NAME

       SSL_CTX_set_generate_session_id, SSL_set_generate_session_id, SSL_has_matching_session_id
       - manipulate generation of SSL session IDs (server only)

SYNOPSIS

        #include <openssl/ssl.h>

        typedef int (*GEN_SESSION_CB)(const SSL *ssl, unsigned char *id,
                                      unsigned int *id_len);

        int SSL_CTX_set_generate_session_id(SSL_CTX *ctx, GEN_SESSION_CB cb);
        int SSL_set_generate_session_id(SSL *ssl, GEN_SESSION_CB, cb);
        int SSL_has_matching_session_id(const SSL *ssl, const unsigned char *id,
                                        unsigned int id_len);

DESCRIPTION

       SSL_CTX_set_generate_session_id() sets the callback function for generating new session
       ids for SSL/TLS sessions for ctx to be cb.

       SSL_set_generate_session_id() sets the callback function for generating new session ids
       for SSL/TLS sessions for ssl to be cb.

       SSL_has_matching_session_id() checks, whether a session with id id (of length id_len) is
       already contained in the internal session cache of the parent context of ssl.

NOTES

       When a new session is established between client and server, the server generates a
       session id. The session id is an arbitrary sequence of bytes.  The length of the session
       id is 16 bytes for SSLv2 sessions and between 1 and 32 bytes for SSLv3/TLSv1. The session
       id is not security critical but must be unique for the server. Additionally, the session
       id is transmitted in the clear when reusing the session so it must not contain sensitive
       information.

       Without a callback being set, an OpenSSL server will generate a unique session id from
       pseudo random numbers of the maximum possible length.  Using the callback function, the
       session id can be changed to contain additional information like e.g. a host id in order
       to improve load balancing or external caching techniques.

       The callback function receives a pointer to the memory location to put id into and a
       pointer to the maximum allowed length id_len. The buffer at location id is only guaranteed
       to have the size id_len.  The callback is only allowed to generate a shorter id and reduce
       id_len; the callback must never increase id_len or write to the location id exceeding the
       given limit.

       If a SSLv2 session id is generated and id_len is reduced, it will be restored after the
       callback has finished and the session id will be padded with 0x00. It is not recommended
       to change the id_len for SSLv2 sessions.  The callback can use the SSL_get_version(3)
       function to check, whether the session is of type SSLv2.

       The location id is filled with 0x00 before the callback is called, so the callback may
       only fill part of the possible length and leave id_len untouched while maintaining
       reproducibility.

       Since the sessions must be distinguished, session ids must be unique.  Without the
       callback a random number is used, so that the probability of generating the same session
       id is extremely small (2^128 possible ids for an SSLv2 session, 2^256 for SSLv3/TLSv1). In
       order to assure the uniqueness of the generated session id, the callback must call
       SSL_has_matching_session_id() and generate another id if a conflict occurs.  If an id
       conflict is not resolved, the handshake will fail.  If the application codes e.g. a unique
       host id, a unique process number, and a unique sequence number into the session id,
       uniqueness could easily be achieved without randomness added (it should however be taken
       care that no confidential information is leaked this way). If the application can not
       guarantee uniqueness, it is recommended to use the maximum id_len and fill in the bytes
       not used to code special information with random data to avoid collisions.

       SSL_has_matching_session_id() will only query the internal session cache, not the external
       one. Since the session id is generated before the handshake is completed, it is not
       immediately added to the cache. If another thread is using the same internal session
       cache, a race condition can occur in that another thread generates the same session id.
       Collisions can also occur when using an external session cache, since the external cache
       is not tested with SSL_has_matching_session_id() and the same race condition applies.

       When calling SSL_has_matching_session_id() for an SSLv2 session with reduced id_len, the
       match operation will be performed using the fixed length required and with a 0x00 padded
       id.

       The callback must return 0 if it cannot generate a session id for whatever reason and
       return 1 on success.

EXAMPLES

       The callback function listed will generate a session id with the server id given, and will
       fill the rest with pseudo random bytes:

        const char session_id_prefix = "www-18";

        #define MAX_SESSION_ID_ATTEMPTS 10
        static int generate_session_id(const SSL *ssl, unsigned char *id,
                                     unsigned int *id_len)
             {
             unsigned int count = 0;
             const char *version;

             version = SSL_get_version(ssl);
             if (!strcmp(version, "SSLv2"))
                 /* we must not change id_len */;

             do      {
                     RAND_pseudo_bytes(id, *id_len);
                     /* Prefix the session_id with the required prefix. NB: If our
                      * prefix is too long, clip it - but there will be worse effects
                      * anyway, eg. the server could only possibly create 1 session
                      * ID (ie. the prefix!) so all future session negotiations will
                      * fail due to conflicts. */
                     memcpy(id, session_id_prefix,
                             (strlen(session_id_prefix) < *id_len) ?
                             strlen(session_id_prefix) : *id_len);
                     }
             while(SSL_has_matching_session_id(ssl, id, *id_len) &&
                     (++count < MAX_SESSION_ID_ATTEMPTS));
             if(count >= MAX_SESSION_ID_ATTEMPTS)
                     return 0;
             return 1;
             }

RETURN VALUES

       SSL_CTX_set_generate_session_id() and SSL_set_generate_session_id() always return 1.

       SSL_has_matching_session_id() returns 1 if another session with the same id is already in
       the cache.

SEE ALSO

       ssl(3), SSL_get_version(3)

HISTORY

       SSL_CTX_set_generate_session_id(), SSL_set_generate_session_id() and
       SSL_has_matching_session_id() have been introduced in OpenSSL 0.9.7.