5 SSL_extension_supported,
6 SSL_custom_ext_add_cb_ex,
7 SSL_custom_ext_free_cb_ex,
8 SSL_custom_ext_parse_cb_ex,
9 SSL_CTX_add_custom_ext,
10 SSL_CTX_add_client_custom_ext, SSL_CTX_add_server_custom_ext,
11 custom_ext_add_cb, custom_ext_free_cb, custom_ext_parse_cb
12 - custom TLS extension handling
16 #include <openssl/ssl.h>
18 typedef int (*SSL_custom_ext_add_cb_ex) (SSL *s, unsigned int ext_type,
20 const unsigned char **out,
21 size_t *outlen, X509 *x,
22 size_t chainidx, int *al,
25 typedef void (*SSL_custom_ext_free_cb_ex) (SSL *s, unsigned int ext_type,
27 const unsigned char *out,
30 typedef int (*SSL_custom_ext_parse_cb_ex) (SSL *s, unsigned int ext_type,
32 const unsigned char *in,
33 size_t inlen, X509 *x,
34 size_t chainidx, int *al,
37 int SSL_CTX_add_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
39 SSL_custom_ext_add_cb_ex add_cb,
40 SSL_custom_ext_free_cb_ex free_cb,
42 SSL_custom_ext_parse_cb_ex parse_cb,
45 typedef int (*custom_ext_add_cb)(SSL *s, unsigned int ext_type,
46 const unsigned char **out,
47 size_t *outlen, int *al,
50 typedef void (*custom_ext_free_cb)(SSL *s, unsigned int ext_type,
51 const unsigned char *out,
54 typedef int (*custom_ext_parse_cb)(SSL *s, unsigned int ext_type,
55 const unsigned char *in,
56 size_t inlen, int *al,
59 int SSL_CTX_add_client_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
60 custom_ext_add_cb add_cb,
61 custom_ext_free_cb free_cb, void *add_arg,
62 custom_ext_parse_cb parse_cb,
65 int SSL_CTX_add_server_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
66 custom_ext_add_cb add_cb,
67 custom_ext_free_cb free_cb, void *add_arg,
68 custom_ext_parse_cb parse_cb,
71 int SSL_extension_supported(unsigned int ext_type);
75 SSL_CTX_add_custom_ext() adds a custom extension for a TLS/DTLS client or server
76 for all supported protocol versions with extension type B<ext_type> and
77 callbacks B<add_cb>, B<free_cb> and B<parse_cb> (see the
78 L</EXTENSION CALLBACKS> section below). The B<context> value determines
79 which messages and under what conditions the extension will be added/parsed (see
80 the L</EXTENSION CONTEXTS> section below).
82 SSL_CTX_add_client_custom_ext() adds a custom extension for a TLS/DTLS client
83 with extension type B<ext_type> and callbacks B<add_cb>, B<free_cb> and
84 B<parse_cb>. This function is similar to SSL_CTX_add_custom_ext() except it only
85 applies to clients, uses the older style of callbacks, and implicitly sets the
88 SSL_EXT_TLS1_2_AND_BELOW_ONLY | SSL_EXT_CLIENT_HELLO
89 | SSL_EXT_TLS1_2_SERVER_HELLO | SSL_EXT_IGNORE_ON_RESUMPTION
91 SSL_CTX_add_server_custom_ext() adds a custom extension for a TLS/DTLS server
92 with extension type B<ext_type> and callbacks B<add_cb>, B<free_cb> and
93 B<parse_cb>. This function is similar to SSL_CTX_add_custom_ext() except it
94 only applies to servers, uses the older style of callbacks, and implicitly sets
95 the B<context> value to the same as for SSL_CTX_add_client_custom_ext() above.
97 The B<ext_type> parameter corresponds to the B<extension_type> field of
98 RFC5246 et al. It is B<not> a NID. In all cases the extension type must not be
99 handled by OpenSSL internally or an error occurs.
101 SSL_extension_supported() returns 1 if the extension B<ext_type> is handled
102 internally by OpenSSL and 0 otherwise.
104 =head1 EXTENSION CALLBACKS
106 The callback B<add_cb> is called to send custom extension data to be
107 included in various TLS messages. The B<ext_type> parameter is set to the
108 extension type which will be added and B<add_arg> to the value set when the
109 extension handler was added. When using the new style callbacks the B<context>
110 parameter will indicate which message is currently being constructed e.g. for
111 the ClientHello it will be set to B<SSL_EXT_CLIENT_HELLO>.
113 If the application wishes to include the extension B<ext_type> it should
114 set B<*out> to the extension data, set B<*outlen> to the length of the
115 extension data and return 1.
117 If the B<add_cb> does not wish to include the extension it must return 0.
119 If B<add_cb> returns -1 a fatal handshake error occurs using the TLS
120 alert value specified in B<*al>.
122 When constructing the ClientHello, if B<add_cb> is set to NULL a zero length
123 extension is added for B<ext_type>. For all other messages if B<add_cb> is set
124 to NULL then no extension is added.
126 When constructing a Certificate message the callback will be called for each
127 certificate in the message. The B<x> parameter will indicate the
128 current certificate and the B<chainidx> parameter will indicate the position
129 of the certificate in the message. The first certificate is always the end
130 entity certificate and has a B<chainidx> value of 0. The certificates are in the
131 order that they were received in the Certificate message.
133 For all messages except the ServerHello and EncryptedExtensions every
134 registered B<add_cb> is always called to see if the application wishes to add an
135 extension (as long as all requirements of the specified B<context> are met).
137 For the ServerHello and EncryptedExtension messages every registered B<add_cb>
138 is called once if and only if the requirements of the specified B<context> are
139 met and the corresponding extension was received in the ClientHello. That is, if
140 no corresponding extension was received in the ClientHello then B<add_cb> will
143 If an extension is added (that is B<add_cb> returns 1) B<free_cb> is called
144 (if it is set) with the value of B<out> set by the add callback. It can be
145 used to free up any dynamic extension data set by B<add_cb>. Since B<out> is
146 constant (to permit use of constant data in B<add_cb>) applications may need to
147 cast away const to free the data.
149 The callback B<parse_cb> receives data for TLS extensions. The callback is only
150 called if the extension is present and relevant for the context (see
151 L</EXTENSION CONTEXTS> below).
153 The extension data consists of B<inlen> bytes in the buffer B<in> for the
154 extension B<ext_type>.
156 If the message being parsed is a TLSv1.3 compatible Certificate message then
157 B<parse_cb> will be called for each certificate contained within the message.
158 The B<x> parameter will indicate the current certificate and the B<chainidx>
159 parameter will indicate the position of the certificate in the message. The
160 first certificate is always the end entity certificate and has a B<chainidx>
163 If the B<parse_cb> considers the extension data acceptable it must return
164 1. If it returns 0 or a negative value a fatal handshake error occurs
165 using the TLS alert value specified in B<*al>.
167 The buffer B<in> is a temporary internal buffer which will not be valid after
168 the callback returns.
170 =head1 EXTENSION CONTEXTS
172 An extension context defines which messages and under which conditions an
173 extension should be added or expected. The context is built up by performing
174 a bitwise OR of multiple pre-defined values together. The valid context values
179 =item SSL_EXT_TLS_ONLY
181 The extension is only allowed in TLS
183 =item SSL_EXT_DTLS_ONLY
185 The extension is only allowed in DTLS
187 =item SSL_EXT_TLS_IMPLEMENTATION_ONLY
189 The extension is allowed in DTLS, but there is only a TLS implementation
190 available (so it is ignored in DTLS).
192 =item SSL_EXT_SSL3_ALLOWED
194 Extensions are not typically defined for SSLv3. Setting this value will allow
195 the extension in SSLv3. Applications will not typically need to use this.
197 =item SSL_EXT_TLS1_2_AND_BELOW_ONLY
199 The extension is only defined for TLSv1.2/DTLSv1.2 and below. Servers will
200 ignore this extension if it is present in the ClientHello and TLSv1.3 is
203 =item SSL_EXT_TLS1_3_ONLY
205 The extension is only defined for TLS1.3 and above. Servers will ignore this
206 extension if it is present in the ClientHello and TLSv1.2 or below is
209 =item SSL_EXT_IGNORE_ON_RESUMPTION
211 The extension will be ignored during parsing if a previous session is being
212 successfully resumed.
214 =item SSL_EXT_CLIENT_HELLO
216 The extension may be present in the ClientHello message.
218 =item SSL_EXT_TLS1_2_SERVER_HELLO
220 The extension may be present in a TLSv1.2 or below compatible ServerHello
223 =item SSL_EXT_TLS1_3_SERVER_HELLO
225 The extension may be present in a TLSv1.3 compatible ServerHello message.
227 =item SSL_EXT_TLS1_3_ENCRYPTED_EXTENSIONS
229 The extension may be present in an EncryptedExtensions message.
231 =item SSL_EXT_TLS1_3_HELLO_RETRY_REQUEST
233 The extension may be present in a HelloRetryRequest message.
235 =item SSL_EXT_TLS1_3_CERTIFICATE
237 The extension may be present in a TLSv1.3 compatible Certificate message.
239 =item SSL_EXT_TLS1_3_NEW_SESSION_TICKET
241 The extension may be present in a TLSv1.3 compatible NewSessionTicket message.
243 =item SSL_EXT_TLS1_3_CERTIFICATE_REQUEST
245 The extension may be present in a TLSv1.3 compatible CertificateRequest message.
249 The context must include at least one message value (otherwise the extension
254 The B<add_arg> and B<parse_arg> parameters can be set to arbitrary values
255 which will be passed to the corresponding callbacks. They can, for example,
256 be used to store the extension data received in a convenient structure or
257 pass the extension data to be added or freed when adding extensions.
259 If the same custom extension type is received multiple times a fatal
260 B<decode_error> alert is sent and the handshake aborts. If a custom extension
261 is received in a ServerHello/EncryptedExtensions message which was not sent in
262 the ClientHello a fatal B<unsupported_extension> alert is sent and the
263 handshake is aborted. The ServerHello/EncryptedExtensions B<add_cb> callback is
264 only called if the corresponding extension was received in the ClientHello. This
265 is compliant with the TLS specifications. This behaviour ensures that each
266 callback is called at most once and that an application can never send
267 unsolicited extensions.
271 SSL_CTX_add_custom_ext(), SSL_CTX_add_client_custom_ext() and
272 SSL_CTX_add_server_custom_ext() return 1 for success and 0 for failure. A
273 failure can occur if an attempt is made to add the same B<ext_type> more than
274 once, if an attempt is made to use an extension type handled internally by
275 OpenSSL or if an internal error occurs (for example a memory allocation
278 SSL_extension_supported() returns 1 if the extension B<ext_type> is handled
279 internally by OpenSSL and 0 otherwise.
287 The SSL_CTX_add_custom_ext() function was added in OpenSSL 1.1.1.
291 Copyright 2014-2017 The OpenSSL Project Authors. All Rights Reserved.
293 Licensed under the Apache License 2.0 (the "License"). You may not use
294 this file except in compliance with the License. You can obtain a copy
295 in the file LICENSE in the source distribution or at
296 L<https://www.openssl.org/source/license.html>.