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_init and HMAC_CTX_init respectively.
43 For new sessions tickets, when the client doesn't present a session
45 ticket, or an attempted retrieval of the ticket failed, or a renew
46 option was indicated, the callback function will be called with enc
47 equal to 1. The OpenSSL library expects that the function will set an
48 arbitrary name, initialize iv, and set the cipher context ctx and the
49 hash context hctx.
50 The name is 16 characters long and is used as a key identifier.
52 The iv length is the length of the IV of the corresponding cipher. The
54 maximum IV length is EVP_MAX_IV_LENGTH bytes defined in evp.h.
55 The initialization vector iv should be a random value. The cipher
57 context ctx should use the initialisation vector iv. The cipher context
58 can be set using EVP_EncryptInit_ex(3). The hmac context can be set
59 using HMAC_Init_ex(3).
60 When the client presents a session ticket, the callback function with
62 be called with enc set to 0 indicating that the cb function should
63 retrieve a set of parameters. In this case name and iv have already
64 been parsed out of the session ticket. The OpenSSL library expects that
65 the name will be used to retrieve a cryptographic parameters and that
66 the cryptographic context ctx will be set with the retrieved parameters
67 and the initialization vector iv. using a function like
68 EVP_DecryptInit_ex(3). The hctx needs to be set using HMAC_Init_ex(3).
69 If the 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
72 again with an argument of enc equal to 1 to set the new ticket.
73 The return value of the cb function is used by OpenSSL to determine
75 what further processing will occur. The following return values have
76 meaning:
77 2 This indicates that the ctx and hctx have been set and the session
79 can continue on those parameters. Additionally it indicates that
80 the session ticket is in a renewal period and should be replaced.
81 The OpenSSL library will call cb again with an enc argument of 1 to
82 set the new ticket (see RFC5077 3.3 paragraph 2).
83 1 This indicates that the ctx and hctx have been set and the session
85 can continue on those parameters.
86 0 This indicates that it was not possible to set/retrieve a session
88 ticket and the SSL/TLS session will continue by negotiating a set
89 of cryptographic parameters or using the alternate SSL/TLS
90 resumption mechanism, session ids.
91 If called with enc equal to 0 the library will call the cb again to
93 get a new set of parameters.
94 less than 0
96 This indicates an error.
97
99 Session resumption shortcuts the TLS so that the client certificate
100 negotiation don't occur. It makes up for this by storing client
101 certificate an all other negotiated state information encrypted within
102 the ticket. In a resumed session the applications will have all this
103 state information available exactly as if a full negotiation had
104 occurred.
105 If an attacker can obtain the key used to encrypt a session ticket,
107 they can obtain the master secret for any ticket using that key and
108 decrypt any traffic using that session: even if the cipher suite
109 supports forward secrecy. As a result applications may wish to use
110 multiple keys and avoid using long term keys stored in files.
111 Applications can use longer keys to maintain a consistent level of
113 security. For example if a cipher suite uses 256 bit ciphers but only
114 a 128 bit ticket key the overall security is only 128 bits because
115 breaking the ticket key will enable an attacker to obtain the session
116 keys.
117
119 Reference Implementation:
120 SSL_CTX_set_tlsext_ticket_key_cb(SSL, ssl_tlsext_ticket_key_cb);
122 ...
123 static int ssl_tlsext_ticket_key_cb(SSL *s, unsigned char key_name[16],
125 unsigned char *iv, EVP_CIPHER_CTX *ctx,
126 HMAC_CTX *hctx, int enc)
127 {
128 if (enc) { /* create new session */
129 if (RAND_bytes(iv, EVP_MAX_IV_LENGTH) <= 0)
130 return -1; /* insufficient random */
131 key = currentkey(); /* something that you need to implement */
133 if (key == NULL) {
134 /* current key doesn't exist or isn't valid */
135 key = createkey(); /*
136 * Something that you need to implement.
137 * createkey needs to initialise a name,
138 * an aes_key, a hmac_key and optionally
139 * an expire time.
140 */
141 if (key == NULL) /* key couldn't be created */
142 return 0;
143 }
144 memcpy(key_name, key->name, 16);
145 EVP_EncryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL, key->aes_key, iv);
147 HMAC_Init_ex(&hctx, key->hmac_key, 16, EVP_sha256(), NULL);
148 return 1;
150 } else { /* retrieve session */
152 key = findkey(name);
153 if (key == NULL || key->expire < now())
155 return 0;
156 HMAC_Init_ex(&hctx, key->hmac_key, 16, EVP_sha256(), NULL);
158 EVP_DecryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL, key->aes_key, iv);
159 if (key->expire < now() - RENEW_TIME) {
161 /*
162 * return 2 - This session will get a new ticket even though the
163 * current one is still valid.
164 */
165 return 2;
166 }
167 return 1;
168 }
169 }
170
172 returns 0 to indicate the callback function was set.
173
175 ssl(7), SSL_set_session(3), SSL_session_reused(3),
176 SSL_CTX_add_session(3), SSL_CTX_sess_number(3),
177 SSL_CTX_sess_set_get_cb(3), SSL_CTX_set_session_id_context(3),
178
180 Copyright 2014-2018 The OpenSSL Project Authors. All Rights Reserved.
181 Licensed under the OpenSSL license (the "License"). You may not use
183 this file except in compliance with the License. You can obtain a copy
184 in the file LICENSE in the source distribution or at
185 <https://www.openssl.org/source/license.html>.
1861.1.1 2018-09-11SSL_CTX_SET_TLSEXT_TICKET_KEY_CB(3)