#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);
_S_S_L___s_e_t___g_e_n_e_r_a_t_e___s_e_s_s_i_o_n___i_d_(_) sets the callback function for generating new session ids for SSL/TLS sessions for ssssll to be ccbb.
_S_S_L___h_a_s___m_a_t_c_h_i_n_g___s_e_s_s_i_o_n___i_d_(_) checks, whether a session with id iidd (of length iidd__lleenn) is already contained in the internal session cache of the parent context of ssssll.
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 iidd into and a pointer to the maximum allowed length iidd__lleenn. The buffer at location iidd is only guaranteed to have the size iidd__lleenn. The callback is only allowed to generate a shorter id and reduce iidd__lleenn; the callback mmuusstt nneevveerr increase iidd__lleenn or write to the location iidd exceeding the given limit.
If a SSLv2 session id is generated and iidd__lleenn 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 iidd__lleenn for SSLv2 sessions. The callback can use the _S_S_L___g_e_t___v_e_r_s_i_o_n(3) function to check, whether the session is of type SSLv2.
The location iidd is filled with 0x00 before the callback is called, so the callback may only fill part of the possible length and leave iidd__lleenn 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 _S_S_L___h_a_s___m_a_t_c_h_i_n_g___s_e_s_s_i_o_n___i_d_(_) 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 iidd__lleenn and fill in the bytes not used to code special information with random data to avoid collisions.
_S_S_L___h_a_s___m_a_t_c_h_i_n_g___s_e_s_s_i_o_n___i_d_(_) 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 _S_S_L___h_a_s___m_a_t_c_h_i_n_g___s_e_s_s_i_o_n___i_d_(_) and the same race condition applies.
When calling _S_S_L___h_a_s___m_a_t_c_h_i_n_g___s_e_s_s_i_o_n___i_d_(_) for an SSLv2 session with reduced iidd__lleenn, 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.
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; }
_S_S_L___h_a_s___m_a_t_c_h_i_n_g___s_e_s_s_i_o_n___i_d_(_) returns 1 if another session with the same id is already in the cache.