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

        #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);

       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.

       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.

       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;

       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.

       ssl(3), SSL_get_version(3)

       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.

0.9.8g                            2001-02SSL_CTX_set_generate_session_id(3SSL)