NAME
SSL_CTX_set_tlsext_ticket_key_cb - set a callback for session ticket processing
LIBRARY
libcrypto, -lcrypto
SYNOPSIS
#include <openssl/tls1.h>
long SSL_CTX_set_tlsext_ticket_key_cb(SSL_CTX sslctx,
int (*cb)(SSL *s, unsigned char key_name[16],
unsigned char iv[EVP_MAX_IV_LENGTH],
EVP_CIPHER_CTX *ctx, HMAC_CTX *hctx, int enc));
DESCRIPTION
SSL_CTX_set_tlsext_ticket_key_cb() sets a callback fuction
cb for
handling session tickets for the ssl context
sslctx. Session tickets,
defined in RFC5077 provide an enhanced session resumption capability where the
server implementation is not required to maintain per session state. It only
applies to TLS and there is no SSLv3 implementation.
The callback is available when the OpenSSL library was built without
OPENSSL_NO_TLSEXT being defined.
The callback function
cb will be called for every client instigated TLS
session when session ticket extension is presented in the TLS hello message.
It is the responsibility of this function to create or retrieve the
cryptographic parameters and to maintain their state.
The OpenSSL library uses your callback function to help implement a common TLS
ticket construction state according to RFC5077 Section 4 such that per session
state is unnecessary and a small set of cryptographic variables needs to be
maintained by the callback function implementation.
In order to reuse a session, a TLS client must send the a session ticket
extension to the server. The client can only send exactly one session ticket.
The server, through the callback function, either agrees to reuse the session
ticket information or it starts a full TLS handshake to create a new session
ticket.
Before the callback function is started
ctx and
hctx have been
initialised with EVP_CIPHER_CTX_init and HMAC_CTX_init respectively.
For new sessions tickets, when the client doesn't present a session ticket, or
an attempted retreival of the ticket failed, or a renew option was indicated,
the callback function will be called with
enc equal to 1. The OpenSSL
library expects that the function will set an arbitary
name, initialize
iv, and set the cipher context
ctx and the hash context
hctx.
The
name is 16 characters long and is used as a key identifier.
The
iv length is the length of the IV of the corresponding cipher. The
maximum IV length is EVP_MAX_IV_LENGTH bytes defined in
evp.h.
The initialization vector
iv should be a random value. The cipher context
ctx should use the initialisation vector
iv. The cipher context
can be set using EVP_EncryptInit_ex. The hmac context can be set using
HMAC_Init_ex.
When the client presents a session ticket, the callback function with be called
with
enc set to 0 indicating that the
cb function should
retreive a set of parameters. In this case
name and
iv have
already been parsed out of the session ticket. The OpenSSL library expects
that the
name will be used to retrieve a cryptographic parameters and
that the cryptographic context
ctx will be set with the retreived
parameters and the initialization vector
iv. using a function like
EVP_DecryptInit_ex. The
hctx needs to be set using HMAC_Init_ex.
If the
name is still valid but a renewal of the ticket is required the
callback function should return 2. The library will call the callback again
with an arguement of enc equal to 1 to set the new ticket.
The return value of the
cb function is used by OpenSSL to determine what
further processing will occur. The following return values have meaning:
- 2
- This indicates that the ctx and hctx have
been set and the session can continue on those parameters. Additionally it
indicates that the session ticket is in a renewal period and should be
replaced. The OpenSSL library will call cb again with an enc
argument of 1 to set the new ticket (see RFC5077 3.3 paragraph 2).
- 1
- This indicates that the ctx and hctx have
been set and the session can continue on those parameters.
- 0
- This indicates that it was not possible to set/retrieve a
session ticket and the SSL/TLS session will continue by by negiotationing
a set of cryptographic parameters or using the alternate SSL/TLS
resumption mechanism, session ids.
If called with enc equal to 0 the library will call the cb again to
get a new set of parameters.
- less than 0
- This indicates an error.
NOTES
Session resumption shortcuts the TLS so that the client certificate negiotation
don't occur. It makes up for this by storing client certificate an all other
negotiated state information encrypted within the ticket. In a resumed session
the applications will have all this state information available exactly as if
a full negiotation had occured.
If an attacker can obtain the key used to encrypt a session ticket, they can
obtain the master secret for any ticket using that key and decrypt any traffic
using that session: even if the ciphersuite supports forward secrecy. As a
result applications may wish to use multiple keys and avoid using long term
keys stored in files.
Applications can use longer keys to maintain a consistent level of security. For
example if a ciphersuite uses 256 bit ciphers but only a 128 bit ticket key
the overall security is only 128 bits because breaking the ticket key will
enable an attacker to obtain the session keys.
EXAMPLES
Reference Implemention:
SSL_CTX_set_tlsext_ticket_key_cb(SSL,ssl_tlsext_ticket_key_cb);
....
static int ssl_tlsext_ticket_key_cb(SSL *s, unsigned char key_name[16], unsigned char *iv, EVP_CIPHER_CTX *ctx, HMAC_CTX *hctx, int enc)
{
if (enc) { /* create new session */
if (RAND_bytes(iv, EVP_MAX_IV_LENGTH) ) {
return -1; /* insufficient random */
}
key = currentkey(); /* something that you need to implement */
if ( !key ) {
/* current key doesn't exist or isn't valid */
key = createkey(); /* something that you need to implement.
* createkey needs to initialise, a name,
* an aes_key, a hmac_key and optionally
* an expire time. */
if ( !key ) { /* key couldn't be created */
return 0;
}
}
memcpy(key_name, key->name, 16);
EVP_EncryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL, key->aes_key, iv);
HMAC_Init_ex(&hctx, key->hmac_key, 16, EVP_sha256(), NULL);
return 1;
} else { /* retrieve session */
key = findkey(name);
if (!key || key->expire < now() ) {
return 0;
}
HMAC_Init_ex(&hctx, key->hmac_key, 16, EVP_sha256(), NULL);
EVP_DecryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL, key->aes_key, iv );
if (key->expire < ( now() - RENEW_TIME ) ) {
/* return 2 - this session will get a new ticket even though the current is still valid */
return 2;
}
return 1;
}
}
RETURN VALUES
returns 0 to indicate the callback function was set.
SEE ALSO
ssl(3),
SSL_set_session(3),
SSL_session_reused(3),
SSL_CTX_add_session(3),
SSL_CTX_sess_number(3),
SSL_CTX_sess_set_get_cb(3),
SSL_CTX_set_session_id_context(3),
HISTORY
This function was introduced in OpenSSL 0.9.8h