1=pod
2
3=head1 NAME
4
5SSL_CTX_set_generate_session_id, SSL_set_generate_session_id, SSL_has_matching_session_id - manipulate generation of SSL session IDs (server only)
6
7=head1 SYNOPSIS
8
9 #include <openssl/ssl.h>
10
11 typedef int (*GEN_SESSION_CB)(const SSL *ssl, unsigned char *id,
12                               unsigned int *id_len);
13
14 int SSL_CTX_set_generate_session_id(SSL_CTX *ctx, GEN_SESSION_CB cb);
15 int SSL_set_generate_session_id(SSL *ssl, GEN_SESSION_CB, cb);
16 int SSL_has_matching_session_id(const SSL *ssl, const unsigned char *id,
17				 unsigned int id_len);
18
19=head1 DESCRIPTION
20
21SSL_CTX_set_generate_session_id() sets the callback function for generating
22new session ids for SSL/TLS sessions for B<ctx> to be B<cb>.
23
24SSL_set_generate_session_id() sets the callback function for generating
25new session ids for SSL/TLS sessions for B<ssl> to be B<cb>.
26
27SSL_has_matching_session_id() checks, whether a session with id B<id>
28(of length B<id_len>) is already contained in the internal session cache
29of the parent context of B<ssl>.
30
31=head1 NOTES
32
33When a new session is established between client and server, the server
34generates a session id. The session id is an arbitrary sequence of bytes.
35The length of the session id is 16 bytes for SSLv2 sessions and between
361 and 32 bytes for SSLv3/TLSv1. The session id is not security critical
37but must be unique for the server. Additionally, the session id is
38transmitted in the clear when reusing the session so it must not contain
39sensitive information.
40
41Without a callback being set, an OpenSSL server will generate a unique
42session id from pseudo random numbers of the maximum possible length.
43Using the callback function, the session id can be changed to contain
44additional information like e.g. a host id in order to improve load balancing
45or external caching techniques.
46
47The callback function receives a pointer to the memory location to put
48B<id> into and a pointer to the maximum allowed length B<id_len>. The
49buffer at location B<id> is only guaranteed to have the size B<id_len>.
50The callback is only allowed to generate a shorter id and reduce B<id_len>;
51the callback B<must never> increase B<id_len> or write to the location
52B<id> exceeding the given limit.
53
54If a SSLv2 session id is generated and B<id_len> is reduced, it will be
55restored after the callback has finished and the session id will be padded
56with 0x00. It is not recommended to change the B<id_len> for SSLv2 sessions.
57The callback can use the L<SSL_get_version(3)|SSL_get_version(3)> function
58to check, whether the session is of type SSLv2.
59
60The location B<id> is filled with 0x00 before the callback is called, so the
61callback may only fill part of the possible length and leave B<id_len>
62untouched while maintaining reproducibility.
63
64Since the sessions must be distinguished, session ids must be unique.
65Without the callback a random number is used, so that the probability
66of generating the same session id is extremely small (2^128 possible ids
67for an SSLv2 session, 2^256 for SSLv3/TLSv1). In order to assure the
68uniqueness of the generated session id, the callback must call
69SSL_has_matching_session_id() and generate another id if a conflict occurs.
70If an id conflict is not resolved, the handshake will fail.
71If the application codes e.g. a unique host id, a unique process number, and
72a unique sequence number into the session id, uniqueness could easily be
73achieved without randomness added (it should however be taken care that
74no confidential information is leaked this way). If the application can not
75guarantee uniqueness, it is recommended to use the maximum B<id_len> and
76fill in the bytes not used to code special information with random data
77to avoid collisions.
78
79SSL_has_matching_session_id() will only query the internal session cache,
80not the external one. Since the session id is generated before the
81handshake is completed, it is not immediately added to the cache. If
82another thread is using the same internal session cache, a race condition
83can occur in that another thread generates the same session id.
84Collisions can also occur when using an external session cache, since
85the external cache is not tested with SSL_has_matching_session_id()
86and the same race condition applies.
87
88When calling SSL_has_matching_session_id() for an SSLv2 session with
89reduced B<id_len>, the match operation will be performed using the
90fixed length required and with a 0x00 padded id.
91
92The callback must return 0 if it cannot generate a session id for whatever
93reason and return 1 on success.
94
95=head1 EXAMPLES
96
97The callback function listed will generate a session id with the
98server id given, and will fill the rest with pseudo random bytes:
99
100 const char session_id_prefix = "www-18";
101
102 #define MAX_SESSION_ID_ATTEMPTS 10
103 static int generate_session_id(const SSL *ssl, unsigned char *id,
104                              unsigned int *id_len)
105      {
106      unsigned int count = 0;
107      const char *version;
108
109      version = SSL_get_version(ssl);
110      if (!strcmp(version, "SSLv2"))
111	  /* we must not change id_len */;
112
113      do      {
114              RAND_pseudo_bytes(id, *id_len);
115              /* Prefix the session_id with the required prefix. NB: If our
116               * prefix is too long, clip it - but there will be worse effects
117               * anyway, eg. the server could only possibly create 1 session
118               * ID (ie. the prefix!) so all future session negotiations will
119               * fail due to conflicts. */
120              memcpy(id, session_id_prefix,
121                      (strlen(session_id_prefix) < *id_len) ?
122                      strlen(session_id_prefix) : *id_len);
123              }
124      while(SSL_has_matching_session_id(ssl, id, *id_len) &&
125              (++count < MAX_SESSION_ID_ATTEMPTS));
126      if(count >= MAX_SESSION_ID_ATTEMPTS)
127              return 0;
128      return 1;
129      }
130
131
132=head1 RETURN VALUES
133
134SSL_CTX_set_generate_session_id() and SSL_set_generate_session_id()
135always return 1.
136
137SSL_has_matching_session_id() returns 1 if another session with the
138same id is already in the cache.
139
140=head1 SEE ALSO
141
142L<ssl(3)|ssl(3)>, L<SSL_get_version(3)|SSL_get_version(3)>
143
144=head1 HISTORY
145
146SSL_CTX_set_generate_session_id(), SSL_set_generate_session_id()
147and SSL_has_matching_session_id() have been introduced in
148OpenSSL 0.9.7.
149
150=cut
151