HP Open Source Security for OpenVMS Volume 2: HP SSL for OpenVMS > SSL Application Programming Interface (API)
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 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:
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.