5 SSL_extension_supported,
6 SSL_CTX_add_custom_ext,
7 SSL_CTX_add_client_custom_ext, SSL_CTX_add_server_custom_ext,
8 custom_ext_add_cb, custom_ext_free_cb, custom_ext_parse_cb
9 - custom TLS extension handling
13 #include <openssl/ssl.h>
15 typedef int (*custom_ext_add_cb_ex) (SSL *s, unsigned int ext_type,
17 const unsigned char **out,
18 size_t *outlen, X509 *x, size_t chainidx,
19 int *al, void *add_arg);
21 typedef void (*custom_ext_free_cb_ex) (SSL *s, unsigned int ext_type,
23 const unsigned char *out,
26 typedef int (*custom_ext_parse_cb_ex) (SSL *s, unsigned int ext_type,
28 const unsigned char *in,
29 size_t inlen, X509 *x, size_t chainidx,
30 int *al, void *parse_arg);
32 int SSL_CTX_add_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
34 custom_ext_add_cb_ex add_cb,
35 custom_ext_free_cb_ex free_cb,
37 custom_ext_parse_cb_ex parse_cb, void *parse_arg);
39 typedef int (*custom_ext_add_cb)(SSL *s, unsigned int ext_type,
40 const unsigned char **out,
41 size_t *outlen, int *al,
44 typedef void (*custom_ext_free_cb)(SSL *s, unsigned int ext_type,
45 const unsigned char *out,
48 typedef int (*custom_ext_parse_cb)(SSL *s, unsigned int ext_type,
49 const unsigned char *in,
50 size_t inlen, int *al,
53 int SSL_CTX_add_client_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
54 custom_ext_add_cb add_cb,
55 custom_ext_free_cb free_cb, void *add_arg,
56 custom_ext_parse_cb parse_cb,
59 int SSL_CTX_add_server_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_extension_supported(unsigned int ext_type);
69 SSL_CTX_add_custom_ext() adds a custom extension for a TLS/DTLS client or server
70 for all supported protocol versions with extension type B<ext_type> and
71 callbacks B<add_cb>, B<free_cb> and B<parse_cb> (see the
72 L</EXTENSION CALLBACKS> section below). The B<context> value determines
73 which messages and under what conditions the extension will be added/parsed (see
74 the L</EXTENSION CONTEXTS> section below).
76 SSL_CTX_add_client_custom_ext() adds a custom extension for a TLS/DTLS client
77 with extension type B<ext_type> and callbacks B<add_cb>, B<free_cb> and
78 B<parse_cb>. This function is similar to SSL_CTX_add_custom_ext() except it only
79 applies to clients, uses the older style of callbacks, and implicitly sets the
82 SSL_EXT_TLS1_2_AND_BELOW_ONLY | SSL_EXT_CLIENT_HELLO
83 | SSL_EXT_TLS1_2_SERVER_HELLO | SSL_EXT_IGNORE_ON_RESUMPTION
85 SSL_CTX_add_server_custom_ext() adds a custom extension for a TLS/DTLS server
86 with extension type B<ext_type> and callbacks B<add_cb>, B<free_cb> and
87 B<parse_cb>. This function is similar to SSL_CTX_add_custom_ext() except it
88 only applies to servers, uses the older style of callbacks, and implicitly sets
89 the B<context> value to the same as for SSL_CTX_add_client_custom_ext() above.
91 The B<ext_type> parameter corresponds to the B<extension_type> field of
92 RFC5246 et al. It is B<not> a NID. In all cases the extension type must not be
93 handled by OpenSSL internally or an error occurs.
95 SSL_extension_supported() returns 1 if the extension B<ext_type> is handled
96 internally by OpenSSL and 0 otherwise.
98 =head1 EXTENSION CALLBACKS
100 The callback B<add_cb> is called to send custom extension data to be
101 included in various TLS messages. The B<ext_type> parameter is set to the
102 extension type which will be added and B<add_arg> to the value set when the
103 extension handler was added. When using the new style callbacks the B<context>
104 parameter will indicate which message is currently being constructed e.g. for
105 the ClientHello it will be set to B<SSL_EXT_CLIENT_HELLO>.
107 If the application wishes to include the extension B<ext_type> it should
108 set B<*out> to the extension data, set B<*outlen> to the length of the
109 extension data and return 1.
111 If the B<add_cb> does not wish to include the extension it must return 0.
113 If B<add_cb> returns -1 a fatal handshake error occurs using the TLS
114 alert value specified in B<*al>.
116 When constructing the ClientHello, if B<add_cb> is set to NULL a zero length
117 extension is added for B<ext_type>. For all other messages if B<add_cb> is set
118 to NULL then no extension is added.
120 When constructing a Certificate message the callback will be called for each
121 certificate in the message. The B<x> parameter will indicate the
122 current certificate and the B<chainidx> parameter will indicate the position
123 of the certificate in the message. The first certificate is always the end
124 entity certificate and has a B<chainidx> value of 0. The certificates are in the
125 order that they were received in the Certificate message.
127 For all messages except the ServerHello and EncryptedExtensions every
128 registered B<add_cb> is always called to see if the application wishes to add an
129 extension (as long as all requirements of the specified B<context> are met).
131 For the ServerHello and EncryptedExtension messages every registered B<add_cb>
132 is called once if and only if the requirements of the specified B<context> are
133 met and the corresponding extension was received in the ClientHello. That is, if
134 no corresponding extension was received in the ClientHello then B<add_cb> will
137 If an extension is added (that is B<add_cb> returns 1) B<free_cb> is called
138 (if it is set) with the value of B<out> set by the add callback. It can be
139 used to free up any dynamic extension data set by B<add_cb>. Since B<out> is
140 constant (to permit use of constant data in B<add_cb>) applications may need to
141 cast away const to free the data.
143 The callback B<parse_cb> receives data for TLS extensions. The callback is only
144 called if the extension is present and relevant for the context (see
145 L</EXTENSION CONTEXTS> below).
147 The extension data consists of B<inlen> bytes in the buffer B<in> for the
148 extension B<ext_type>.
150 If the message being parsed is a TLSv1.3 compatible Certificate message then
151 B<parse_cb> will be called for each certificate contained within the message.
152 The B<x> parameter will indicate the current certificate and the B<chainidx>
153 parameter will indicate the position of the certificate in the message. The
154 first certificate is always the end entity certificate and has a B<chainidx>
157 If the B<parse_cb> considers the extension data acceptable it must return
158 1. If it returns 0 or a negative value a fatal handshake error occurs
159 using the TLS alert value specified in B<*al>.
161 The buffer B<in> is a temporary internal buffer which will not be valid after
162 the callback returns.
164 =head1 EXTENSION CONTEXTS
166 An extension context defines which messages and under which conditions an
167 extension should be added or expected. The context is built up by performing
168 a bitwise OR of multiple pre-defined values together. The valid context values
173 =item SSL_EXT_TLS_ONLY
175 The extension is only allowed in TLS
177 =item SSL_EXT_DTLS_ONLY
179 The extension is only allowed in DTLS
181 =item SSL_EXT_TLS_IMPLEMENTATION_ONLY
183 The extension is allowed in DTLS, but there is only a TLS implementation
184 available (so it is ignored in DTLS).
186 =item SSL_EXT_SSL3_ALLOWED
188 Extensions are not typically defined for SSLv3. Setting this value will allow
189 the extension in SSLv3. Applications will not typically need to use this.
191 =item SSL_EXT_TLS1_2_AND_BELOW_ONLY
193 The extension is only defined for TLSv1.2/DTLSv1.2 and below. Servers will
194 ignore this extension if it is present in the ClientHello and TLSv1.3 is
197 =item SSL_EXT_TLS1_3_ONLY
199 The extension is only defined for TLS1.3 and above. Servers will ignore this
200 extension if it is present in the ClientHello and TLSv1.2 or below is
203 =item SSL_EXT_IGNORE_ON_RESUMPTION
205 The extension will be ignored during parsing if a previous session is being
206 successfully resumed.
208 =item SSL_EXT_CLIENT_HELLO
210 The extension may be present in the ClientHello message.
212 =item SSL_EXT_TLS1_2_SERVER_HELLO
214 The extension may be present in a TLSv1.2 or below compatible ServerHello
217 =item SSL_EXT_TLS1_3_SERVER_HELLO
219 The extension may be present in a TLSv1.3 compatible ServerHello message.
221 =item SSL_EXT_TLS1_3_ENCRYPTED_EXTENSIONS
223 The extension may be present in an EncryptedExtensions message.
225 =item SSL_EXT_TLS1_3_HELLO_RETRY_REQUEST
227 The extension may be present in a HelloRetryRequest message.
229 =item SSL_EXT_TLS1_3_CERTIFICATE
231 The extension may be present in a TLSv1.3 compatible Certificate message.
233 =item SSL_EXT_TLS1_3_NEW_SESSION_TICKET
235 The extension may be present in a TLSv1.3 compatible NewSessionTicket message.
237 =item SSL_EXT_TLS1_3_CERTIFICATE_REQUEST
239 The extension may be present in a TLSv1.3 compatible CertificateRequest message.
243 The context must include at least one message value (otherwise the extension
248 The B<add_arg> and B<parse_arg> parameters can be set to arbitrary values
249 which will be passed to the corresponding callbacks. They can, for example,
250 be used to store the extension data received in a convenient structure or
251 pass the extension data to be added or freed when adding extensions.
253 If the same custom extension type is received multiple times a fatal
254 B<decode_error> alert is sent and the handshake aborts. If a custom extension
255 is received in a ServerHello/EncryptedExtensions message which was not sent in
256 the ClientHello a fatal B<unsupported_extension> alert is sent and the
257 handshake is aborted. The ServerHello/EncryptedExtensions B<add_cb> callback is
258 only called if the corresponding extension was received in the ClientHello. This
259 is compliant with the TLS specifications. This behaviour ensures that each
260 callback is called at most once and that an application can never send
261 unsolicited extensions.
265 SSL_CTX_add_custom_ext(), SSL_CTX_add_client_custom_ext() and
266 SSL_CTX_add_server_custom_ext() return 1 for success and 0 for failure. A
267 failure can occur if an attempt is made to add the same B<ext_type> more than
268 once, if an attempt is made to use an extension type handled internally by
269 OpenSSL or if an internal error occurs (for example a memory allocation
272 SSL_extension_supported() returns 1 if the extension B<ext_type> is handled
273 internally by OpenSSL and 0 otherwise.
277 The function SSL_CTX_add_custom_ext() was added in OpenSSL version 1.1.1.
281 Copyright 2014-2017 The OpenSSL Project Authors. All Rights Reserved.
283 Licensed under the OpenSSL license (the "License"). You may not use
284 this file except in compliance with the License. You can obtain a copy
285 in the file LICENSE in the source distribution or at
286 L<https://www.openssl.org/source/license.html>.