SSL_CTX_SET_GENERATE_SESSION_OpSSL_CTX_SET_GENERATE_SESSION_ID(3)NAME
SSL_CTX_set_generate_session_id,
SSL_set_generate_session_id, SSL_has_matching_session_id -
manipulate generation of SSL session IDs (server only)
SYNOPSIS
#include <openssl/ssl.h>
typedef int (*GEN_SESSION_CB)(const 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);
DESCRIPTIONSSL_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 inter-
nal session cache of the parent context of ssl.
NOTES
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
16 bytes for SSLv2 sessions and between 1 and 32 bytes for
SSLv3/TLSv1. 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 gen-
erate 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 loca-
tion 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
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SSL_CTX_SET_GENERATE_SESSION_OpSSL_CTX_SET_GENERATE_SESSION_ID(3)
the given limit.
If a SSLv2 session id is generated and id_len is reduced, it
will be restored after the callback has finished and the
session id will be padded with 0x00. It is not recommended
to change the id_len for SSLv2 sessions. The callback can
use the SSL_get_version(3) function to check, whether the
session is of type SSLv2.
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 reprodu-
cibility.
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 (2^128 possible ids for an SSLv2 session,
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. Colli-
sions 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.
When calling SSL_has_matching_session_id() for an SSLv2 ses-
sion with reduced id_len, the match operation will be per-
formed using the fixed length required and with a 0x00 pad-
ded id.
The callback must return 0 if it cannot generate a session
id for whatever reason and return 1 on success.
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SSL_CTX_SET_GENERATE_SESSION_OpSSL_CTX_SET_GENERATE_SESSION_ID(3)EXAMPLES
The callback function listed will generate a session id with
the server id given, and will fill the rest with pseudo ran-
dom bytes:
const char session_id_prefix = "www-18";
#define MAX_SESSION_ID_ATTEMPTS 10
static int generate_session_id(const SSL *ssl, unsigned char *id,
unsigned int *id_len)
{
unsigned int count = 0;
const char *version;
version = SSL_get_version(ssl);
if (!strcmp(version, "SSLv2"))
/* we must not change id_len */;
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, eg. the server could only possibly create 1 session
* ID (ie. 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;
}
RETURN VALUESSSL_CTX_set_generate_session_id() and
SSL_set_generate_session_id() always return 1.
SSL_has_matching_session_id() returns 1 if another session
with the same id is already in the cache.
SEE ALSOssl(3), SSL_get_version(3)HISTORYSSL_CTX_set_generate_session_id(),
SSL_set_generate_session_id() and
SSL_has_matching_session_id() have been introduced in
OpenSSL 0.9.7.
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