1=pod
2
3=head1 NAME
4
5SSL_CTX_set_generate_session_id, SSL_set_generate_session_id,
6SSL_has_matching_session_id, GEN_SESSION_CB
7- manipulate generation of SSL session IDs (server only)
8
9=head1 SYNOPSIS
10
11 #include <openssl/ssl.h>
12
13 typedef int (*GEN_SESSION_CB)(SSL *ssl, unsigned char *id,
14                               unsigned int *id_len);
15
16 int SSL_CTX_set_generate_session_id(SSL_CTX *ctx, GEN_SESSION_CB cb);
17 int SSL_set_generate_session_id(SSL *ssl, GEN_SESSION_CB, cb);
18 int SSL_has_matching_session_id(const SSL *ssl, const unsigned char *id,
19                                 unsigned int id_len);
20
21=head1 DESCRIPTION
22
23SSL_CTX_set_generate_session_id() sets the callback function for generating
24new session ids for SSL/TLS sessions for B<ctx> to be B<cb>.
25
26SSL_set_generate_session_id() sets the callback function for generating
27new session ids for SSL/TLS sessions for B<ssl> to be B<cb>.
28
29SSL_has_matching_session_id() checks, whether a session with id B<id>
30(of length B<id_len>) is already contained in the internal session cache
31of the parent context of B<ssl>.
32
33=head1 NOTES
34
35When a new session is established between client and server, the server
36generates a session id. The session id is an arbitrary sequence of bytes.
37The length of the session id is between 1 and 32 bytes.  The session id is not
38security critical but must be unique for the server. Additionally, the session id is
39transmitted in the clear when reusing the session so it must not contain
40sensitive information.
41
42Without a callback being set, an OpenSSL server will generate a unique
43session id from pseudo random numbers of the maximum possible length.
44Using the callback function, the session id can be changed to contain
45additional information like e.g. a host id in order to improve load balancing
46or external caching techniques.
47
48The callback function receives a pointer to the memory location to put
49B<id> into and a pointer to the maximum allowed length B<id_len>. The
50buffer at location B<id> is only guaranteed to have the size B<id_len>.
51The callback is only allowed to generate a shorter id and reduce B<id_len>;
52the callback B<must never> increase B<id_len> or write to the location
53B<id> exceeding the given limit.
54
55The location B<id> is filled with 0x00 before the callback is called, so the
56callback may only fill part of the possible length and leave B<id_len>
57untouched while maintaining reproducibility.
58
59Since the sessions must be distinguished, session ids must be unique.
60Without the callback a random number is used, so that the probability
61of generating the same session id is extremely small (2^256 for SSLv3/TLSv1).
62In order to assure the uniqueness of the generated session id, the callback must call
63SSL_has_matching_session_id() and generate another id if a conflict occurs.
64If an id conflict is not resolved, the handshake will fail.
65If the application codes e.g. a unique host id, a unique process number, and
66a unique sequence number into the session id, uniqueness could easily be
67achieved without randomness added (it should however be taken care that
68no confidential information is leaked this way). If the application can not
69guarantee uniqueness, it is recommended to use the maximum B<id_len> and
70fill in the bytes not used to code special information with random data
71to avoid collisions.
72
73SSL_has_matching_session_id() will only query the internal session cache,
74not the external one. Since the session id is generated before the
75handshake is completed, it is not immediately added to the cache. If
76another thread is using the same internal session cache, a race condition
77can occur in that another thread generates the same session id.
78Collisions can also occur when using an external session cache, since
79the external cache is not tested with SSL_has_matching_session_id()
80and the same race condition applies.
81
82The callback must return 0 if it cannot generate a session id for whatever
83reason and return 1 on success.
84
85=head1 RETURN VALUES
86
87SSL_CTX_set_generate_session_id() and SSL_set_generate_session_id()
88return 1 on success and 0 for failure.
89
90SSL_has_matching_session_id() returns 1 if another session with the
91same id is already in the cache, or 0 otherwise.
92
93=head1 EXAMPLES
94
95The callback function listed will generate a session id with the
96server id given, and will fill the rest with pseudo random bytes:
97
98 const char session_id_prefix = "www-18";
99
100 #define MAX_SESSION_ID_ATTEMPTS 10
101 static int generate_session_id(SSL *ssl, unsigned char *id,
102                                unsigned int *id_len)
103 {
104     unsigned int count = 0;
105
106     do {
107         RAND_pseudo_bytes(id, *id_len);
108         /*
109          * Prefix the session_id with the required prefix. NB: If our
110          * prefix is too long, clip it - but there will be worse effects
111          * anyway, e.g. the server could only possibly create 1 session
112          * ID (i.e. the prefix!) so all future session negotiations will
113          * fail due to conflicts.
114          */
115         memcpy(id, session_id_prefix, strlen(session_id_prefix) < *id_len ?
116                                       strlen(session_id_prefix) : *id_len);
117     } while (SSL_has_matching_session_id(ssl, id, *id_len)
118               && ++count < MAX_SESSION_ID_ATTEMPTS);
119     if (count >= MAX_SESSION_ID_ATTEMPTS)
120         return 0;
121     return 1;
122 }
123
124
125=head1 SEE ALSO
126
127L<ssl(7)>, L<SSL_get_version(3)>
128
129=head1 COPYRIGHT
130
131Copyright 2001-2020 The OpenSSL Project Authors. All Rights Reserved.
132
133Licensed under the Apache License 2.0 (the "License").  You may not use
134this file except in compliance with the License.  You can obtain a copy
135in the file LICENSE in the source distribution or at
136L<https://www.openssl.org/source/license.html>.
137
138=cut
139