293 lines
10 KiB
Groff
293 lines
10 KiB
Groff
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.\" $NetBSD: SSL_CTX_set_generate_session_id.3,v 1.1 2003/07/24 14:16:44 itojun Exp $
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.\"
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.\" Thu Jul 24 13:08:12 2003
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.rm #[ #] #H #V #F C
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.\" ======================================================================
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.\"
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.IX Title "SSL_CTX_set_generate_session_id 3"
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.TH SSL_CTX_set_generate_session_id 3 "0.9.7b" "2001-02-24" "OpenSSL"
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.UC
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.SH "NAME"
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SSL_CTX_set_generate_session_id, SSL_set_generate_session_id, SSL_has_matching_session_id \- manipulate generation of \s-1SSL\s0 session IDs (server only)
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.SH "LIBRARY"
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libcrypto, -lcrypto
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/ssl.h>
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.Ve
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.Vb 2
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\& typedef int (*GEN_SESSION_CB)(const SSL *ssl, unsigned char *id,
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\& unsigned int *id_len);
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.Ve
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.Vb 4
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\& int SSL_CTX_set_generate_session_id(SSL_CTX *ctx, GEN_SESSION_CB cb);
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\& int SSL_set_generate_session_id(SSL *ssl, GEN_SESSION_CB, cb);
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\& int SSL_has_matching_session_id(const SSL *ssl, const unsigned char *id,
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\& unsigned int id_len);
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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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\&\fISSL_CTX_set_generate_session_id()\fR sets the callback function for generating
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new session ids for \s-1SSL/TLS\s0 sessions for \fBctx\fR to be \fBcb\fR.
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.PP
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\&\fISSL_set_generate_session_id()\fR sets the callback function for generating
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new session ids for \s-1SSL/TLS\s0 sessions for \fBssl\fR to be \fBcb\fR.
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.PP
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\&\fISSL_has_matching_session_id()\fR checks, whether a session with id \fBid\fR
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(of length \fBid_len\fR) is already contained in the internal session cache
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of the parent context of \fBssl\fR.
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.SH "NOTES"
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.IX Header "NOTES"
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When a new session is established between client and server, the server
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generates a session id. The session id is an arbitrary sequence of bytes.
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The length of the session id is 16 bytes for SSLv2 sessions and between
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1 and 32 bytes for SSLv3/TLSv1. The session id is not security critical
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but must be unique for the server. Additionally, the session id is
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transmitted in the clear when reusing the session so it must not contain
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sensitive information.
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.PP
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Without a callback being set, an OpenSSL server will generate a unique
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session id from pseudo random numbers of the maximum possible length.
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Using the callback function, the session id can be changed to contain
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additional information like e.g. a host id in order to improve load balancing
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or external caching techniques.
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.PP
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The callback function receives a pointer to the memory location to put
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\&\fBid\fR into and a pointer to the maximum allowed length \fBid_len\fR. The
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buffer at location \fBid\fR is only guaranteed to have the size \fBid_len\fR.
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The callback is only allowed to generate a shorter id and reduce \fBid_len\fR;
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the callback \fBmust never\fR increase \fBid_len\fR or write to the location
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\&\fBid\fR exceeding the given limit.
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.PP
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If a SSLv2 session id is generated and \fBid_len\fR is reduced, it will be
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restored after the callback has finished and the session id will be padded
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with 0x00. It is not recommended to change the \fBid_len\fR for SSLv2 sessions.
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The callback can use the SSL_get_version(3) function
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to check, whether the session is of type SSLv2.
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.PP
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The location \fBid\fR is filled with 0x00 before the callback is called, so the
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callback may only fill part of the possible length and leave \fBid_len\fR
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untouched while maintaining reproducibility.
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.PP
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Since the sessions must be distinguished, session ids must be unique.
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Without the callback a random number is used, so that the probability
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of generating the same session id is extremely small (2^128 possible ids
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for an SSLv2 session, 2^256 for SSLv3/TLSv1). In order to assure the
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uniqueness of the generated session id, the callback must call
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\&\fISSL_has_matching_session_id()\fR and generate another id if a conflict occurs.
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If an id conflict is not resolved, the handshake will fail.
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If the application codes e.g. a unique host id, a unique process number, and
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a unique sequence number into the session id, uniqueness could easily be
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achieved without randomness added (it should however be taken care that
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no confidential information is leaked this way). If the application can not
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guarantee uniqueness, it is recommended to use the maximum \fBid_len\fR and
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fill in the bytes not used to code special information with random data
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to avoid collisions.
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.PP
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\&\fISSL_has_matching_session_id()\fR will only query the internal session cache,
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not the external one. Since the session id is generated before the
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handshake is completed, it is not immediately added to the cache. If
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another thread is using the same internal session cache, a race condition
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can occur in that another thread generates the same session id.
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Collisions can also occur when using an external session cache, since
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the external cache is not tested with \fISSL_has_matching_session_id()\fR
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and the same race condition applies.
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.PP
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When calling \fISSL_has_matching_session_id()\fR for an SSLv2 session with
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reduced \fBid_len\fR, the match operation will be performed using the
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fixed length required and with a 0x00 padded id.
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.PP
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The callback must return 0 if it cannot generate a session id for whatever
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reason and return 1 on success.
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.SH "EXAMPLES"
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.IX Header "EXAMPLES"
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The callback function listed will generate a session id with the
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server id given, and will fill the rest with pseudo random bytes:
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.PP
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.Vb 1
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\& const char session_id_prefix = "www-18";
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.Ve
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.Vb 6
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\& #define MAX_SESSION_ID_ATTEMPTS 10
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\& static int generate_session_id(const SSL *ssl, unsigned char *id,
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\& unsigned int *id_len)
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\& {
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\& unsigned int count = 0;
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\& const char *version;
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.Ve
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.Vb 3
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\& version = SSL_get_version(ssl);
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\& if (!strcmp(version, "SSLv2"))
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\& /* we must not change id_len */;
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.Ve
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.Vb 17
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\& do {
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\& RAND_pseudo_bytes(id, *id_len);
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\& /* Prefix the session_id with the required prefix. NB: If our
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\& * prefix is too long, clip it - but there will be worse effects
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\& * anyway, eg. the server could only possibly create 1 session
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\& * ID (ie. the prefix!) so all future session negotiations will
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\& * fail due to conflicts. */
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\& memcpy(id, session_id_prefix,
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\& (strlen(session_id_prefix) < *id_len) ?
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\& strlen(session_id_prefix) : *id_len);
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\& }
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\& while(SSL_has_matching_session_id(ssl, id, *id_len) &&
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\& (++count < MAX_SESSION_ID_ATTEMPTS));
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\& if(count >= MAX_SESSION_ID_ATTEMPTS)
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\& return 0;
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\& return 1;
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\& }
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.Ve
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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\&\fISSL_CTX_set_generate_session_id()\fR and \fISSL_set_generate_session_id()\fR
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always return 1.
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.PP
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\&\fISSL_has_matching_session_id()\fR returns 1 if another session with the
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same id is already in the cache.
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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ssl(3), SSL_get_version(3)
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.SH "HISTORY"
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.IX Header "HISTORY"
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\&\fISSL_CTX_set_generate_session_id()\fR, \fISSL_set_generate_session_id()\fR
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and \fISSL_has_matching_session_id()\fR have been introduced in
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OpenSSL 0.9.7.
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