Update files for OpenSSL-1.0.0f import.
[dragonfly.git] / secure / lib / libssl / man / SSL_CTX_set_generate_session_id.3
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124.\" ========================================================================
125.\"
126.IX Title "SSL_CTX_set_generate_session_id 3"
e3261593 127.TH SSL_CTX_set_generate_session_id 3 "2012-01-04" "1.0.0f" "OpenSSL"
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128.\" For nroff, turn off justification. Always turn off hyphenation; it makes
129.\" way too many mistakes in technical documents.
130.if n .ad l
131.nh
984263bc 132.SH "NAME"
a7d27d5a 133SSL_CTX_set_generate_session_id, SSL_set_generate_session_id, SSL_has_matching_session_id \- manipulate generation of SSL session IDs (server only)
984263bc 134.SH "SYNOPSIS"
e056f0e0 135.IX Header "SYNOPSIS"
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136.Vb 1
137\& #include <openssl/ssl.h>
e257b235 138\&
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139\& typedef int (*GEN_SESSION_CB)(const SSL *ssl, unsigned char *id,
140\& unsigned int *id_len);
e257b235 141\&
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142\& int SSL_CTX_set_generate_session_id(SSL_CTX *ctx, GEN_SESSION_CB cb);
143\& int SSL_set_generate_session_id(SSL *ssl, GEN_SESSION_CB, cb);
144\& int SSL_has_matching_session_id(const SSL *ssl, const unsigned char *id,
145\& unsigned int id_len);
146.Ve
147.SH "DESCRIPTION"
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148.IX Header "DESCRIPTION"
149\&\fISSL_CTX_set_generate_session_id()\fR sets the callback function for generating
150new session ids for \s-1SSL/TLS\s0 sessions for \fBctx\fR to be \fBcb\fR.
984263bc 151.PP
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152\&\fISSL_set_generate_session_id()\fR sets the callback function for generating
153new session ids for \s-1SSL/TLS\s0 sessions for \fBssl\fR to be \fBcb\fR.
984263bc 154.PP
e056f0e0 155\&\fISSL_has_matching_session_id()\fR checks, whether a session with id \fBid\fR
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156(of length \fBid_len\fR) is already contained in the internal session cache
157of the parent context of \fBssl\fR.
158.SH "NOTES"
e056f0e0 159.IX Header "NOTES"
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160When a new session is established between client and server, the server
161generates a session id. The session id is an arbitrary sequence of bytes.
162The length of the session id is 16 bytes for SSLv2 sessions and between
1631 and 32 bytes for SSLv3/TLSv1. The session id is not security critical
164but must be unique for the server. Additionally, the session id is
165transmitted in the clear when reusing the session so it must not contain
166sensitive information.
167.PP
168Without a callback being set, an OpenSSL server will generate a unique
169session id from pseudo random numbers of the maximum possible length.
170Using the callback function, the session id can be changed to contain
171additional information like e.g. a host id in order to improve load balancing
172or external caching techniques.
173.PP
174The callback function receives a pointer to the memory location to put
e056f0e0 175\&\fBid\fR into and a pointer to the maximum allowed length \fBid_len\fR. The
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176buffer at location \fBid\fR is only guaranteed to have the size \fBid_len\fR.
177The callback is only allowed to generate a shorter id and reduce \fBid_len\fR;
178the callback \fBmust never\fR increase \fBid_len\fR or write to the location
e056f0e0 179\&\fBid\fR exceeding the given limit.
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180.PP
181If a SSLv2 session id is generated and \fBid_len\fR is reduced, it will be
182restored after the callback has finished and the session id will be padded
183with 0x00. It is not recommended to change the \fBid_len\fR for SSLv2 sessions.
e056f0e0 184The callback can use the \fISSL_get_version\fR\|(3) function
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185to check, whether the session is of type SSLv2.
186.PP
187The location \fBid\fR is filled with 0x00 before the callback is called, so the
188callback may only fill part of the possible length and leave \fBid_len\fR
189untouched while maintaining reproducibility.
190.PP
191Since the sessions must be distinguished, session ids must be unique.
192Without the callback a random number is used, so that the probability
193of generating the same session id is extremely small (2^128 possible ids
194for an SSLv2 session, 2^256 for SSLv3/TLSv1). In order to assure the
195uniqueness of the generated session id, the callback must call
e056f0e0 196\&\fISSL_has_matching_session_id()\fR and generate another id if a conflict occurs.
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197If an id conflict is not resolved, the handshake will fail.
198If the application codes e.g. a unique host id, a unique process number, and
199a unique sequence number into the session id, uniqueness could easily be
200achieved without randomness added (it should however be taken care that
201no confidential information is leaked this way). If the application can not
202guarantee uniqueness, it is recommended to use the maximum \fBid_len\fR and
203fill in the bytes not used to code special information with random data
204to avoid collisions.
205.PP
e056f0e0 206\&\fISSL_has_matching_session_id()\fR will only query the internal session cache,
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207not the external one. Since the session id is generated before the
208handshake is completed, it is not immediately added to the cache. If
209another thread is using the same internal session cache, a race condition
210can occur in that another thread generates the same session id.
211Collisions can also occur when using an external session cache, since
212the external cache is not tested with \fISSL_has_matching_session_id()\fR
213and the same race condition applies.
214.PP
215When calling \fISSL_has_matching_session_id()\fR for an SSLv2 session with
216reduced \fBid_len\fR, the match operation will be performed using the
217fixed length required and with a 0x00 padded id.
218.PP
219The callback must return 0 if it cannot generate a session id for whatever
220reason and return 1 on success.
221.SH "EXAMPLES"
e056f0e0 222.IX Header "EXAMPLES"
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223The callback function listed will generate a session id with the
224server id given, and will fill the rest with pseudo random bytes:
225.PP
226.Vb 1
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227\& const char session_id_prefix = "www\-18";
228\&
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229\& #define MAX_SESSION_ID_ATTEMPTS 10
230\& static int generate_session_id(const SSL *ssl, unsigned char *id,
231\& unsigned int *id_len)
232\& {
233\& unsigned int count = 0;
234\& const char *version;
e257b235 235\&
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236\& version = SSL_get_version(ssl);
237\& if (!strcmp(version, "SSLv2"))
238\& /* we must not change id_len */;
e257b235 239\&
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240\& do {
241\& RAND_pseudo_bytes(id, *id_len);
242\& /* Prefix the session_id with the required prefix. NB: If our
e257b235 243\& * prefix is too long, clip it \- but there will be worse effects
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244\& * anyway, eg. the server could only possibly create 1 session
245\& * ID (ie. the prefix!) so all future session negotiations will
246\& * fail due to conflicts. */
247\& memcpy(id, session_id_prefix,
248\& (strlen(session_id_prefix) < *id_len) ?
249\& strlen(session_id_prefix) : *id_len);
250\& }
251\& while(SSL_has_matching_session_id(ssl, id, *id_len) &&
252\& (++count < MAX_SESSION_ID_ATTEMPTS));
253\& if(count >= MAX_SESSION_ID_ATTEMPTS)
254\& return 0;
255\& return 1;
256\& }
257.Ve
258.SH "RETURN VALUES"
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259.IX Header "RETURN VALUES"
260\&\fISSL_CTX_set_generate_session_id()\fR and \fISSL_set_generate_session_id()\fR
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261always return 1.
262.PP
e056f0e0 263\&\fISSL_has_matching_session_id()\fR returns 1 if another session with the
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264same id is already in the cache.
265.SH "SEE ALSO"
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266.IX Header "SEE ALSO"
267\&\fIssl\fR\|(3), \fISSL_get_version\fR\|(3)
984263bc 268.SH "HISTORY"
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269.IX Header "HISTORY"
270\&\fISSL_CTX_set_generate_session_id()\fR, \fISSL_set_generate_session_id()\fR
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271and \fISSL_has_matching_session_id()\fR have been introduced in
272OpenSSL 0.9.7.