1=pod 2 3=head1 NAME 4 5SSL_extension_supported, 6SSL_custom_ext_add_cb_ex, 7SSL_custom_ext_free_cb_ex, 8SSL_custom_ext_parse_cb_ex, 9SSL_CTX_add_custom_ext, 10SSL_CTX_add_client_custom_ext, SSL_CTX_add_server_custom_ext, 11custom_ext_add_cb, custom_ext_free_cb, custom_ext_parse_cb 12- custom TLS extension handling 13 14=head1 SYNOPSIS 15 16 #include <openssl/ssl.h> 17 18 typedef int (*SSL_custom_ext_add_cb_ex)(SSL *s, unsigned int ext_type, 19 unsigned int context, 20 const unsigned char **out, 21 size_t *outlen, X509 *x, 22 size_t chainidx, int *al, 23 void *add_arg); 24 25 typedef void (*SSL_custom_ext_free_cb_ex)(SSL *s, unsigned int ext_type, 26 unsigned int context, 27 const unsigned char *out, 28 void *add_arg); 29 30 typedef int (*SSL_custom_ext_parse_cb_ex)(SSL *s, unsigned int ext_type, 31 unsigned int context, 32 const unsigned char *in, 33 size_t inlen, X509 *x, 34 size_t chainidx, int *al, 35 void *parse_arg); 36 37 int SSL_CTX_add_custom_ext(SSL_CTX *ctx, unsigned int ext_type, 38 unsigned int context, 39 SSL_custom_ext_add_cb_ex add_cb, 40 SSL_custom_ext_free_cb_ex free_cb, 41 void *add_arg, 42 SSL_custom_ext_parse_cb_ex parse_cb, 43 void *parse_arg); 44 45 typedef int (*custom_ext_add_cb)(SSL *s, unsigned int ext_type, 46 const unsigned char **out, 47 size_t *outlen, int *al, 48 void *add_arg); 49 50 typedef void (*custom_ext_free_cb)(SSL *s, unsigned int ext_type, 51 const unsigned char *out, 52 void *add_arg); 53 54 typedef int (*custom_ext_parse_cb)(SSL *s, unsigned int ext_type, 55 const unsigned char *in, 56 size_t inlen, int *al, 57 void *parse_arg); 58 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, 63 void *parse_arg); 64 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, 69 void *parse_arg); 70 71 int SSL_extension_supported(unsigned int ext_type); 72 73=head1 DESCRIPTION 74 75SSL_CTX_add_custom_ext() adds a custom extension for a TLS/DTLS client or server 76for all supported protocol versions with extension type B<ext_type> and 77callbacks B<add_cb>, B<free_cb> and B<parse_cb> (see the 78L</EXTENSION CALLBACKS> section below). The B<context> value determines 79which messages and under what conditions the extension will be added/parsed (see 80the L</EXTENSION CONTEXTS> section below). 81 82SSL_CTX_add_client_custom_ext() adds a custom extension for a TLS/DTLS client 83with extension type B<ext_type> and callbacks B<add_cb>, B<free_cb> and 84B<parse_cb>. This function is similar to SSL_CTX_add_custom_ext() except it only 85applies to clients, uses the older style of callbacks, and implicitly sets the 86B<context> value to: 87 88 SSL_EXT_TLS1_2_AND_BELOW_ONLY | SSL_EXT_CLIENT_HELLO 89 | SSL_EXT_TLS1_2_SERVER_HELLO | SSL_EXT_IGNORE_ON_RESUMPTION 90 91SSL_CTX_add_server_custom_ext() adds a custom extension for a TLS/DTLS server 92with extension type B<ext_type> and callbacks B<add_cb>, B<free_cb> and 93B<parse_cb>. This function is similar to SSL_CTX_add_custom_ext() except it 94only applies to servers, uses the older style of callbacks, and implicitly sets 95the B<context> value to the same as for SSL_CTX_add_client_custom_ext() above. 96 97The B<ext_type> parameter corresponds to the B<extension_type> field of 98RFC5246 et al. It is B<not> a NID. In all cases the extension type must not be 99handled by OpenSSL internally or an error occurs. 100 101SSL_extension_supported() returns 1 if the extension B<ext_type> is handled 102internally by OpenSSL and 0 otherwise. 103 104=head1 EXTENSION CALLBACKS 105 106The callback B<add_cb> is called to send custom extension data to be 107included in various TLS messages. The B<ext_type> parameter is set to the 108extension type which will be added and B<add_arg> to the value set when the 109extension handler was added. When using the new style callbacks the B<context> 110parameter will indicate which message is currently being constructed e.g. for 111the ClientHello it will be set to B<SSL_EXT_CLIENT_HELLO>. 112 113If the application wishes to include the extension B<ext_type> it should 114set B<*out> to the extension data, set B<*outlen> to the length of the 115extension data and return 1. 116 117If the B<add_cb> does not wish to include the extension it must return 0. 118 119If B<add_cb> returns -1 a fatal handshake error occurs using the TLS 120alert value specified in B<*al>. 121 122When constructing the ClientHello, if B<add_cb> is set to NULL a zero length 123extension is added for B<ext_type>. For all other messages if B<add_cb> is set 124to NULL then no extension is added. 125 126When constructing a Certificate message the callback will be called for each 127certificate in the message. The B<x> parameter will indicate the 128current certificate and the B<chainidx> parameter will indicate the position 129of the certificate in the message. The first certificate is always the end 130entity certificate and has a B<chainidx> value of 0. The certificates are in the 131order that they were received in the Certificate message. 132 133For all messages except the ServerHello and EncryptedExtensions every 134registered B<add_cb> is always called to see if the application wishes to add an 135extension (as long as all requirements of the specified B<context> are met). 136 137For the ServerHello and EncryptedExtension messages every registered B<add_cb> 138is called once if and only if the requirements of the specified B<context> are 139met and the corresponding extension was received in the ClientHello. That is, if 140no corresponding extension was received in the ClientHello then B<add_cb> will 141not be called. 142 143If 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 145used to free up any dynamic extension data set by B<add_cb>. Since B<out> is 146constant (to permit use of constant data in B<add_cb>) applications may need to 147cast away const to free the data. 148 149The callback B<parse_cb> receives data for TLS extensions. The callback is only 150called if the extension is present and relevant for the context (see 151L</EXTENSION CONTEXTS> below). 152 153The extension data consists of B<inlen> bytes in the buffer B<in> for the 154extension B<ext_type>. 155 156If the message being parsed is a TLSv1.3 compatible Certificate message then 157B<parse_cb> will be called for each certificate contained within the message. 158The B<x> parameter will indicate the current certificate and the B<chainidx> 159parameter will indicate the position of the certificate in the message. The 160first certificate is always the end entity certificate and has a B<chainidx> 161value of 0. 162 163If the B<parse_cb> considers the extension data acceptable it must return 1641. If it returns 0 or a negative value a fatal handshake error occurs 165using the TLS alert value specified in B<*al>. 166 167The buffer B<in> is a temporary internal buffer which will not be valid after 168the callback returns. 169 170=head1 EXTENSION CONTEXTS 171 172An extension context defines which messages and under which conditions an 173extension should be added or expected. The context is built up by performing 174a bitwise OR of multiple pre-defined values together. The valid context values 175are: 176 177=over 4 178 179=item SSL_EXT_TLS_ONLY 180 181The extension is only allowed in TLS 182 183=item SSL_EXT_DTLS_ONLY 184 185The extension is only allowed in DTLS 186 187=item SSL_EXT_TLS_IMPLEMENTATION_ONLY 188 189The extension is allowed in DTLS, but there is only a TLS implementation 190available (so it is ignored in DTLS). 191 192=item SSL_EXT_SSL3_ALLOWED 193 194Extensions are not typically defined for SSLv3. Setting this value will allow 195the extension in SSLv3. Applications will not typically need to use this. 196 197=item SSL_EXT_TLS1_2_AND_BELOW_ONLY 198 199The extension is only defined for TLSv1.2/DTLSv1.2 and below. Servers will 200ignore this extension if it is present in the ClientHello and TLSv1.3 is 201negotiated. 202 203=item SSL_EXT_TLS1_3_ONLY 204 205The extension is only defined for TLS1.3 and above. Servers will ignore this 206extension if it is present in the ClientHello and TLSv1.2 or below is 207negotiated. 208 209=item SSL_EXT_IGNORE_ON_RESUMPTION 210 211The extension will be ignored during parsing if a previous session is being 212successfully resumed. 213 214=item SSL_EXT_CLIENT_HELLO 215 216The extension may be present in the ClientHello message. 217 218=item SSL_EXT_TLS1_2_SERVER_HELLO 219 220The extension may be present in a TLSv1.2 or below compatible ServerHello 221message. 222 223=item SSL_EXT_TLS1_3_SERVER_HELLO 224 225The extension may be present in a TLSv1.3 compatible ServerHello message. 226 227=item SSL_EXT_TLS1_3_ENCRYPTED_EXTENSIONS 228 229The extension may be present in an EncryptedExtensions message. 230 231=item SSL_EXT_TLS1_3_HELLO_RETRY_REQUEST 232 233The extension may be present in a HelloRetryRequest message. 234 235=item SSL_EXT_TLS1_3_CERTIFICATE 236 237The extension may be present in a TLSv1.3 compatible Certificate message. 238 239=item SSL_EXT_TLS1_3_NEW_SESSION_TICKET 240 241The extension may be present in a TLSv1.3 compatible NewSessionTicket message. 242 243=item SSL_EXT_TLS1_3_CERTIFICATE_REQUEST 244 245The extension may be present in a TLSv1.3 compatible CertificateRequest message. 246 247=back 248 249The context must include at least one message value (otherwise the extension 250will never be used). 251 252=head1 NOTES 253 254The B<add_arg> and B<parse_arg> parameters can be set to arbitrary values 255which will be passed to the corresponding callbacks. They can, for example, 256be used to store the extension data received in a convenient structure or 257pass the extension data to be added or freed when adding extensions. 258 259If the same custom extension type is received multiple times a fatal 260B<decode_error> alert is sent and the handshake aborts. If a custom extension 261is received in a ServerHello/EncryptedExtensions message which was not sent in 262the ClientHello a fatal B<unsupported_extension> alert is sent and the 263handshake is aborted. The ServerHello/EncryptedExtensions B<add_cb> callback is 264only called if the corresponding extension was received in the ClientHello. This 265is compliant with the TLS specifications. This behaviour ensures that each 266callback is called at most once and that an application can never send 267unsolicited extensions. 268 269=head1 RETURN VALUES 270 271SSL_CTX_add_custom_ext(), SSL_CTX_add_client_custom_ext() and 272SSL_CTX_add_server_custom_ext() return 1 for success and 0 for failure. A 273failure can occur if an attempt is made to add the same B<ext_type> more than 274once, if an attempt is made to use an extension type handled internally by 275OpenSSL or if an internal error occurs (for example a memory allocation 276failure). 277 278SSL_extension_supported() returns 1 if the extension B<ext_type> is handled 279internally by OpenSSL and 0 otherwise. 280 281=head1 SEE ALSO 282 283L<ssl(7)> 284 285=head1 HISTORY 286 287The SSL_CTX_add_custom_ext() function was added in OpenSSL 1.1.1. 288 289=head1 COPYRIGHT 290 291Copyright 2014-2020 The OpenSSL Project Authors. All Rights Reserved. 292 293Licensed under the Apache License 2.0 (the "License"). You may not use 294this file except in compliance with the License. You can obtain a copy 295in the file LICENSE in the source distribution or at 296L<https://www.openssl.org/source/license.html>. 297 298=cut 299