1SSL_CTX_SET_TLSEXT_TICKET_KEY_CB(3ossOlp)enSSSSLL_CTX_SET_TLSEXT_TICKET_KEY_CB(3ossl)
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NAME

6       SSL_CTX_set_tlsext_ticket_key_evp_cb, SSL_CTX_set_tlsext_ticket_key_cb
7       - set a callback for session ticket processing
8

SYNOPSIS

10        #include <openssl/tls1.h>
11
12        int SSL_CTX_set_tlsext_ticket_key_evp_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, EVP_MAC_CTX *hctx, int enc));
16
17       The following function has been deprecated since OpenSSL 3.0, and can
18       be hidden entirely by defining OPENSSL_API_COMPAT with a suitable
19       version value, see openssl_user_macros(7):
20
21        int SSL_CTX_set_tlsext_ticket_key_cb(SSL_CTX sslctx,
22            int (*cb)(SSL *s, unsigned char key_name[16],
23                      unsigned char iv[EVP_MAX_IV_LENGTH],
24                      EVP_CIPHER_CTX *ctx, HMAC_CTX *hctx, int enc));
25

DESCRIPTION

27       SSL_CTX_set_tlsext_ticket_key_evp_cb() sets a callback function cb for
28       handling session tickets for the ssl context sslctx. Session tickets,
29       defined in RFC5077 provide an enhanced session resumption capability
30       where the server implementation is not required to maintain per session
31       state. It only applies to TLS and there is no SSLv3 implementation.
32
33       The callback function cb will be called for every client instigated TLS
34       session when session ticket extension is presented in the TLS hello
35       message. It is the responsibility of this function to create or
36       retrieve the cryptographic parameters and to maintain their state.
37
38       The OpenSSL library uses your callback function to help implement a
39       common TLS ticket construction state according to RFC5077 Section 4
40       such that per session state is unnecessary and a small set of
41       cryptographic variables needs to be maintained by the callback function
42       implementation.
43
44       In order to reuse a session, a TLS client must send the a session
45       ticket extension to the server. The client can only send exactly one
46       session ticket.  The server, through the callback function, either
47       agrees to reuse the session ticket information or it starts a full TLS
48       handshake to create a new session ticket.
49
50       Before the callback function is started ctx and hctx have been
51       initialised with EVP_CIPHER_CTX_reset(3) and EVP_MAC_CTX_new(3)
52       respectively.
53
54       For new sessions tickets, when the client doesn't present a session
55       ticket, or an attempted retrieval of the ticket failed, or a renew
56       option was indicated, the callback function will be called with enc
57       equal to 1. The OpenSSL library expects that the function will set an
58       arbitrary name, initialize iv, and set the cipher context ctx and the
59       hash context hctx.
60
61       The name is 16 characters long and is used as a key identifier.
62
63       The iv length is the length of the IV of the corresponding cipher. The
64       maximum IV length is EVP_MAX_IV_LENGTH bytes defined in
65       <openssl/evp.h>.
66
67       The initialization vector iv should be a random value. The cipher
68       context ctx should use the initialisation vector iv. The cipher context
69       can be set using EVP_EncryptInit_ex(3). The hmac context and digest can
70       be set using EVP_MAC_CTX_set_params(3) with the OSSL_MAC_PARAM_KEY and
71       OSSL_MAC_PARAM_DIGEST parameters respectively.
72
73       When the client presents a session ticket, the callback function with
74       be called with enc set to 0 indicating that the cb function should
75       retrieve a set of parameters. In this case name and iv have already
76       been parsed out of the session ticket. The OpenSSL library expects that
77       the name will be used to retrieve a cryptographic parameters and that
78       the cryptographic context ctx will be set with the retrieved parameters
79       and the initialization vector iv. using a function like
80       EVP_DecryptInit_ex(3). The key material and digest for hctx need to be
81       set using EVP_MAC_CTX_set_params(3) with the OSSL_MAC_PARAM_KEY and
82       OSSL_MAC_PARAM_DIGEST parameters respectively.
83
84       If the name is still valid but a renewal of the ticket is required the
85       callback function should return 2. The library will call the callback
86       again with an argument of enc equal to 1 to set the new ticket.
87
88       The return value of the cb function is used by OpenSSL to determine
89       what further processing will occur. The following return values have
90       meaning:
91
92       2   This indicates that the ctx and hctx have been set and the session
93           can continue on those parameters. Additionally it indicates that
94           the session ticket is in a renewal period and should be replaced.
95           The OpenSSL library will call cb again with an enc argument of 1 to
96           set the new ticket (see RFC5077 3.3 paragraph 2).
97
98       1   This indicates that the ctx and hctx have been set and the session
99           can continue on those parameters.
100
101       0   This indicates that it was not possible to set/retrieve a session
102           ticket and the SSL/TLS session will continue by negotiating a set
103           of cryptographic parameters or using the alternate SSL/TLS
104           resumption mechanism, session ids.
105
106           If called with enc equal to 0 the library will call the cb again to
107           get a new set of parameters.
108
109       less than 0
110           This indicates an error.
111
112       The SSL_CTX_set_tlsext_ticket_key_cb() function is identical to
113       SSL_CTX_set_tlsext_ticket_key_evp_cb() except that it takes a
114       deprecated HMAC_CTX pointer instead of an EVP_MAC_CTX one.  Before this
115       callback function is started hctx will have been initialised with
116       EVP_MAC_CTX_new(3) and the digest set with EVP_MAC_CTX_set_params(3).
117       The hctx key material can be set using HMAC_Init_ex(3).
118

NOTES

120       Session resumption shortcuts the TLS so that the client certificate
121       negotiation don't occur. It makes up for this by storing client
122       certificate an all other negotiated state information encrypted within
123       the ticket. In a resumed session the applications will have all this
124       state information available exactly as if a full negotiation had
125       occurred.
126
127       If an attacker can obtain the key used to encrypt a session ticket,
128       they can obtain the master secret for any ticket using that key and
129       decrypt any traffic using that session: even if the cipher suite
130       supports forward secrecy. As a result applications may wish to use
131       multiple keys and avoid using long term keys stored in files.
132
133       Applications can use longer keys to maintain a consistent level of
134       security.  For example if a cipher suite uses 256 bit ciphers but only
135       a 128 bit ticket key the overall security is only 128 bits because
136       breaking the ticket key will enable an attacker to obtain the session
137       keys.
138

RETURN VALUES

140       Returns 1 to indicate the callback function was set and 0 otherwise.
141

EXAMPLES

143       Reference Implementation:
144
145        SSL_CTX_set_tlsext_ticket_key_evp_cb(SSL, ssl_tlsext_ticket_key_cb);
146        ...
147
148        static int ssl_tlsext_ticket_key_cb(SSL *s, unsigned char key_name[16],
149                                            unsigned char *iv, EVP_CIPHER_CTX *ctx,
150                                            EVP_MAC_CTX *hctx, int enc)
151        {
152            OSSL_PARAM params[3];
153            your_type_t *key; /* something that you need to implement */
154
155            if (enc) { /* create new session */
156                if (RAND_bytes(iv, EVP_MAX_IV_LENGTH) <= 0)
157                    return -1; /* insufficient random */
158
159                key = currentkey(); /* something that you need to implement */
160                if (key == NULL) {
161                    /* current key doesn't exist or isn't valid */
162                    key = createkey(); /*
163                                        * Something that you need to implement.
164                                        * createkey needs to initialise a name,
165                                        * an aes_key, a hmac_key and optionally
166                                        * an expire time.
167                                        */
168                    if (key == NULL) /* key couldn't be created */
169                        return 0;
170                }
171                memcpy(key_name, key->name, 16);
172
173                if (EVP_EncryptInit_ex(&ctx, EVP_aes_256_cbc(), NULL, key->aes_key,
174                                       iv) == 0)
175                   return -1; /* error in cipher initialisation */
176
177                params[0] = OSSL_PARAM_construct_octet_string(OSSL_MAC_PARAM_KEY,
178                                                              key->hmac_key, 32);
179                params[1] = OSSL_PARAM_construct_utf8_string(OSSL_MAC_PARAM_DIGEST,
180                                                             "sha256", 0);
181                params[2] = OSSL_PARAM_construct_end();
182                if (EVP_MAC_CTX_set_params(hctx, params) == 0)
183                   return -1; /* error in mac initialisation */
184
185                return 1;
186
187            } else { /* retrieve session */
188                time_t t = time(NULL);
189                key = findkey(key_name); /* something that you need to implement */
190
191                if (key == NULL || key->expire < t)
192                    return 0;
193
194                params[0] = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
195                                                              key->hmac_key, 32);
196                params[1] = OSSL_PARAM_construct_utf8_string(OSSL_MAC_PARAM_DIGEST,
197                                                             "sha256", 0);
198                params[2] = OSSL_PARAM_construct_end();
199                if (EVP_MAC_CTX_set_params(hctx, params) == 0)
200                   return -1; /* error in mac initialisation */
201
202                if (EVP_DecryptInit_ex(&ctx, EVP_aes_256_cbc(), NULL, key->aes_key,
203                                       iv) == 0)
204                   return -1; /* error in cipher initialisation */
205
206                if (key->expire < t - RENEW_TIME) { /* RENEW_TIME: implement */
207                    /*
208                     * return 2 - This session will get a new ticket even though the
209                     * current one is still valid.
210                     */
211                    return 2;
212                }
213                return 1;
214            }
215        }
216

SEE ALSO

218       ssl(7), SSL_set_session(3), SSL_session_reused(3),
219       SSL_CTX_add_session(3), SSL_CTX_sess_number(3),
220       SSL_CTX_sess_set_get_cb(3), SSL_CTX_set_session_id_context(3),
221

HISTORY

223       The SSL_CTX_set_tlsext_ticket_key_cb() function was deprecated in
224       OpenSSL 3.0.
225
226       The SSL_CTX_set_tlsext_ticket_key_evp_cb() function was introduced in
227       OpenSSL 3.0.
228
230       Copyright 2014-2021 The OpenSSL Project Authors. All Rights Reserved.
231
232       Licensed under the Apache License 2.0 (the "License").  You may not use
233       this file except in compliance with the License.  You can obtain a copy
234       in the file LICENSE in the source distribution or at
235       <https://www.openssl.org/source/license.html>.
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2393.1.1                             2023-0S8S-L3_1CTX_SET_TLSEXT_TICKET_KEY_CB(3ossl)
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