1=pod
2
3=head1 NAME
4
5SSL_CTX_set_tlsext_ticket_key_evp_cb,
6SSL_CTX_set_tlsext_ticket_key_cb
7- set a callback for session ticket processing
8
9=head1 SYNOPSIS
10
11 #include <openssl/tls1.h>
12
13 int SSL_CTX_set_tlsext_ticket_key_evp_cb(SSL_CTX sslctx,
14     int (*cb)(SSL *s, unsigned char key_name[16],
15               unsigned char iv[EVP_MAX_IV_LENGTH],
16               EVP_CIPHER_CTX *ctx, EVP_MAC_CTX *hctx, int enc));
17
18The following function has been deprecated since OpenSSL 3.0, and can be
19hidden entirely by defining B<OPENSSL_API_COMPAT> with a suitable version value,
20see L<openssl_user_macros(7)>:
21
22 int SSL_CTX_set_tlsext_ticket_key_cb(SSL_CTX sslctx,
23     int (*cb)(SSL *s, unsigned char key_name[16],
24               unsigned char iv[EVP_MAX_IV_LENGTH],
25               EVP_CIPHER_CTX *ctx, HMAC_CTX *hctx, int enc));
26
27=head1 DESCRIPTION
28
29SSL_CTX_set_tlsext_ticket_key_evp_cb() sets a callback function I<cb> for handling
30session tickets for the ssl context I<sslctx>. Session tickets, defined in
31RFC5077 provide an enhanced session resumption capability where the server
32implementation is not required to maintain per session state. It only applies
33to TLS and there is no SSLv3 implementation.
34
35The callback function I<cb> will be called for every client instigated TLS
36session when session ticket extension is presented in the TLS hello
37message. It is the responsibility of this function to create or retrieve the
38cryptographic parameters and to maintain their state.
39
40The OpenSSL library uses your callback function to help implement a common TLS
41ticket construction state according to RFC5077 Section 4 such that per session
42state is unnecessary and a small set of cryptographic variables needs to be
43maintained by the callback function implementation.
44
45In order to reuse a session, a TLS client must send the session ticket
46extension to the server. The client must send exactly one session ticket.
47The server, through the callback function, either agrees to reuse the session
48ticket information or it starts a full TLS handshake to create a new session
49ticket.
50
51Before the callback function is started I<ctx> and I<hctx> have been
52initialised with L<EVP_CIPHER_CTX_reset(3)> and L<EVP_MAC_CTX_new(3)>
53respectively.
54
55For new sessions tickets, when the client doesn't present a session ticket, or
56an attempted retrieval of the ticket failed, or a renew option was indicated,
57the callback function will be called with I<enc> equal to 1. The OpenSSL
58library expects that the function will set an arbitrary I<name>, initialize
59I<iv>, and set the cipher context I<ctx> and the hash context I<hctx>.
60
61The I<name> is 16 characters long and is used as a key identifier.
62
63The I<iv> length is the length of the IV of the corresponding cipher. The
64maximum IV length is B<EVP_MAX_IV_LENGTH> bytes defined in F<< <openssl/evp.h> >>.
65
66The initialization vector I<iv> should be a random value. The cipher context
67I<ctx> should use the initialisation vector I<iv>. The cipher context can be
68set using L<EVP_EncryptInit_ex(3)>. The hmac context and digest can be set using
69L<EVP_MAC_CTX_set_params(3)> with the B<OSSL_MAC_PARAM_KEY> and
70B<OSSL_MAC_PARAM_DIGEST> parameters respectively.
71
72When the client presents a session ticket, the callback function with be called
73with I<enc> set to 0 indicating that the I<cb> function should retrieve a set
74of parameters. In this case I<name> and I<iv> have already been parsed out of
75the session ticket. The OpenSSL library expects that the I<name> will be used
76to retrieve a cryptographic parameters and that the cryptographic context
77I<ctx> will be set with the retrieved parameters and the initialization vector
78I<iv>. using a function like L<EVP_DecryptInit_ex(3)>. The key material and
79digest for I<hctx> need to be set using L<EVP_MAC_CTX_set_params(3)> with the
80B<OSSL_MAC_PARAM_KEY> and B<OSSL_MAC_PARAM_DIGEST> parameters respectively.
81
82If the I<name> is still valid but a renewal of the ticket is required the
83callback function should return 2. The library will call the callback again
84with an argument of enc equal to 1 to set the new ticket.
85
86The return value of the I<cb> function is used by OpenSSL to determine what
87further processing will occur. The following return values have meaning:
88
89=over 4
90
91=item Z<>2
92
93This indicates that the I<ctx> and I<hctx> have been set and the session can
94continue on those parameters. Additionally it indicates that the session
95ticket is in a renewal period and should be replaced. The OpenSSL library will
96call I<cb> again with an enc argument of 1 to set the new ticket (see RFC5077
973.3 paragraph 2).
98
99=item Z<>1
100
101This indicates that the I<ctx> and I<hctx> have been set and the session can
102continue on those parameters.
103
104=item Z<>0
105
106This indicates that it was not possible to set/retrieve a session ticket and
107the SSL/TLS session will continue by negotiating a set of cryptographic
108parameters or using the alternate SSL/TLS resumption mechanism, session ids.
109
110If called with enc equal to 0 the library will call the I<cb> again to get
111a new set of parameters.
112
113=item less than 0
114
115This indicates an error.
116
117=back
118
119The SSL_CTX_set_tlsext_ticket_key_cb() function is identical to
120SSL_CTX_set_tlsext_ticket_key_evp_cb() except that it takes a deprecated
121HMAC_CTX pointer instead of an EVP_MAC_CTX one.
122Before this callback function is started I<hctx> will have been
123initialised with L<EVP_MAC_CTX_new(3)> and the digest set with
124L<EVP_MAC_CTX_set_params(3)>.
125The I<hctx> key material can be set using L<HMAC_Init_ex(3)>.
126
127=head1 NOTES
128
129Session resumption shortcuts the TLS handshake so that the client certificate
130negotiation doesn't occur. It makes up for this by storing the client certificate
131and all other negotiated state information encrypted within the ticket. In a
132resumed session the applications will have all this state information available
133exactly as if a full negotiation had occurred.
134
135If an attacker can obtain the key used to encrypt a session ticket, they can
136obtain the master secret for any ticket using that key and decrypt any traffic
137using that session: even if the cipher suite supports forward secrecy. As
138a result applications may wish to use multiple keys and avoid using long term
139keys stored in files.
140
141Applications can use longer keys to maintain a consistent level of security.
142For example if a cipher suite uses 256 bit ciphers but only a 128 bit ticket key
143the overall security is only 128 bits because breaking the ticket key will
144enable an attacker to obtain the session keys.
145
146=head1 RETURN VALUES
147
148Returns 1 to indicate the callback function was set and 0 otherwise.
149
150=head1 EXAMPLES
151
152Reference Implementation:
153
154 SSL_CTX_set_tlsext_ticket_key_evp_cb(SSL, ssl_tlsext_ticket_key_cb);
155 ...
156
157 static int ssl_tlsext_ticket_key_cb(SSL *s, unsigned char key_name[16],
158                                     unsigned char *iv, EVP_CIPHER_CTX *ctx,
159                                     EVP_MAC_CTX *hctx, int enc)
160 {
161     OSSL_PARAM params[3];
162     your_type_t *key; /* something that you need to implement */
163
164     if (enc) { /* create new session */
165         if (RAND_bytes(iv, EVP_MAX_IV_LENGTH) <= 0)
166             return -1; /* insufficient random */
167
168         key = currentkey(); /* something that you need to implement */
169         if (key == NULL) {
170             /* current key doesn't exist or isn't valid */
171             key = createkey(); /*
172                                 * Something that you need to implement.
173                                 * createkey needs to initialise a name,
174                                 * an aes_key, a hmac_key and optionally
175                                 * an expire time.
176                                 */
177             if (key == NULL) /* key couldn't be created */
178                 return 0;
179         }
180         memcpy(key_name, key->name, 16);
181
182         if (EVP_EncryptInit_ex(&ctx, EVP_aes_256_cbc(), NULL, key->aes_key,
183                                iv) == 0)
184            return -1; /* error in cipher initialisation */
185
186         params[0] = OSSL_PARAM_construct_octet_string(OSSL_MAC_PARAM_KEY,
187                                                       key->hmac_key, 32);
188         params[1] = OSSL_PARAM_construct_utf8_string(OSSL_MAC_PARAM_DIGEST,
189                                                      "sha256", 0);
190         params[2] = OSSL_PARAM_construct_end();
191         if (EVP_MAC_CTX_set_params(hctx, params) == 0)
192            return -1; /* error in mac initialisation */
193
194         return 1;
195
196     } else { /* retrieve session */
197         time_t t = time(NULL);
198         key = findkey(key_name); /* something that you need to implement */
199
200         if (key == NULL || key->expire < t)
201             return 0;
202
203         params[0] = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
204                                                       key->hmac_key, 32);
205         params[1] = OSSL_PARAM_construct_utf8_string(OSSL_MAC_PARAM_DIGEST,
206                                                      "sha256", 0);
207         params[2] = OSSL_PARAM_construct_end();
208         if (EVP_MAC_CTX_set_params(hctx, params) == 0)
209            return -1; /* error in mac initialisation */
210
211         if (EVP_DecryptInit_ex(&ctx, EVP_aes_256_cbc(), NULL, key->aes_key,
212                                iv) == 0)
213            return -1; /* error in cipher initialisation */
214
215         if (key->expire < t - RENEW_TIME) { /* RENEW_TIME: implement */
216             /*
217              * return 2 - This session will get a new ticket even though the
218              * current one is still valid.
219              */
220             return 2;
221         }
222         return 1;
223     }
224 }
225
226=head1 SEE ALSO
227
228L<ssl(7)>, L<SSL_set_session(3)>,
229L<SSL_session_reused(3)>,
230L<SSL_CTX_add_session(3)>,
231L<SSL_CTX_sess_number(3)>,
232L<SSL_CTX_sess_set_get_cb(3)>,
233L<SSL_CTX_set_session_id_context(3)>,
234
235=head1 HISTORY
236
237The SSL_CTX_set_tlsext_ticket_key_cb() function was deprecated in OpenSSL 3.0.
238
239The SSL_CTX_set_tlsext_ticket_key_evp_cb() function was introduced in
240OpenSSL 3.0.
241
242=head1 COPYRIGHT
243
244Copyright 2014-2024 The OpenSSL Project Authors. All Rights Reserved.
245
246Licensed under the Apache License 2.0 (the "License").  You may not use
247this file except in compliance with the License.  You can obtain a copy
248in the file LICENSE in the source distribution or at
249L<https://www.openssl.org/source/license.html>.
250
251=cut
252