Provided by: libssl-doc_1.1.1c-1ubuntu3_all bug

NAME

       SSL_CTX_set_tlsext_ticket_key_cb - set a callback for session ticket processing

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 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 HMAC_CTX_reset(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 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 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.

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 0 to indicate the callback function was set.

EXAMPLES

       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)
        {
            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_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 == NULL || 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 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),

COPYRIGHT

       Copyright 2014-2019 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 <https://www.openssl.org/source/license.html>.