1SSL_CTX_SET_TLSEXT_TICKET_KEY_CB(3) OpenSSLSSL_CTX_SET_TLSEXT_TICKET_KEY_CB(3)
2
6 SSL_CTX_set_tlsext_ticket_key_cb - set a callback for session ticket
7 processing
8
10 #include <openssl/tls1.h>
11 long SSL_CTX_set_tlsext_ticket_key_cb(SSL_CTX sslctx,
13 int (*cb)(SSL *s, unsigned char key_name[16],
14 unsigned char iv[EVP_MAX_IV_LENGTH],
15 EVP_CIPHER_CTX *ctx, HMAC_CTX *hctx, int enc));
16
18 SSL_CTX_set_tlsext_ticket_key_cb() sets a callback function cb for
19 handling session tickets for the ssl context sslctx. Session tickets,
20 defined in RFC5077 provide an enhanced session resumption capability
21 where the server implementation is not required to maintain per session
22 state. It only applies to TLS and there is no SSLv3 implementation.
23 The callback function cb will be called for every client instigated TLS
25 session when session ticket extension is presented in the TLS hello
26 message. It is the responsibility of this function to create or
27 retrieve the cryptographic parameters and to maintain their state.
28 The OpenSSL library uses your callback function to help implement a
30 common TLS ticket construction state according to RFC5077 Section 4
31 such that per session state is unnecessary and a small set of
32 cryptographic variables needs to be maintained by the callback function
33 implementation.
34 In order to reuse a session, a TLS client must send the a session
36 ticket extension to the server. The client can only send exactly one
37 session ticket. The server, through the callback function, either
38 agrees to reuse the session ticket information or it starts a full TLS
39 handshake to create a new session ticket.
40 Before the callback function is started ctx and hctx have been
42 initialised with EVP_CIPHER_CTX_reset(3) and HMAC_CTX_reset(3)
43 respectively.
44 For new sessions tickets, when the client doesn't present a session
46 ticket, or an attempted retrieval of the ticket failed, or a renew
47 option was indicated, the callback function will be called with enc
48 equal to 1. The OpenSSL library expects that the function will set an
49 arbitrary name, initialize iv, and set the cipher context ctx and the
50 hash context hctx.
51 The name is 16 characters long and is used as a key identifier.
53 The iv length is the length of the IV of the corresponding cipher. The
55 maximum IV length is EVP_MAX_IV_LENGTH bytes defined in evp.h.
56 The initialization vector iv should be a random value. The cipher
58 context ctx should use the initialisation vector iv. The cipher context
59 can be set using EVP_EncryptInit_ex(3). The hmac context can be set
60 using HMAC_Init_ex(3).
61 When the client presents a session ticket, the callback function with
63 be called with enc set to 0 indicating that the cb function should
64 retrieve a set of parameters. In this case name and iv have already
65 been parsed out of the session ticket. The OpenSSL library expects that
66 the name will be used to retrieve a cryptographic parameters and that
67 the cryptographic context ctx will be set with the retrieved parameters
68 and the initialization vector iv. using a function like
69 EVP_DecryptInit_ex(3). The hctx needs to be set using HMAC_Init_ex(3).
70 If the name is still valid but a renewal of the ticket is required the
72 callback function should return 2. The library will call the callback
73 again with an argument of enc equal to 1 to set the new ticket.
74 The return value of the cb function is used by OpenSSL to determine
76 what further processing will occur. The following return values have
77 meaning:
78 2 This indicates that the ctx and hctx have been set and the session
80 can continue on those parameters. Additionally it indicates that
81 the session ticket is in a renewal period and should be replaced.
82 The OpenSSL library will call cb again with an enc argument of 1 to
83 set the new ticket (see RFC5077 3.3 paragraph 2).
84 1 This indicates that the ctx and hctx have been set and the session
86 can continue on those parameters.
87 0 This indicates that it was not possible to set/retrieve a session
89 ticket and the SSL/TLS session will continue by negotiating a set
90 of cryptographic parameters or using the alternate SSL/TLS
91 resumption mechanism, session ids.
92 If called with enc equal to 0 the library will call the cb again to
94 get a new set of parameters.
95 less than 0
97 This indicates an error.
98
100 Session resumption shortcuts the TLS so that the client certificate
101 negotiation don't occur. It makes up for this by storing client
102 certificate an all other negotiated state information encrypted within
103 the ticket. In a resumed session the applications will have all this
104 state information available exactly as if a full negotiation had
105 occurred.
106 If an attacker can obtain the key used to encrypt a session ticket,
108 they can obtain the master secret for any ticket using that key and
109 decrypt any traffic using that session: even if the cipher suite
110 supports forward secrecy. As a result applications may wish to use
111 multiple keys and avoid using long term keys stored in files.
112 Applications can use longer keys to maintain a consistent level of
114 security. For example if a cipher suite uses 256 bit ciphers but only
115 a 128 bit ticket key the overall security is only 128 bits because
116 breaking the ticket key will enable an attacker to obtain the session
117 keys.
118
120 returns 0 to indicate the callback function was set.
121
123 Reference Implementation:
124 SSL_CTX_set_tlsext_ticket_key_cb(SSL, ssl_tlsext_ticket_key_cb);
126 ...
127 static int ssl_tlsext_ticket_key_cb(SSL *s, unsigned char key_name[16],
129 unsigned char *iv, EVP_CIPHER_CTX *ctx,
130 HMAC_CTX *hctx, int enc)
131 {
132 your_type_t *key; /* something that you need to implement */
133 if (enc) { /* create new session */
135 if (RAND_bytes(iv, EVP_MAX_IV_LENGTH) <= 0)
136 return -1; /* insufficient random */
137 key = currentkey(); /* something that you need to implement */
139 if (key == NULL) {
140 /* current key doesn't exist or isn't valid */
141 key = createkey(); /*
142 * 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 */
147 if (key == NULL) /* key couldn't be created */
148 return 0;
149 }
150 memcpy(key_name, key->name, 16);
151 EVP_EncryptInit_ex(&ctx, EVP_aes_256_cbc(), NULL, key->aes_key, iv);
153 HMAC_Init_ex(&hctx, key->hmac_key, 32, EVP_sha256(), NULL);
154 return 1;
156 } else { /* retrieve session */
158 time_t t = time(NULL);
159 key = findkey(key_name); /* something that you need to implement */
160 if (key == NULL || key->expire < t)
162 return 0;
163 HMAC_Init_ex(&hctx, key->hmac_key, 32, EVP_sha256(), NULL);
165 EVP_DecryptInit_ex(&ctx, EVP_aes_256_cbc(), NULL, key->aes_key, iv);
166 if (key->expire < t - RENEW_TIME) { /* RENEW_TIME: implement */
168 /*
169 * return 2 - This session will get a new ticket even though the
170 * current one is still valid.
171 */
172 return 2;
173 }
174 return 1;
175 }
176 }
177
179 ssl(7), SSL_set_session(3), SSL_session_reused(3),
180 SSL_CTX_add_session(3), SSL_CTX_sess_number(3),
181 SSL_CTX_sess_set_get_cb(3), SSL_CTX_set_session_id_context(3),
182
184 Copyright 2014-2020 The OpenSSL Project Authors. All Rights Reserved.
185 Licensed under the OpenSSL license (the "License"). You may not use
187 this file except in compliance with the License. You can obtain a copy
188 in the file LICENSE in the source distribution or at
189 <https://www.openssl.org/source/license.html>.
1901.1.1i 2021-07-22SSL_CTX_SET_TLSEXT_TICKET_KEY_CB(3)