1=pod 2 3=head1 NAME 4 5SSL_CTX_dane_enable, SSL_CTX_dane_mtype_set, SSL_dane_enable, 6SSL_dane_tlsa_add, SSL_get0_dane_authority, SSL_get0_dane_tlsa, 7SSL_CTX_dane_set_flags, SSL_CTX_dane_clear_flags, 8SSL_dane_set_flags, SSL_dane_clear_flags 9- enable DANE TLS authentication of the remote TLS server in the local 10TLS client 11 12=head1 SYNOPSIS 13 14 #include <openssl/ssl.h> 15 16 int SSL_CTX_dane_enable(SSL_CTX *ctx); 17 int SSL_CTX_dane_mtype_set(SSL_CTX *ctx, const EVP_MD *md, 18 uint8_t mtype, uint8_t ord); 19 int SSL_dane_enable(SSL *s, const char *basedomain); 20 int SSL_dane_tlsa_add(SSL *s, uint8_t usage, uint8_t selector, 21 uint8_t mtype, unsigned const char *data, size_t dlen); 22 int SSL_get0_dane_authority(SSL *s, X509 **mcert, EVP_PKEY **mspki); 23 int SSL_get0_dane_tlsa(SSL *s, uint8_t *usage, uint8_t *selector, 24 uint8_t *mtype, unsigned const char **data, 25 size_t *dlen); 26 unsigned long SSL_CTX_dane_set_flags(SSL_CTX *ctx, unsigned long flags); 27 unsigned long SSL_CTX_dane_clear_flags(SSL_CTX *ctx, unsigned long flags); 28 unsigned long SSL_dane_set_flags(SSL *ssl, unsigned long flags); 29 unsigned long SSL_dane_clear_flags(SSL *ssl, unsigned long flags); 30 31=head1 DESCRIPTION 32 33These functions implement support for DANE TLSA (RFC6698 and RFC7671) 34peer authentication. 35 36SSL_CTX_dane_enable() must be called first to initialize the shared state 37required for DANE support. 38Individual connections associated with the context can then enable 39per-connection DANE support as appropriate. 40DANE authentication is implemented in the L<X509_verify_cert(3)> function, and 41applications that override L<X509_verify_cert(3)> via 42L<SSL_CTX_set_cert_verify_callback(3)> are responsible to authenticate the peer 43chain in whatever manner they see fit. 44 45SSL_CTX_dane_mtype_set() may then be called zero or more times to adjust the 46supported digest algorithms. 47This must be done before any SSL handles are created for the context. 48 49The B<mtype> argument specifies a DANE TLSA matching type and the B<md> 50argument specifies the associated digest algorithm handle. 51The B<ord> argument specifies a strength ordinal. 52Algorithms with a larger strength ordinal are considered more secure. 53Strength ordinals are used to implement RFC7671 digest algorithm agility. 54Specifying a B<NULL> digest algorithm for a matching type disables 55support for that matching type. 56Matching type Full(0) cannot be modified or disabled. 57 58By default, matching type C<SHA2-256(1)> (see RFC7218 for definitions 59of the DANE TLSA parameter acronyms) is mapped to C<EVP_sha256()> 60with a strength ordinal of C<1> and matching type C<SHA2-512(2)> 61is mapped to C<EVP_sha512()> with a strength ordinal of C<2>. 62 63SSL_dane_enable() must be called before the SSL handshake is initiated with 64L<SSL_connect(3)> if (and only if) you want to enable DANE for that connection. 65(The connection must be associated with a DANE-enabled SSL context). 66The B<basedomain> argument specifies the RFC7671 TLSA base domain, 67which will be the primary peer reference identifier for certificate 68name checks. 69Additional server names can be specified via L<SSL_add1_host(3)>. 70The B<basedomain> is used as the default SNI hint if none has yet been 71specified via L<SSL_set_tlsext_host_name(3)>. 72 73SSL_dane_tlsa_add() may then be called one or more times, to load each of the 74TLSA records that apply to the remote TLS peer. 75(This too must be done prior to the beginning of the SSL handshake). 76The arguments specify the fields of the TLSA record. 77The B<data> field is provided in binary (wire RDATA) form, not the hexadecimal 78ASCII presentation form, with an explicit length passed via B<dlen>. 79The library takes a copy of the B<data> buffer contents and the caller may 80free the original B<data> buffer when convenient. 81A return value of 0 indicates that "unusable" TLSA records (with invalid or 82unsupported parameters) were provided. 83A negative return value indicates an internal error in processing the record. 84 85The caller is expected to check the return value of each SSL_dane_tlsa_add() 86call and take appropriate action if none are usable or an internal error 87is encountered in processing some records. 88 89If no TLSA records are added successfully, DANE authentication is not enabled, 90and authentication will be based on any configured traditional trust-anchors; 91authentication success in this case does not mean that the peer was 92DANE-authenticated. 93 94SSL_get0_dane_authority() can be used to get more detailed information about 95the matched DANE trust-anchor after successful connection completion. 96The return value is negative if DANE verification failed (or was not enabled), 970 if an EE TLSA record directly matched the leaf certificate, or a positive 98number indicating the depth at which a TA record matched an issuer certificate. 99The complete verified chain can be retrieved via L<SSL_get0_verified_chain(3)>. 100The return value is an index into this verified chain, rather than the list of 101certificates sent by the peer as returned by L<SSL_get_peer_cert_chain(3)>. 102 103If the B<mcert> argument is not B<NULL> and a TLSA record matched a chain 104certificate, a pointer to the matching certificate is returned via B<mcert>. 105The returned address is a short-term internal reference to the certificate and 106must not be freed by the application. 107Applications that want to retain access to the certificate can call 108L<X509_up_ref(3)> to obtain a long-term reference which must then be freed via 109L<X509_free(3)> once no longer needed. 110 111If no TLSA records directly matched any elements of the certificate chain, but 112a DANE-TA(2) SPKI(1) Full(0) record provided the public key that signed an 113element of the chain, then that key is returned via B<mspki> argument (if not 114NULL). 115In this case the return value is the depth of the top-most element of the 116validated certificate chain. 117As with B<mcert> this is a short-term internal reference, and 118L<EVP_PKEY_up_ref(3)> and L<EVP_PKEY_free(3)> can be used to acquire and 119release long-term references respectively. 120 121SSL_get0_dane_tlsa() can be used to retrieve the fields of the TLSA record that 122matched the peer certificate chain. 123The return value indicates the match depth or failure to match just as with 124SSL_get0_dane_authority(). 125When the return value is nonnegative, the storage pointed to by the B<usage>, 126B<selector>, B<mtype> and B<data> parameters is updated to the corresponding 127TLSA record fields. 128The B<data> field is in binary wire form, and is therefore not NUL-terminated, 129its length is returned via the B<dlen> parameter. 130If any of these parameters is NULL, the corresponding field is not returned. 131The B<data> parameter is set to a short-term internal-copy of the associated 132data field and must not be freed by the application. 133Applications that need long-term access to this field need to copy the content. 134 135SSL_CTX_dane_set_flags() and SSL_dane_set_flags() can be used to enable 136optional DANE verification features. 137SSL_CTX_dane_clear_flags() and SSL_dane_clear_flags() can be used to disable 138the same features. 139The B<flags> argument is a bit mask of the features to enable or disable. 140The B<flags> set for an B<SSL_CTX> context are copied to each B<SSL> handle 141associated with that context at the time the handle is created. 142Subsequent changes in the context's B<flags> have no effect on the B<flags> set 143for the handle. 144 145At present, the only available option is B<DANE_FLAG_NO_DANE_EE_NAMECHECKS> 146which can be used to disable server name checks when authenticating via 147DANE-EE(3) TLSA records. 148For some applications, primarily web browsers, it is not safe to disable name 149checks due to "unknown key share" attacks, in which a malicious server can 150convince a client that a connection to a victim server is instead a secure 151connection to the malicious server. 152The malicious server may then be able to violate cross-origin scripting 153restrictions. 154Thus, despite the text of RFC7671, name checks are by default enabled for 155DANE-EE(3) TLSA records, and can be disabled in applications where it is safe 156to do so. 157In particular, SMTP and XMPP clients should set this option as SRV and MX 158records already make it possible for a remote domain to redirect client 159connections to any server of its choice, and in any case SMTP and XMPP clients 160do not execute scripts downloaded from remote servers. 161 162=head1 RETURN VALUES 163 164The functions SSL_CTX_dane_enable(), SSL_CTX_dane_mtype_set(), 165SSL_dane_enable() and SSL_dane_tlsa_add() return a positive value on success. 166Negative return values indicate resource problems (out of memory, etc.) in the 167SSL library, while a return value of B<0> indicates incorrect usage or invalid 168input, such as an unsupported TLSA record certificate usage, selector or 169matching type. 170Invalid input also includes malformed data, either a digest length that does 171not match the digest algorithm, or a C<Full(0)> (binary ASN.1 DER form) 172certificate or a public key that fails to parse. 173 174The functions SSL_get0_dane_authority() and SSL_get0_dane_tlsa() return a 175negative value when DANE authentication failed or was not enabled, a 176nonnegative value indicates the chain depth at which the TLSA record matched a 177chain certificate, or the depth of the top-most certificate, when the TLSA 178record is a full public key that is its signer. 179 180The functions SSL_CTX_dane_set_flags(), SSL_CTX_dane_clear_flags(), 181SSL_dane_set_flags() and SSL_dane_clear_flags() return the B<flags> in effect 182before they were called. 183 184=head1 EXAMPLES 185 186Suppose "smtp.example.com" is the MX host of the domain "example.com", and has 187DNSSEC-validated TLSA records. 188The calls below will perform DANE authentication and arrange to match either 189the MX hostname or the destination domain name in the SMTP server certificate. 190Wildcards are supported, but must match the entire label. 191The actual name matched in the certificate (which might be a wildcard) is 192retrieved, and must be copied by the application if it is to be retained beyond 193the lifetime of the SSL connection. 194 195 SSL_CTX *ctx; 196 SSL *ssl; 197 int (*verify_cb)(int ok, X509_STORE_CTX *sctx) = NULL; 198 int num_usable = 0; 199 const char *nexthop_domain = "example.com"; 200 const char *dane_tlsa_domain = "smtp.example.com"; 201 uint8_t usage, selector, mtype; 202 203 if ((ctx = SSL_CTX_new(TLS_client_method())) == NULL) 204 /* error */ 205 if (SSL_CTX_dane_enable(ctx) <= 0) 206 /* error */ 207 if ((ssl = SSL_new(ctx)) == NULL) 208 /* error */ 209 if (SSL_dane_enable(ssl, dane_tlsa_domain) <= 0) 210 /* error */ 211 212 /* 213 * For many applications it is safe to skip DANE-EE(3) namechecks. Do not 214 * disable the checks unless "unknown key share" attacks pose no risk for 215 * your application. 216 */ 217 SSL_dane_set_flags(ssl, DANE_FLAG_NO_DANE_EE_NAMECHECKS); 218 219 if (!SSL_add1_host(ssl, nexthop_domain)) 220 /* error */ 221 SSL_set_hostflags(ssl, X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS); 222 223 for (... each TLSA record ...) { 224 unsigned char *data; 225 size_t len; 226 int ret; 227 228 /* set usage, selector, mtype, data, len */ 229 230 /* 231 * Opportunistic DANE TLS clients support only DANE-TA(2) or DANE-EE(3). 232 * They treat all other certificate usages, and in particular PKIX-TA(0) 233 * and PKIX-EE(1), as unusable. 234 */ 235 switch (usage) { 236 default: 237 case 0: /* PKIX-TA(0) */ 238 case 1: /* PKIX-EE(1) */ 239 continue; 240 case 2: /* DANE-TA(2) */ 241 case 3: /* DANE-EE(3) */ 242 break; 243 } 244 245 ret = SSL_dane_tlsa_add(ssl, usage, selector, mtype, data, len); 246 /* free data as appropriate */ 247 248 if (ret < 0) 249 /* handle SSL library internal error */ 250 else if (ret == 0) 251 /* handle unusable TLSA record */ 252 else 253 ++num_usable; 254 } 255 256 /* 257 * At this point, the verification mode is still the default SSL_VERIFY_NONE. 258 * Opportunistic DANE clients use unauthenticated TLS when all TLSA records 259 * are unusable, so continue the handshake even if authentication fails. 260 */ 261 if (num_usable == 0) { 262 /* Log all records unusable? */ 263 264 /* Optionally set verify_cb to a suitable non-NULL callback. */ 265 SSL_set_verify(ssl, SSL_VERIFY_NONE, verify_cb); 266 } else { 267 /* At least one usable record. We expect to verify the peer */ 268 269 /* Optionally set verify_cb to a suitable non-NULL callback. */ 270 271 /* 272 * Below we elect to fail the handshake when peer verification fails. 273 * Alternatively, use the permissive SSL_VERIFY_NONE verification mode, 274 * complete the handshake, check the verification status, and if not 275 * verified disconnect gracefully at the application layer, especially if 276 * application protocol supports informing the server that authentication 277 * failed. 278 */ 279 SSL_set_verify(ssl, SSL_VERIFY_PEER, verify_cb); 280 } 281 282 /* 283 * Load any saved session for resumption, making sure that the previous 284 * session applied the same security and authentication requirements that 285 * would be expected of a fresh connection. 286 */ 287 288 /* Perform SSL_connect() handshake and handle errors here */ 289 290 if (SSL_session_reused(ssl)) { 291 if (SSL_get_verify_result(ssl) == X509_V_OK) { 292 /* 293 * Resumed session was originally verified, this connection is 294 * authenticated. 295 */ 296 } else { 297 /* 298 * Resumed session was not originally verified, this connection is not 299 * authenticated. 300 */ 301 } 302 } else if (SSL_get_verify_result(ssl) == X509_V_OK) { 303 const char *peername = SSL_get0_peername(ssl); 304 EVP_PKEY *mspki = NULL; 305 306 int depth = SSL_get0_dane_authority(ssl, NULL, &mspki); 307 if (depth >= 0) { 308 (void) SSL_get0_dane_tlsa(ssl, &usage, &selector, &mtype, NULL, NULL); 309 printf("DANE TLSA %d %d %d %s at depth %d\n", usage, selector, mtype, 310 (mspki != NULL) ? "TA public key verified certificate" : 311 depth ? "matched TA certificate" : "matched EE certificate", 312 depth); 313 } 314 if (peername != NULL) { 315 /* Name checks were in scope and matched the peername */ 316 printf("Verified peername: %s\n", peername); 317 } 318 } else { 319 /* 320 * Not authenticated, presumably all TLSA rrs unusable, but possibly a 321 * callback suppressed connection termination despite the presence of 322 * usable TLSA RRs none of which matched. Do whatever is appropriate for 323 * fresh unauthenticated connections. 324 */ 325 } 326 327=head1 NOTES 328 329It is expected that the majority of clients employing DANE TLS will be doing 330"opportunistic DANE TLS" in the sense of RFC7672 and RFC7435. 331That is, they will use DANE authentication when DNSSEC-validated TLSA records 332are published for a given peer, and otherwise will use unauthenticated TLS or 333even cleartext. 334 335Such applications should generally treat any TLSA records published by the peer 336with usages PKIX-TA(0) and PKIX-EE(1) as "unusable", and should not include 337them among the TLSA records used to authenticate peer connections. 338In addition, some TLSA records with supported usages may be "unusable" as a 339result of invalid or unsupported parameters. 340 341When a peer has TLSA records, but none are "usable", an opportunistic 342application must avoid cleartext, but cannot authenticate the peer, 343and so should generally proceed with an unauthenticated connection. 344Opportunistic applications need to note the return value of each 345call to SSL_dane_tlsa_add(), and if all return 0 (due to invalid 346or unsupported parameters) disable peer authentication by calling 347L<SSL_set_verify(3)> with B<mode> equal to B<SSL_VERIFY_NONE>. 348 349=head1 SEE ALSO 350 351L<SSL_new(3)>, 352L<SSL_add1_host(3)>, 353L<SSL_set_hostflags(3)>, 354L<SSL_set_tlsext_host_name(3)>, 355L<SSL_set_verify(3)>, 356L<SSL_CTX_set_cert_verify_callback(3)>, 357L<SSL_get0_verified_chain(3)>, 358L<SSL_get_peer_cert_chain(3)>, 359L<SSL_get_verify_result(3)>, 360L<SSL_connect(3)>, 361L<SSL_get0_peername(3)>, 362L<X509_verify_cert(3)>, 363L<X509_up_ref(3)>, 364L<X509_free(3)>, 365L<EVP_get_digestbyname(3)>, 366L<EVP_PKEY_up_ref(3)>, 367L<EVP_PKEY_free(3)> 368 369=head1 HISTORY 370 371These functions were added in OpenSSL 1.1.0. 372 373=head1 COPYRIGHT 374 375Copyright 2016-2020 The OpenSSL Project Authors. All Rights Reserved. 376 377Licensed under the OpenSSL license (the "License"). You may not use 378this file except in compliance with the License. You can obtain a copy 379in the file LICENSE in the source distribution or at 380L<https://www.openssl.org/source/license.html>. 381 382=cut 383