1=pod 2 3=head1 NAME 4 5SSL_CTX_set_tlsext_ticket_key_cb - set a callback for session ticket processing 6 7=head1 SYNOPSIS 8 9 #include <openssl/tls1.h> 10 11 long SSL_CTX_set_tlsext_ticket_key_cb(SSL_CTX sslctx, 12 int (*cb)(SSL *s, unsigned char key_name[16], 13 unsigned char iv[EVP_MAX_IV_LENGTH], 14 EVP_CIPHER_CTX *ctx, HMAC_CTX *hctx, int enc)); 15 16=head1 DESCRIPTION 17 18SSL_CTX_set_tlsext_ticket_key_cb() sets a callback fuction I<cb> for handling 19session tickets for the ssl context I<sslctx>. Session tickets, defined in 20RFC5077 provide an enhanced session resumption capability where the server 21implementation is not required to maintain per session state. It only applies 22to TLS and there is no SSLv3 implementation. 23 24The callback is available when the OpenSSL library was built without 25I<OPENSSL_NO_TLSEXT> being defined. 26 27The callback function I<cb> will be called for every client instigated TLS 28session when session ticket extension is presented in the TLS hello 29message. It is the responsibility of this function to create or retrieve the 30cryptographic parameters and to maintain their state. 31 32The OpenSSL library uses your callback function to help implement a common TLS 33ticket construction state according to RFC5077 Section 4 such that per session 34state is unnecessary and a small set of cryptographic variables needs to be 35maintained by the callback function implementation. 36 37In order to reuse a session, a TLS client must send the a session ticket 38extension to the server. The client can only send exactly one session ticket. 39The server, through the callback function, either agrees to reuse the session 40ticket information or it starts a full TLS handshake to create a new session 41ticket. 42 43Before the callback function is started I<ctx> and I<hctx> have been 44initialised with EVP_CIPHER_CTX_init and HMAC_CTX_init respectively. 45 46For new sessions tickets, when the client doesn't present a session ticket, or 47an attempted retreival of the ticket failed, or a renew option was indicated, 48the callback function will be called with I<enc> equal to 1. The OpenSSL 49library expects that the function will set an arbitary I<name>, initialize 50I<iv>, and set the cipher context I<ctx> and the hash context I<hctx>. 51 52The I<name> is 16 characters long and is used as a key identifier. 53 54The I<iv> length is the length of the IV of the corresponding cipher. The 55maximum IV length is L<EVP_MAX_IV_LENGTH> bytes defined in B<evp.h>. 56 57The initialization vector I<iv> should be a random value. The cipher context 58I<ctx> should use the initialisation vector I<iv>. The cipher context can be 59set using L<EVP_EncryptInit_ex>. The hmac context can be set using L<HMAC_Init_ex>. 60 61When the client presents a session ticket, the callback function with be called 62with I<enc> set to 0 indicating that the I<cb> function should retreive a set 63of parameters. In this case I<name> and I<iv> have already been parsed out of 64the session ticket. The OpenSSL library expects that the I<name> will be used 65to retrieve a cryptographic parameters and that the cryptographic context 66I<ctx> will be set with the retreived parameters and the initialization vector 67I<iv>. using a function like L<EVP_DecryptInit_ex>. The I<hctx> needs to be set 68using L<HMAC_Init_ex>. 69 70If the I<name> is still valid but a renewal of the ticket is required the 71callback function should return 2. The library will call the callback again 72with an arguement of enc equal to 1 to set the new ticket. 73 74The return value of the I<cb> function is used by OpenSSL to determine what 75further processing will occur. The following return values have meaning: 76 77=over 4 78 79=item Z<>2 80 81This indicates that the I<ctx> and I<hctx> have been set and the session can 82continue on those parameters. Additionally it indicates that the session 83ticket is in a renewal period and should be replaced. The OpenSSL library will 84call I<cb> again with an enc argument of 1 to set the new ticket (see RFC5077 853.3 paragraph 2). 86 87=item Z<>1 88 89This indicates that the I<ctx> and I<hctx> have been set and the session can 90continue on those parameters. 91 92=item Z<>0 93 94This indicates that it was not possible to set/retrieve a session ticket and 95the SSL/TLS session will continue by by negiotationing a set of cryptographic 96parameters or using the alternate SSL/TLS resumption mechanism, session ids. 97 98If called with enc equal to 0 the library will call the I<cb> again to get 99a new set of parameters. 100 101=item less than 0 102 103This indicates an error. 104 105=back 106 107=head1 NOTES 108 109Session resumption shortcuts the TLS so that the client certificate 110negiotation don't occur. It makes up for this by storing client certificate 111an all other negotiated state information encrypted within the ticket. In a 112resumed session the applications will have all this state information available 113exactly as if a full negiotation had occured. 114 115If an attacker can obtain the key used to encrypt a session ticket, they can 116obtain the master secret for any ticket using that key and decrypt any traffic 117using that session: even if the ciphersuite supports forward secrecy. As 118a result applications may wish to use multiple keys and avoid using long term 119keys stored in files. 120 121Applications can use longer keys to maintain a consistent level of security. 122For example if a ciphersuite uses 256 bit ciphers but only a 128 bit ticket key 123the overall security is only 128 bits because breaking the ticket key will 124enable an attacker to obtain the session keys. 125 126=head1 EXAMPLES 127 128Reference Implemention: 129 SSL_CTX_set_tlsext_ticket_key_cb(SSL,ssl_tlsext_ticket_key_cb); 130 .... 131 132 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) 133 { 134 if (enc) { /* create new session */ 135 if (RAND_bytes(iv, EVP_MAX_IV_LENGTH) ) { 136 return -1; /* insufficient random */ 137 } 138 139 key = currentkey(); /* something that you need to implement */ 140 if ( !key ) { 141 /* current key doesn't exist or isn't valid */ 142 key = createkey(); /* something that you need to implement. 143 * createkey needs to initialise, a name, 144 * an aes_key, a hmac_key and optionally 145 * an expire time. */ 146 if ( !key ) { /* key couldn't be created */ 147 return 0; 148 } 149 } 150 memcpy(key_name, key->name, 16); 151 152 EVP_EncryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL, key->aes_key, iv); 153 HMAC_Init_ex(&hctx, key->hmac_key, 16, EVP_sha256(), NULL); 154 155 return 1; 156 157 } else { /* retrieve session */ 158 key = findkey(name); 159 160 if (!key || key->expire < now() ) { 161 return 0; 162 } 163 164 HMAC_Init_ex(&hctx, key->hmac_key, 16, EVP_sha256(), NULL); 165 EVP_DecryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL, key->aes_key, iv ); 166 167 if (key->expire < ( now() - RENEW_TIME ) ) { 168 /* return 2 - this session will get a new ticket even though the current is still valid */ 169 return 2; 170 } 171 return 1; 172 173 } 174 } 175 176 177 178=head1 RETURN VALUES 179 180returns 0 to indicate the callback function was set. 181 182=head1 SEE ALSO 183 184L<ssl(3)|ssl(3)>, L<SSL_set_session(3)|SSL_set_session(3)>, 185L<SSL_session_reused(3)|SSL_session_reused(3)>, 186L<SSL_CTX_add_session(3)|SSL_CTX_add_session(3)>, 187L<SSL_CTX_sess_number(3)|SSL_CTX_sess_number(3)>, 188L<SSL_CTX_sess_set_get_cb(3)|SSL_CTX_sess_set_get_cb(3)>, 189L<SSL_CTX_set_session_id_context(3)|SSL_CTX_set_session_id_context(3)>, 190 191=head1 HISTORY 192 193This function was introduced in OpenSSL 0.9.8h 194 195=cut 196