Manual Pages for UNIX Operating System command usage for man SSL_CTX_set_generate_session_id

OpenSSL                 SSL_CTX_set_generate_session_id(3openssl)



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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)

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      #include 

      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);


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

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

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

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     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 iiiidddd into and a pointer to the maximum allowed
     length iiiidddd_lllleeeennnn. The buffer at location iiiidddd is only guaranteed
     to have the size iiiidddd_lllleeeennnn.  The callback is only allowed to
     generate a shorter id and reduce iiiidddd_lllleeeennnn; the callback mmmmuuuusssstttt



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OpenSSL                 SSL_CTX_set_generate_session_id(3openssl)



     nnnneeeevvvveeeerrrr increase iiiidddd_lllleeeennnn or write to the location iiiidddd exceeding
     the given limit.

     If a SSLv2 session id is generated and iiiidddd_lllleeeennnn 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 iiiidddd_lllleeeennnn for SSLv2 sessions.  The callback can
     use the SSL_get_version(3) function to check, whether the
     session is of type SSLv2.

     The location iiiidddd is filled with 0x00 before the callback is
     called, so the callback may only fill part of the possible
     length and leave iiiidddd_lllleeeennnn 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 iiiidddd_lllleeeennnn 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 iiiidddd_lllleeeennnn, 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.




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OpenSSL                 SSL_CTX_set_generate_session_id(3openssl)



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     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;
           }


RRRREEEETTTTUUUURRRRNNNN VVVVAAAALLLLUUUUEEEESSSS
     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.

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     ssl(3), SSL_get_version(3)

HHHHIIIISSSSTTTTOOOORRRRYYYY
     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.



23/Feb/2001            Last change: 0.9.8o                      3






OpenSSL                 SSL_CTX_set_generate_session_id(3openssl)























































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