build - Rewire secure, remove conflicts from libmd, libcrypt
[dragonfly.git] / lib / libssl / man / SSL_CTX_set_generate_session_id.3
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133.\" ========================================================================
134.\"
135.IX Title "SSL_CTX_set_generate_session_id 3"
57eefc0b 136.TH SSL_CTX_set_generate_session_id 3 "2016-05-03" "1.0.2h" "OpenSSL"
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137.\" For nroff, turn off justification. Always turn off hyphenation; it makes
138.\" way too many mistakes in technical documents.
139.if n .ad l
140.nh
984263bc 141.SH "NAME"
a7d27d5a 142SSL_CTX_set_generate_session_id, SSL_set_generate_session_id, SSL_has_matching_session_id \- manipulate generation of SSL session IDs (server only)
984263bc 143.SH "SYNOPSIS"
e056f0e0 144.IX Header "SYNOPSIS"
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145.Vb 1
146\& #include <openssl/ssl.h>
e257b235 147\&
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148\& typedef int (*GEN_SESSION_CB)(const SSL *ssl, unsigned char *id,
149\& unsigned int *id_len);
e257b235 150\&
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151\& int SSL_CTX_set_generate_session_id(SSL_CTX *ctx, GEN_SESSION_CB cb);
152\& int SSL_set_generate_session_id(SSL *ssl, GEN_SESSION_CB, cb);
153\& int SSL_has_matching_session_id(const SSL *ssl, const unsigned char *id,
154\& unsigned int id_len);
155.Ve
156.SH "DESCRIPTION"
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157.IX Header "DESCRIPTION"
158\&\fISSL_CTX_set_generate_session_id()\fR sets the callback function for generating
159new session ids for \s-1SSL/TLS\s0 sessions for \fBctx\fR to be \fBcb\fR.
984263bc 160.PP
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161\&\fISSL_set_generate_session_id()\fR sets the callback function for generating
162new session ids for \s-1SSL/TLS\s0 sessions for \fBssl\fR to be \fBcb\fR.
984263bc 163.PP
e056f0e0 164\&\fISSL_has_matching_session_id()\fR checks, whether a session with id \fBid\fR
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165(of length \fBid_len\fR) is already contained in the internal session cache
166of the parent context of \fBssl\fR.
167.SH "NOTES"
e056f0e0 168.IX Header "NOTES"
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169When a new session is established between client and server, the server
170generates a session id. The session id is an arbitrary sequence of bytes.
171The length of the session id is 16 bytes for SSLv2 sessions and between
1721 and 32 bytes for SSLv3/TLSv1. The session id is not security critical
173but must be unique for the server. Additionally, the session id is
174transmitted in the clear when reusing the session so it must not contain
175sensitive information.
176.PP
177Without a callback being set, an OpenSSL server will generate a unique
178session id from pseudo random numbers of the maximum possible length.
179Using the callback function, the session id can be changed to contain
180additional information like e.g. a host id in order to improve load balancing
181or external caching techniques.
182.PP
183The callback function receives a pointer to the memory location to put
e056f0e0 184\&\fBid\fR into and a pointer to the maximum allowed length \fBid_len\fR. The
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185buffer at location \fBid\fR is only guaranteed to have the size \fBid_len\fR.
186The callback is only allowed to generate a shorter id and reduce \fBid_len\fR;
187the callback \fBmust never\fR increase \fBid_len\fR or write to the location
e056f0e0 188\&\fBid\fR exceeding the given limit.
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189.PP
190If a SSLv2 session id is generated and \fBid_len\fR is reduced, it will be
191restored after the callback has finished and the session id will be padded
192with 0x00. It is not recommended to change the \fBid_len\fR for SSLv2 sessions.
e056f0e0 193The callback can use the \fISSL_get_version\fR\|(3) function
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194to check, whether the session is of type SSLv2.
195.PP
196The location \fBid\fR is filled with 0x00 before the callback is called, so the
197callback may only fill part of the possible length and leave \fBid_len\fR
198untouched while maintaining reproducibility.
199.PP
200Since the sessions must be distinguished, session ids must be unique.
201Without the callback a random number is used, so that the probability
202of generating the same session id is extremely small (2^128 possible ids
203for an SSLv2 session, 2^256 for SSLv3/TLSv1). In order to assure the
204uniqueness of the generated session id, the callback must call
e056f0e0 205\&\fISSL_has_matching_session_id()\fR and generate another id if a conflict occurs.
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206If an id conflict is not resolved, the handshake will fail.
207If the application codes e.g. a unique host id, a unique process number, and
208a unique sequence number into the session id, uniqueness could easily be
209achieved without randomness added (it should however be taken care that
210no confidential information is leaked this way). If the application can not
211guarantee uniqueness, it is recommended to use the maximum \fBid_len\fR and
212fill in the bytes not used to code special information with random data
213to avoid collisions.
214.PP
e056f0e0 215\&\fISSL_has_matching_session_id()\fR will only query the internal session cache,
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216not the external one. Since the session id is generated before the
217handshake is completed, it is not immediately added to the cache. If
218another thread is using the same internal session cache, a race condition
219can occur in that another thread generates the same session id.
220Collisions can also occur when using an external session cache, since
221the external cache is not tested with \fISSL_has_matching_session_id()\fR
222and the same race condition applies.
223.PP
224When calling \fISSL_has_matching_session_id()\fR for an SSLv2 session with
225reduced \fBid_len\fR, the match operation will be performed using the
226fixed length required and with a 0x00 padded id.
227.PP
228The callback must return 0 if it cannot generate a session id for whatever
229reason and return 1 on success.
230.SH "EXAMPLES"
e056f0e0 231.IX Header "EXAMPLES"
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232The callback function listed will generate a session id with the
233server id given, and will fill the rest with pseudo random bytes:
234.PP
235.Vb 1
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236\& const char session_id_prefix = "www\-18";
237\&
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238\& #define MAX_SESSION_ID_ATTEMPTS 10
239\& static int generate_session_id(const SSL *ssl, unsigned char *id,
240\& unsigned int *id_len)
241\& {
242\& unsigned int count = 0;
243\& const char *version;
e257b235 244\&
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245\& version = SSL_get_version(ssl);
246\& if (!strcmp(version, "SSLv2"))
247\& /* we must not change id_len */;
e257b235 248\&
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249\& do {
250\& RAND_pseudo_bytes(id, *id_len);
251\& /* Prefix the session_id with the required prefix. NB: If our
e257b235 252\& * prefix is too long, clip it \- but there will be worse effects
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253\& * anyway, eg. the server could only possibly create 1 session
254\& * ID (ie. the prefix!) so all future session negotiations will
255\& * fail due to conflicts. */
256\& memcpy(id, session_id_prefix,
257\& (strlen(session_id_prefix) < *id_len) ?
258\& strlen(session_id_prefix) : *id_len);
259\& }
260\& while(SSL_has_matching_session_id(ssl, id, *id_len) &&
261\& (++count < MAX_SESSION_ID_ATTEMPTS));
262\& if(count >= MAX_SESSION_ID_ATTEMPTS)
263\& return 0;
264\& return 1;
265\& }
266.Ve
267.SH "RETURN VALUES"
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268.IX Header "RETURN VALUES"
269\&\fISSL_CTX_set_generate_session_id()\fR and \fISSL_set_generate_session_id()\fR
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270always return 1.
271.PP
e056f0e0 272\&\fISSL_has_matching_session_id()\fR returns 1 if another session with the
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273same id is already in the cache.
274.SH "SEE ALSO"
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275.IX Header "SEE ALSO"
276\&\fIssl\fR\|(3), \fISSL_get_version\fR\|(3)
984263bc 277.SH "HISTORY"
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278.IX Header "HISTORY"
279\&\fISSL_CTX_set_generate_session_id()\fR, \fISSL_set_generate_session_id()\fR
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280and \fISSL_has_matching_session_id()\fR have been introduced in
281OpenSSL 0.9.7.