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NetBSD 6.1.5 - man page for ssl_ctx_set_tlsext_ticket_key_cb (netbsd section 3)

SSL_CTX_set_tlsext_ticket_key_cb(3)	     OpenSSL	      SSL_CTX_set_tlsext_ticket_key_cb(3)

       SSL_CTX_set_tlsext_ticket_key_cb - set a callback for session ticket processing

       libcrypto, -lcrypto

	#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));

       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

       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

       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

       less than 0
	   This indicates an error.

       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.

       Reference Implemention:

	 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;


       returns 0 to indicate the callback function was set.

       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),

       This function was introduced in OpenSSL 0.9.8h

1.0.1i					    2014-08-10	      SSL_CTX_set_tlsext_ticket_key_cb(3)

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