SSL_CTX_SET_GENERATE_SESSION_OpeSnSSL_CTX_SET_GENERATE_SESSION_ID(3)
SSL_CTX_set_generate_session_id, SSL_set_generate_session_id,
SSL_has_matching_session_id - manipulate generation
of SSL session IDs (server only)
#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);
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 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 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.
SSL_CTX_SET_GENERATE_SESSION_OpeSnSSL_CTX_SET_GENERATE_SESSION_ID(3)
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 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 (2^128 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. 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.
When calling SSL_has_matching_session_id() for an SSLv2
session with reduced id_len, the match operation will be
performed using the fixed length required and with a 0x00
padded id.
The callback must return 0 if it cannot generate a session
id for whatever reason and return 1 on success.
The callback function listed will generate a session id
with the server id given, and will fill the rest with
pseudo random bytes:
SSL_CTX_SET_GENERATE_SESSION_OpeSnSSL_CTX_SET_GENERATE_SESSION_ID(3)
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;
}
SSL_CTX_set_generate_session_id() and SSL_set_gener-
ate_session_id() always return 1.
SSL_has_matching_session_id() returns 1 if another session
with the same id is already in the cache.
ssl(3), SSL_get_version(3)
SSL_CTX_set_generate_session_id(), SSL_set_generate_ses-
sion_id() and SSL_has_matching_session_id() have been
introduced in OpenSSL 0.9.7.
OpenBSD 3.6 2002-05-14 3 [ Back ] |
