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       SSL_CTX_set_tlsext_ticket_key_cb	- set a	callback for session ticket

	#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 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

       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 HMAC_CTX_reset(3)

       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 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	can be set
       using HMAC_Init_ex(3).

       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 hctx needs to	be set using HMAC_Init_ex(3).

       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

       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.

       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

       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

       returns 0 to indicate the callback function was set.

       Reference Implementation:

	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)
	    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);

		EVP_EncryptInit_ex(&ctx, EVP_aes_256_cbc(), NULL, key->aes_key,	iv);
		HMAC_Init_ex(&hctx, key->hmac_key, 32, EVP_sha256(), NULL);

		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;

		HMAC_Init_ex(&hctx, key->hmac_key, 32, EVP_sha256(), NULL);
		EVP_DecryptInit_ex(&ctx, EVP_aes_256_cbc(), NULL, key->aes_key,	iv);

		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;

       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),

       Copyright 2014-2020 The OpenSSL Project Authors.	All Rights Reserved.

       Licensed	under the OpenSSL license (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

1.1.1h				  2020-09-2SSL_CTX_SET_TLSEXT_TICKET_KEY_CB(3)


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