5 SSL_CTX_set_tlsext_ticket_key_cb - set a callback for session ticket processing
9 #include <openssl/tls1.h>
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));
18 SSL_CTX_set_tlsext_ticket_key_cb() sets a callback fuction I<cb> for handling
19 session tickets for the ssl context I<sslctx>. Session tickets, defined in
20 RFC5077 provide an enhanced session resumption capability where the server
21 implementation is not required to maintain per session state. It only applies
22 to TLS and there is no SSLv3 implementation.
24 The callback is available when the OpenSSL library was built without
25 I<OPENSSL_NO_TLSEXT> being defined.
27 The callback function I<cb> will be called for every client instigated TLS
28 session when session ticket extension is presented in the TLS hello
29 message. It is the responsibility of this function to create or retrieve the
30 cryptographic parameters and to maintain their state.
32 The OpenSSL library uses your callback function to help implement a common TLS
33 ticket construction state according to RFC5077 Section 4 such that per session
34 state is unnecessary and a small set of cryptographic variables needs to be
35 maintained by the callback function implementation.
37 In order to reuse a session, a TLS client must send the a session ticket
38 extension to the server. The client can only send exactly one session ticket.
39 The server, through the callback function, either agrees to reuse the session
40 ticket information or it starts a full TLS handshake to create a new session
43 Before the callback function is started I<ctx> and I<hctx> have been
44 initialised with EVP_CIPHER_CTX_init and HMAC_CTX_init respectively.
46 For new sessions tickets, when the client doesn't present a session ticket, or
47 an attempted retreival of the ticket failed, or a renew option was indicated,
48 the callback function will be called with I<enc> equal to 1. The OpenSSL
49 library expects that the function will set an arbitary I<name>, initialize
50 I<iv>, and set the cipher context I<ctx> and the hash context I<hctx>.
52 The I<name> is 16 characters long and is used as a key identifier.
54 The I<iv> length is the length of the IV of the corresponding cipher. The
55 maximum IV length is L<EVP_MAX_IV_LENGTH> bytes defined in B<evp.h>.
57 The initialization vector I<iv> should be a random value. The cipher context
58 I<ctx> should use the initialisation vector I<iv>. The cipher context can be
59 set using L<EVP_EncryptInit_ex>. The hmac context can be set using L<HMAC_Init_ex>.
61 When the client presents a session ticket, the callback function with be called
62 with I<enc> set to 0 indicating that the I<cb> function should retreive a set
63 of parameters. In this case I<name> and I<iv> have already been parsed out of
64 the session ticket. The OpenSSL library expects that the I<name> will be used
65 to retrieve a cryptographic parameters and that the cryptographic context
66 I<ctx> will be set with the retreived parameters and the initialization vector
67 I<iv>. using a function like L<EVP_DecryptInit_ex>. The I<hctx> needs to be set
68 using L<HMAC_Init_ex>.
70 If the I<name> is still valid but a renewal of the ticket is required the
71 callback function should return 2. The library will call the callback again
72 with an arguement of enc equal to 1 to set the new ticket.
74 The return value of the I<cb> function is used by OpenSSL to determine what
75 further processing will occur. The following return values have meaning:
81 This indicates that the I<ctx> and I<hctx> have been set and the session can
82 continue on those parameters. Additionally it indicates that the session
83 ticket is in a renewal period and should be replaced. The OpenSSL library will
84 call I<cb> again with an enc argument of 1 to set the new ticket (see RFC5077
89 This indicates that the I<ctx> and I<hctx> have been set and the session can
90 continue on those parameters.
94 This indicates that it was not possible to set/retrieve a session ticket and
95 the SSL/TLS session will continue by by negiotationing a set of cryptographic
96 parameters or using the alternate SSL/TLS resumption mechanism, session ids.
98 If called with enc equal to 0 the library will call the I<cb> again to get
99 a new set of parameters.
103 This indicates an error.
109 Session resumption shortcuts the TLS so that the client certificate
110 negiotation don't occur. It makes up for this by storing client certificate
111 an all other negotiated state information encrypted within the ticket. In a
112 resumed session the applications will have all this state information available
113 exactly as if a full negiotation had occured.
115 If an attacker can obtain the key used to encrypt a session ticket, they can
116 obtain the master secret for any ticket using that key and decrypt any traffic
117 using that session: even if the ciphersuite supports forward secrecy. As
118 a result applications may wish to use multiple keys and avoid using long term
119 keys stored in files.
121 Applications can use longer keys to maintain a consistent level of security.
122 For example if a ciphersuite uses 256 bit ciphers but only a 128 bit ticket key
123 the overall security is only 128 bits because breaking the ticket key will
124 enable an attacker to obtain the session keys.
128 Reference Implemention:
129 SSL_CTX_set_tlsext_ticket_key_cb(SSL,ssl_tlsext_ticket_key_cb);
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)
134 if (enc) { /* create new session */
135 if (RAND_bytes(iv, EVP_MAX_IV_LENGTH) ) {
136 return -1; /* insufficient random */
139 key = currentkey(); /* something that you need to implement */
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
146 if ( !key ) { /* key couldn't be created */
150 memcpy(key_name, key->name, 16);
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);
157 } else { /* retrieve session */
160 if (!key || key->expire < now() ) {
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 );
167 if (key->expire < ( now() - RENEW_TIME ) ) {
168 /* return 2 - this session will get a new ticket even though the current is still valid */
180 returns 0 to indicate the callback function was set.
184 L<ssl(3)|ssl(3)>, L<SSL_set_session(3)|SSL_set_session(3)>,
185 L<SSL_session_reused(3)|SSL_session_reused(3)>,
186 L<SSL_CTX_add_session(3)|SSL_CTX_add_session(3)>,
187 L<SSL_CTX_sess_number(3)|SSL_CTX_sess_number(3)>,
188 L<SSL_CTX_sess_set_get_cb(3)|SSL_CTX_sess_set_get_cb(3)>,
189 L<SSL_CTX_set_session_id_context(3)|SSL_CTX_set_session_id_context(3)>,
193 This function was introduced in OpenSSL 0.9.8h