equal to 1 to set the new ticket.

       The return value of the cb function is used by OpenSSL to determine
       what further processing will occur. The following return values have
       meaning:

       2   This indicates that the ctx and hctx have been set and the session
           can continue on those parameters. Additionally it indicates that
           the session ticket is in a renewal period and should be replaced.
           The OpenSSL library will call cb again with an enc argument of 1 to
           set the new ticket (see RFC5077 3.3 paragraph 2).

       1   This indicates that the ctx and hctx have been set and the session
           can continue on those parameters.

       0   This indicates that it was not possible to set/retrieve a session
           ticket and the SSL/TLS session will continue by by negiotationing a
           set of cryptographic parameters or using the alternate SSL/TLS
           resumption mechanism, session ids.

           If called with enc equal to 0 the library will call the cb again to
           get a new set of parameters.

       less than 0
           This indicates an error.

NOTES
       Session resumption shortcuts the TLS so that the client certificate
       negiotation don't occur. It makes up for this by storing client
       certificate an all other negotiated state information encrypted within
       the ticket. In a resumed session the applications will have all this
       state information available exactly as if a full negiotation had
       occured.

       If an attacker can obtain the key used to encrypt a session ticket,
       they can obtain the master secret for any ticket using that key and
       decrypt any traffic using that session: even if the ciphersuite
       supports forward secrecy. As a result applications may wish to use
       multiple keys and avoid using long term keys stored in files.

       Applications can use longer keys to maintain a consistent level of
       security.  For example if a ciphersuite uses 256 bit ciphers but only a
       128 bit ticket key the overall security is only 128 bits because
       breaking the ticket key will enable an attacker to obtain the session
       keys.

EXAMPLES
       Reference Implemention:
         SSL_CTX_set_tlsext_ticket_key_cb(SSL,ssl_tlsext_ticket_key_cb);
         ....

         static int ssl_tlsext_ticket_key_cb(SSL *s, unsigned char key_name[16], unsigned char *iv, EVP_CIPHER_CTX *ctx, HMAC_CTX *hctx, int enc)
         {
             if (enc) { /* create new session */
                 if (RAND_bytes(iv, EVP_MAX_IV_LENGTH) ) {
                     return -1; /* insufficient random */
                 }

                 key = currentkey(); /* something that you need to implement */
                 if ( !key ) {
                     /* current key doesn't exist or isn't valid */
                     key = createkey(); /* something that you need to implement.
                                          * createkey needs to initialise, a name,
                                          * an aes_key, a hmac_key and optionally
                                          * an expire time. */
                     if ( !key ) { /* key couldn't be created */
                         return 0;
                     }
                 }
                 memcpy(key_name, key->name, 16);

                 EVP_EncryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL, key->aes_key, iv);
                 HMAC_Init_ex(&hctx, key->hmac_key, 16, EVP_sha256(), NULL);

                 return 1;

             } else { /* retrieve session */
                 key = findkey(name);

                 if  (!key || key->expire < now() ) {
                     return 0;
                 }

                 HMAC_Init_ex(&hctx, key->hmac_key, 16, EVP_sha256(), NULL);
                 EVP_DecryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL, key->aes_key, iv );

                 if (key->expire < ( now() - RENEW_TIME ) ) {
                     /* return 2 - this session will get a new ticket even though the current is still valid */
                     return 2;
                 }
                 return 1;

             }
         }

RETURN VALUES
       returns 0 to indicate the callback function was set.

SEE ALSO
       ssl(3), SSL_set_session(3), SSL_session_reused(3),
       SSL_CTX_add_session(3), SSL_CTX_sess_number(3),
       SSL_CTX_sess_set_get_cb(3), SSL_CTX_set_session_id_context(3),

HISTORY
       This function was introduced in OpenSSL 0.9.8h



1.0.1i                            2014-08-10
                                            SSL_CTX_set_tlsext_ticket_key_cb(3)