SSL_CTX_set_generate_session_id, SSL_set_generate_session_id,
SSL_has_matching_session_id, GEN_SESSION_CB - manipulate generation of SSL
session IDs (server only)
#include <openssl/ssl.h>
typedef int (*GEN_SESSION_CB)(SSL *ssl, unsigned char *id,
unsigned int *id_len);
int SSL_CTX_set_generate_session_id(SSL_CTX *ctx, GEN_SESSION_CB cb);
int SSL_set_generate_session_id(SSL *ssl, GEN_SESSION_CB, cb);
int SSL_has_matching_session_id(const SSL *ssl, const unsigned char *id,
unsigned int id_len);
SSL_CTX_set_generate_session_id() sets the callback function for
generating new session ids for SSL/TLS sessions for
ctx to be
cb.
SSL_set_generate_session_id() sets the callback function for generating
new session ids for SSL/TLS sessions for
ssl to be
cb.
SSL_has_matching_session_id() checks, whether a session with id
id
(of length
id_len) is already contained in the internal session cache
of the parent context of
ssl.
When a new session is established between client and server, the server
generates a session id. The session id is an arbitrary sequence of bytes. The
length of the session id is between 1 and 32 bytes. The session id is not
security critical but must be unique for the server. Additionally, the session
id is transmitted in the clear when reusing the session so it must not contain
sensitive information.
Without a callback being set, an OpenSSL server will generate a unique session
id from pseudo random numbers of the maximum possible length. Using the
callback function, the session id can be changed to contain additional
information like e.g. a host id in order to improve load balancing or external
caching techniques.
The callback function receives a pointer to the memory location to put
id
into and a pointer to the maximum allowed length
id_len. The buffer at
location
id is only guaranteed to have the size
id_len. The
callback is only allowed to generate a shorter id and reduce
id_len;
the callback
must never increase
id_len or write to the location
id exceeding the given limit.
The location
id is filled with 0x00 before the callback is called, so the
callback may only fill part of the possible length and leave
id_len
untouched while maintaining reproducibility.
Since the sessions must be distinguished, session ids must be unique. Without
the callback a random number is used, so that the probability of generating
the same session id is extremely small (2^256 for SSLv3/TLSv1). In order to
assure the uniqueness of the generated session id, the callback must call
SSL_has_matching_session_id() and generate another id if a conflict
occurs. If an id conflict is not resolved, the handshake will fail. If the
application codes e.g. a unique host id, a unique process number, and a unique
sequence number into the session id, uniqueness could easily be achieved
without randomness added (it should however be taken care that no confidential
information is leaked this way). If the application can not guarantee
uniqueness, it is recommended to use the maximum
id_len and fill in the
bytes not used to code special information with random data to avoid
collisions.
SSL_has_matching_session_id() will only query the internal session cache,
not the external one. Since the session id is generated before the handshake
is completed, it is not immediately added to the cache. If another thread is
using the same internal session cache, a race condition can occur in that
another thread generates the same session id. Collisions can also occur when
using an external session cache, since the external cache is not tested with
SSL_has_matching_session_id() and the same race condition applies.
The callback must return 0 if it cannot generate a session id for whatever
reason and return 1 on success.
SSL_CTX_set_generate_session_id() and
SSL_set_generate_session_id() return 1 on success and 0 for failure.
SSL_has_matching_session_id() returns 1 if another session with the same
id is already in the cache, or 0 otherwise.
The callback function listed will generate a session id with the server id
given, and will fill the rest with pseudo random bytes:
const char session_id_prefix = "www-18";
#define MAX_SESSION_ID_ATTEMPTS 10
static int generate_session_id(SSL *ssl, unsigned char *id,
unsigned int *id_len)
{
unsigned int count = 0;
do {
RAND_pseudo_bytes(id, *id_len);
/*
* Prefix the session_id with the required prefix. NB: If our
* prefix is too long, clip it - but there will be worse effects
* anyway, e.g. the server could only possibly create 1 session
* ID (i.e. the prefix!) so all future session negotiations will
* fail due to conflicts.
*/
memcpy(id, session_id_prefix, strlen(session_id_prefix) < *id_len ?
strlen(session_id_prefix) : *id_len);
} while (SSL_has_matching_session_id(ssl, id, *id_len)
&& ++count < MAX_SESSION_ID_ATTEMPTS);
if (count >= MAX_SESSION_ID_ATTEMPTS)
return 0;
return 1;
}
ssl(7),
SSL_get_version(3)
Copyright 2001-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy in the
file LICENSE in the source distribution or at
<
https://www.openssl.org/source/license.html>.