Provided by: libssl-doc_3.0.8-1ubuntu1_all 
NAME
SSL_CTX_set_tlsext_ticket_key_evp_cb, SSL_CTX_set_tlsext_ticket_key_cb - set a callback
for session ticket processing
SYNOPSIS
#include <openssl/tls1.h>
int SSL_CTX_set_tlsext_ticket_key_evp_cb(SSL_CTX sslctx,
int (*cb)(SSL *s, unsigned char key_name[16],
unsigned char iv[EVP_MAX_IV_LENGTH],
EVP_CIPHER_CTX *ctx, EVP_MAC_CTX *hctx, int enc));
The following function has been deprecated since OpenSSL 3.0, and can be hidden entirely
by defining OPENSSL_API_COMPAT with a suitable version value, see openssl_user_macros(7):
int 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_evp_cb() sets a callback function 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 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_reset(3) and EVP_MAC_CTX_new(3) respectively.
For new sessions tickets, when the client doesn't present a session ticket, or an
attempted retrieval 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 arbitrary 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 <openssl/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(3).
The hmac context and digest can be set using EVP_MAC_CTX_set_params(3) with the
OSSL_MAC_PARAM_KEY and OSSL_MAC_PARAM_DIGEST parameters respectively.
When the client presents a session ticket, the callback function with be called with enc
set to 0 indicating that the cb function should retrieve 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 retrieved parameters and the initialization
vector iv. using a function like EVP_DecryptInit_ex(3). The key material and digest for
hctx need to be set using EVP_MAC_CTX_set_params(3) with the OSSL_MAC_PARAM_KEY and
OSSL_MAC_PARAM_DIGEST parameters respectively.
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 argument 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 negotiating 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.
The SSL_CTX_set_tlsext_ticket_key_cb() function is identical to
SSL_CTX_set_tlsext_ticket_key_evp_cb() except that it takes a deprecated HMAC_CTX pointer
instead of an EVP_MAC_CTX one. Before this callback function is started hctx will have
been initialised with EVP_MAC_CTX_new(3) and the digest set with
EVP_MAC_CTX_set_params(3). The hctx key material can be set using HMAC_Init_ex(3).
NOTES
Session resumption shortcuts the TLS so that the client certificate negotiation 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 negotiation had occurred.
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 cipher suite 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 cipher suite 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.
RETURN VALUES
Returns 1 to indicate the callback function was set and 0 otherwise.
EXAMPLES
Reference Implementation:
SSL_CTX_set_tlsext_ticket_key_evp_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,
EVP_MAC_CTX *hctx, int enc)
{
OSSL_PARAM params[3];
your_type_t *key; /* something that you need to implement */
if (enc) { /* create new session */
if (RAND_bytes(iv, EVP_MAX_IV_LENGTH) <= 0)
return -1; /* insufficient random */
key = currentkey(); /* something that you need to implement */
if (key == NULL) {
/* 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 == NULL) /* key couldn't be created */
return 0;
}
memcpy(key_name, key->name, 16);
if (EVP_EncryptInit_ex(&ctx, EVP_aes_256_cbc(), NULL, key->aes_key,
iv) == 0)
return -1; /* error in cipher initialisation */
params[0] = OSSL_PARAM_construct_octet_string(OSSL_MAC_PARAM_KEY,
key->hmac_key, 32);
params[1] = OSSL_PARAM_construct_utf8_string(OSSL_MAC_PARAM_DIGEST,
"sha256", 0);
params[2] = OSSL_PARAM_construct_end();
if (EVP_MAC_CTX_set_params(hctx, params) == 0)
return -1; /* error in mac initialisation */
return 1;
} else { /* retrieve session */
time_t t = time(NULL);
key = findkey(key_name); /* something that you need to implement */
if (key == NULL || key->expire < t)
return 0;
params[0] = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
key->hmac_key, 32);
params[1] = OSSL_PARAM_construct_utf8_string(OSSL_MAC_PARAM_DIGEST,
"sha256", 0);
params[2] = OSSL_PARAM_construct_end();
if (EVP_MAC_CTX_set_params(hctx, params) == 0)
return -1; /* error in mac initialisation */
if (EVP_DecryptInit_ex(&ctx, EVP_aes_256_cbc(), NULL, key->aes_key,
iv) == 0)
return -1; /* error in cipher initialisation */
if (key->expire < t - RENEW_TIME) { /* RENEW_TIME: implement */
/*
* return 2 - This session will get a new ticket even though the
* current one is still valid.
*/
return 2;
}
return 1;
}
}
SEE ALSO
ssl(7), 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
The SSL_CTX_set_tlsext_ticket_key_cb() function was deprecated in OpenSSL 3.0.
The SSL_CTX_set_tlsext_ticket_key_evp_cb() function was introduced in OpenSSL 3.0.
COPYRIGHT
Copyright 2014-2021 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>.