5 SSL_CONF_cmd_value_type,
6 SSL_CONF_cmd - send configuration command
10 #include <openssl/ssl.h>
12 int SSL_CONF_cmd(SSL_CONF_CTX *cctx, const char *cmd, const char *value);
13 int SSL_CONF_cmd_value_type(SSL_CONF_CTX *cctx, const char *cmd);
17 The function SSL_CONF_cmd() performs configuration operation B<cmd> with
18 optional parameter B<value> on B<ctx>. Its purpose is to simplify application
19 configuration of B<SSL_CTX> or B<SSL> structures by providing a common
20 framework for command line options or configuration files.
22 SSL_CONF_cmd_value_type() returns the type of value that B<cmd> refers to.
24 =head1 SUPPORTED COMMAND LINE COMMANDS
26 Currently supported B<cmd> names for command lines (i.e. when the
27 flag B<SSL_CONF_CMDLINE> is set) are listed below. Note: all B<cmd> names
28 are case sensitive. Unless otherwise stated commands can be used by
29 both clients and servers and the B<value> parameter is not used. The default
30 prefix for command line commands is B<-> and that is reflected below.
36 This sets the supported signature algorithms for TLS v1.2. For clients this
37 value is used directly for the supported signature algorithms extension. For
38 servers it is used to determine which signature algorithms to support.
40 The B<value> argument should be a colon separated list of signature algorithms
41 in order of decreasing preference of the form B<algorithm+hash>. B<algorithm>
42 is one of B<RSA>, B<DSA> or B<ECDSA> and B<hash> is a supported algorithm
43 OID short name such as B<SHA1>, B<SHA224>, B<SHA256>, B<SHA384> of B<SHA512>.
44 Note: algorithm and hash names are case sensitive.
46 If this option is not set then all signature algorithms supported by the
47 OpenSSL library are permissible.
49 =item B<-client_sigalgs>
51 This sets the supported signature algorithms associated with client
52 authentication for TLS v1.2. For servers the value is used in the supported
53 signature algorithms field of a certificate request. For clients it is
54 used to determine which signature algorithm to with the client certificate.
55 If a server does not request a certificate this option has no effect.
57 The syntax of B<value> is identical to B<-sigalgs>. If not set then
58 the value set for B<-sigalgs> will be used instead.
62 This sets the supported groups. For clients, the groups are
63 sent using the supported groups extension. For servers, it is used
64 to determine which group to use. This setting affects groups used for both
65 signatures and key exchange, if applicable. It also affects the preferred
66 key_share sent by a client in a TLSv1.3 compatible connection.
68 The B<value> argument is a colon separated list of groups. The group can be
69 either the B<NIST> name (e.g. B<P-256>), some other commonly used name where
70 applicable (e.g. B<X25519>) or an OpenSSL OID name (e.g B<prime256v1>). Group
71 names are case sensitive. The list should be in order of preference with the
72 most preferred group first. The first listed group will be the one used for a
73 key_share by a TLSv1.3 client.
77 This is a synonym for the "-groups" command.
82 This sets the temporary curve used for ephemeral ECDH modes. Only used by
85 The B<value> argument is a curve name or the special value B<auto> which
86 picks an appropriate curve based on client and server preferences. The curve
87 can be either the B<NIST> name (e.g. B<P-256>) or an OpenSSL OID name
88 (e.g B<prime256v1>). Curve names are case sensitive.
92 Sets the TLSv1.2 and below ciphersuite list to B<value>. This list will be
93 combined with any configured TLSv1.3 ciphersuites. Note: syntax checking
94 of B<value> is currently not performed unless a B<SSL> or B<SSL_CTX> structure is
95 associated with B<cctx>.
97 =item B<-ciphersuites>
99 Sets the available ciphersuites for TLSv1.3 to value. This is a simple colon
100 (":") separated list of TLSv1.3 ciphersuite names in order of preference. This
101 list will be combined any configured TLSv1.2 and below ciphersuites.
106 Attempts to use the file B<value> as the certificate for the appropriate
107 context. It currently uses SSL_CTX_use_certificate_chain_file() if an B<SSL_CTX>
108 structure is set or SSL_use_certificate_file() with filetype PEM if an B<SSL>
109 structure is set. This option is only supported if certificate operations
114 Attempts to use the file B<value> as the private key for the appropriate
115 context. This option is only supported if certificate operations
116 are permitted. Note: if no B<-key> option is set then a private key is
117 not loaded unless the flag B<SSL_CONF_FLAG_REQUIRE_PRIVATE> is set.
121 Attempts to use the file B<value> as the set of temporary DH parameters for
122 the appropriate context. This option is only supported if certificate
123 operations are permitted.
125 =item B<-record_padding>
127 Attempts to pad TLS 1.3 records so that they are a multiple of B<value> in
128 length on send. A B<value> of 0 or 1 turns off padding. Otherwise, the
129 B<value> must be >1 or <=16384.
131 =item B<-no_renegotiation>
133 Disables all attempts at renegotiation in TLSv1.2 and earlier, same as setting
134 B<SSL_OP_NO_RENEGOTIATION>.
136 =item B<-min_protocol>, B<-max_protocol>
138 Sets the minimum and maximum supported protocol.
139 Currently supported protocol values are B<SSLv3>, B<TLSv1>,
140 B<TLSv1.1>, B<TLSv1.2> for TLS and B<DTLSv1>, B<DTLSv1.2> for DTLS,
141 and B<None> for no limit.
142 If the either bound is not specified then only the other bound applies,
144 To restrict the supported protocol versions use these commands rather
145 than the deprecated alternative commands below.
147 =item B<-no_ssl3>, B<-no_tls1>, B<-no_tls1_1>, B<-no_tls1_2>, B<-no_tls1_3>
149 Disables protocol support for SSLv3, TLSv1.0, TLSv1.1, TLSv1.2 or TLSv1.3 by
150 setting the corresponding options B<SSL_OP_NO_SSLv3>, B<SSL_OP_NO_TLSv1>,
151 B<SSL_OP_NO_TLSv1_1>, B<SSL_OP_NO_TLSv1_2> and B<SSL_OP_NO_TLSv1_3>
152 respectively. These options are deprecated, instead use B<-min_protocol> and
157 Various bug workarounds are set, same as setting B<SSL_OP_ALL>.
161 Enables support for SSL/TLS compression, same as clearing
162 B<SSL_OP_NO_COMPRESSION>.
163 This command was introduced in OpenSSL 1.1.0.
164 As of OpenSSL 1.1.0, compression is off by default.
168 Disables support for SSL/TLS compression, same as setting
169 B<SSL_OP_NO_COMPRESSION>.
170 As of OpenSSL 1.1.0, compression is off by default.
174 Disables support for session tickets, same as setting B<SSL_OP_NO_TICKET>.
178 Use server and not client preference order when determining which cipher suite,
179 signature algorithm or elliptic curve to use for an incoming connection.
180 Equivalent to B<SSL_OP_CIPHER_SERVER_PREFERENCE>. Only used by servers.
182 =item B<-prioritize_chacha>
184 Prioritize ChaCha ciphers when the client has a ChaCha20 cipher at the top of
185 its preference list. This usually indicates a client without AES hardware
186 acceleration (e.g. mobile) is in use. Equivalent to B<SSL_OP_PRIORITIZE_CHACHA>.
187 Only used by servers. Requires B<-serverpref>.
189 =item B<-no_resumption_on_reneg>
191 set SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION flag. Only used by servers.
193 =item B<-legacyrenegotiation>
195 permits the use of unsafe legacy renegotiation. Equivalent to setting
196 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>.
198 =item B<-legacy_server_connect>, B<-no_legacy_server_connect>
200 permits or prohibits the use of unsafe legacy renegotiation for OpenSSL
201 clients only. Equivalent to setting or clearing B<SSL_OP_LEGACY_SERVER_CONNECT>.
204 =item B<-allow_no_dhe_kex>
206 In TLSv1.3 allow a non-(ec)dhe based key exchange mode on resumption. This means
207 that there will be no forward secrecy for the resumed session.
211 enables strict mode protocol handling. Equivalent to setting
212 B<SSL_CERT_FLAG_TLS_STRICT>.
216 =head1 SUPPORTED CONFIGURATION FILE COMMANDS
218 Currently supported B<cmd> names for configuration files (i.e. when the
219 flag B<SSL_CONF_FLAG_FILE> is set) are listed below. All configuration file
220 B<cmd> names are case insensitive so B<signaturealgorithms> is recognised
221 as well as B<SignatureAlgorithms>. Unless otherwise stated the B<value> names
222 are also case insensitive.
224 Note: the command prefix (if set) alters the recognised B<cmd> values.
228 =item B<CipherString>
230 Sets the ciphersuite list for TLSv1.2 and below to B<value>. This list will be
231 combined with any configured TLSv1.3 ciphersuites. Note: syntax
232 checking of B<value> is currently not performed unless an B<SSL> or B<SSL_CTX>
233 structure is associated with B<cctx>.
235 =item B<Ciphersuites>
237 Sets the available ciphersuites for TLSv1.3 to B<value>. This is a simple colon
238 (":") separated list of TLSv1.3 ciphersuite names in order of preference. This
239 list will be combined any configured TLSv1.2 and below ciphersuites.
243 Attempts to use the file B<value> as the certificate for the appropriate
244 context. It currently uses SSL_CTX_use_certificate_chain_file() if an B<SSL_CTX>
245 structure is set or SSL_use_certificate_file() with filetype PEM if an B<SSL>
246 structure is set. This option is only supported if certificate operations
251 Attempts to use the file B<value> as the private key for the appropriate
252 context. This option is only supported if certificate operations
253 are permitted. Note: if no B<PrivateKey> option is set then a private key is
254 not loaded unless the B<SSL_CONF_FLAG_REQUIRE_PRIVATE> is set.
256 =item B<ChainCAFile>, B<ChainCAPath>, B<VerifyCAFile>, B<VerifyCAPath>
258 These options indicate a file or directory used for building certificate
259 chains or verifying certificate chains. These options are only supported
260 if certificate operations are permitted.
262 =item B<RequestCAFile>
264 This option indicates a file containing a set of certificates in PEM form.
265 The subject names of the certificates are sent to the peer in the
266 B<certificate_authorities> extension for TLS 1.3 (in ClientHello or
267 CertificateRequest) or in a certificate request for previous versions or
270 =item B<ServerInfoFile>
272 Attempts to use the file B<value> in the "serverinfo" extension using the
273 function SSL_CTX_use_serverinfo_file.
275 =item B<DHParameters>
277 Attempts to use the file B<value> as the set of temporary DH parameters for
278 the appropriate context. This option is only supported if certificate
279 operations are permitted.
281 =item B<RecordPadding>
283 Attempts to pad TLS 1.3 records so that they are a multiple of B<value> in
284 length on send. A B<value> of 0 or 1 turns off padding. Otherwise, the
285 B<value> must be >1 or <=16384.
287 =item B<NoRenegotiation>
289 Disables all attempts at renegotiation in TLSv1.2 and earlier, same as setting
290 B<SSL_OP_NO_RENEGOTIATION>.
292 =item B<SignatureAlgorithms>
294 This sets the supported signature algorithms for TLS v1.2. For clients this
295 value is used directly for the supported signature algorithms extension. For
296 servers it is used to determine which signature algorithms to support.
298 The B<value> argument should be a colon separated list of signature algorithms
299 in order of decreasing preference of the form B<algorithm+hash>. B<algorithm>
300 is one of B<RSA>, B<DSA> or B<ECDSA> and B<hash> is a supported algorithm
301 OID short name such as B<SHA1>, B<SHA224>, B<SHA256>, B<SHA384> of B<SHA512>.
302 Note: algorithm and hash names are case sensitive.
304 If this option is not set then all signature algorithms supported by the
305 OpenSSL library are permissible.
307 =item B<ClientSignatureAlgorithms>
309 This sets the supported signature algorithms associated with client
310 authentication for TLS v1.2. For servers the value is used in the supported
311 signature algorithms field of a certificate request. For clients it is
312 used to determine which signature algorithm to with the client certificate.
314 The syntax of B<value> is identical to B<SignatureAlgorithms>. If not set then
315 the value set for B<SignatureAlgorithms> will be used instead.
319 This sets the supported groups. For clients, the groups are
320 sent using the supported groups extension. For servers, it is used
321 to determine which group to use. This setting affects groups used for both
322 signatures and key exchange, if applicable. It also affects the preferred
323 key_share sent by a client in a TLSv1.3 compatible connection.
325 The B<value> argument is a colon separated list of groups. The group can be
326 either the B<NIST> name (e.g. B<P-256>), some other commonly used name where
327 applicable (e.g. B<X25519>) or an OpenSSL OID name (e.g B<prime256v1>). Group
328 names are case sensitive. The list should be in order of preference with the
329 most preferred group first. The first listed group will be the one used for a
330 key_share by a TLSv1.3 client.
334 This is a synonym for the "Groups" command.
338 This sets the minimum supported SSL, TLS or DTLS version.
340 Currently supported protocol values are B<SSLv3>, B<TLSv1>, B<TLSv1.1>,
341 B<TLSv1.2>, B<DTLSv1> and B<DTLSv1.2>.
342 The value B<None> will disable the limit.
346 This sets the maximum supported SSL, TLS or DTLS version.
348 Currently supported protocol values are B<SSLv3>, B<TLSv1>, B<TLSv1.1>,
349 B<TLSv1.2>, B<DTLSv1> and B<DTLSv1.2>.
350 The value B<None> will disable the limit.
354 This can be used to enable or disable certain versions of the SSL,
355 TLS or DTLS protocol.
357 The B<value> argument is a comma separated list of supported protocols
358 to enable or disable.
359 If a protocol is preceded by B<-> that version is disabled.
361 All protocol versions are enabled by default.
362 You need to disable at least one protocol version for this setting have any
364 Only enabling some protocol versions does not disable the other protocol
367 Currently supported protocol values are B<SSLv3>, B<TLSv1>, B<TLSv1.1>,
368 B<TLSv1.2>, B<DTLSv1> and B<DTLSv1.2>.
369 The special value B<ALL> refers to all supported versions.
371 This can't enable protocols that are disabled using B<MinProtocol>
372 or B<MaxProtocol>, but can disable protocols that are still allowed
375 The B<Protocol> command is fragile and deprecated; do not use it.
376 Use B<MinProtocol> and B<MaxProtocol> instead.
377 If you do use B<Protocol>, make sure that the resulting range of enabled
378 protocols has no "holes", e.g. if TLS 1.0 and TLS 1.2 are both enabled, make
379 sure to also leave TLS 1.1 enabled.
383 The B<value> argument is a comma separated list of various flags to set.
384 If a flag string is preceded B<-> it is disabled.
385 See the L<SSL_CTX_set_options(3)> function for more details of
388 Each option is listed below. Where an operation is enabled by default
389 the B<-flag> syntax is needed to disable it.
391 B<SessionTicket>: session ticket support, enabled by default. Inverse of
392 B<SSL_OP_NO_TICKET>: that is B<-SessionTicket> is the same as setting
395 B<Compression>: SSL/TLS compression support, enabled by default. Inverse
396 of B<SSL_OP_NO_COMPRESSION>.
398 B<EmptyFragments>: use empty fragments as a countermeasure against a
399 SSL 3.0/TLS 1.0 protocol vulnerability affecting CBC ciphers. It
400 is set by default. Inverse of B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS>.
402 B<Bugs>: enable various bug workarounds. Same as B<SSL_OP_ALL>.
404 B<DHSingle>: enable single use DH keys, set by default. Inverse of
405 B<SSL_OP_DH_SINGLE>. Only used by servers.
407 B<ECDHSingle>: enable single use ECDH keys, set by default. Inverse of
408 B<SSL_OP_ECDH_SINGLE>. Only used by servers.
410 B<ServerPreference>: use server and not client preference order when
411 determining which cipher suite, signature algorithm or elliptic curve
412 to use for an incoming connection. Equivalent to
413 B<SSL_OP_CIPHER_SERVER_PREFERENCE>. Only used by servers.
415 B<PrioritizeChaCha>: prioritizes ChaCha ciphers when the client has a
416 ChaCha20 cipher at the top of its preference list. This usually indicates
417 a mobile client is in use. Equivalent to B<SSL_OP_PRIORITIZE_CHACHA>.
418 Only used by servers.
420 B<NoResumptionOnRenegotiation>: set
421 B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> flag. Only used by servers.
423 B<UnsafeLegacyRenegotiation>: permits the use of unsafe legacy renegotiation.
424 Equivalent to B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>.
426 B<UnsafeLegacyServerConnect>: permits the use of unsafe legacy renegotiation
427 for OpenSSL clients only. Equivalent to B<SSL_OP_LEGACY_SERVER_CONNECT>.
430 B<EncryptThenMac>: use encrypt-then-mac extension, enabled by
431 default. Inverse of B<SSL_OP_NO_ENCRYPT_THEN_MAC>: that is,
432 B<-EncryptThenMac> is the same as setting B<SSL_OP_NO_ENCRYPT_THEN_MAC>.
434 B<AllowNoDHEKEX>: In TLSv1.3 allow a non-(ec)dhe based key exchange mode on
435 resumption. This means that there will be no forward secrecy for the resumed
436 session. Equivalent to B<SSL_OP_ALLOW_NO_DHE_KEX>.
438 B<MiddleboxCompat>: If set then dummy Change Cipher Spec (CCS) messages are sent
439 in TLSv1.3. This has the effect of making TLSv1.3 look more like TLSv1.2 so that
440 middleboxes that do not understand TLSv1.3 will not drop the connection. This
441 option is set by default. A future version of OpenSSL may not set this by
442 default. Equivalent to B<SSL_OP_ENABLE_MIDDLEBOX_COMPAT>.
446 The B<value> argument is a comma separated list of flags to set.
448 B<Peer> enables peer verification: for clients only.
450 B<Request> requests but does not require a certificate from the client.
453 B<Require> requests and requires a certificate from the client: an error
454 occurs if the client does not present a certificate. Servers only.
456 B<Once> requests a certificate from a client only on the initial connection:
457 not when renegotiating. Servers only.
459 B<RequestPostHandshake> configures the connection to support requests but does
460 not require a certificate from the client post-handshake. A certificate will
461 not be requested during the initial handshake. The server application must
462 provide a mechanism to request a certificate post-handshake. Servers only.
465 B<RequiresPostHandshake> configures the connection to support requests and
466 requires a certificate from the client post-handshake: an error occurs if the
467 client does not present a certificate. A certificate will not be requested
468 during the initial handshake. The server application must provide a mechanism
469 to request a certificate post-handshake. Servers only. TLSv1.3 only.
471 =item B<ClientCAFile>, B<ClientCAPath>
473 A file or directory of certificates in PEM format whose names are used as the
474 set of acceptable names for client CAs. Servers only. This option is only
475 supported if certificate operations are permitted.
479 =head1 SUPPORTED COMMAND TYPES
481 The function SSL_CONF_cmd_value_type() currently returns one of the following
486 =item B<SSL_CONF_TYPE_UNKNOWN>
488 The B<cmd> string is unrecognised, this return value can be use to flag
491 =item B<SSL_CONF_TYPE_STRING>
493 The value is a string without any specific structure.
495 =item B<SSL_CONF_TYPE_FILE>
497 The value is a file name.
499 =item B<SSL_CONF_TYPE_DIR>
501 The value is a directory name.
503 =item B<SSL_CONF_TYPE_NONE>
505 The value string is not used e.g. a command line option which doesn't take an
512 The order of operations is significant. This can be used to set either defaults
513 or values which cannot be overridden. For example if an application calls:
515 SSL_CONF_cmd(ctx, "Protocol", "-SSLv3");
516 SSL_CONF_cmd(ctx, userparam, uservalue);
518 it will disable SSLv3 support by default but the user can override it. If
519 however the call sequence is:
521 SSL_CONF_cmd(ctx, userparam, uservalue);
522 SSL_CONF_cmd(ctx, "Protocol", "-SSLv3");
524 SSLv3 is B<always> disabled and attempt to override this by the user are
527 By checking the return code of SSL_CONF_cmd() it is possible to query if a
528 given B<cmd> is recognised, this is useful if SSL_CONF_cmd() values are
529 mixed with additional application specific operations.
531 For example an application might call SSL_CONF_cmd() and if it returns
532 -2 (unrecognised command) continue with processing of application specific
535 Applications can also use SSL_CONF_cmd() to process command lines though the
536 utility function SSL_CONF_cmd_argv() is normally used instead. One way
537 to do this is to set the prefix to an appropriate value using
538 SSL_CONF_CTX_set1_prefix(), pass the current argument to B<cmd> and the
539 following argument to B<value> (which may be NULL).
541 In this case if the return value is positive then it is used to skip that
542 number of arguments as they have been processed by SSL_CONF_cmd(). If -2 is
543 returned then B<cmd> is not recognised and application specific arguments
544 can be checked instead. If -3 is returned a required argument is missing
545 and an error is indicated. If 0 is returned some other error occurred and
546 this can be reported back to the user.
548 The function SSL_CONF_cmd_value_type() can be used by applications to
549 check for the existence of a command or to perform additional syntax
550 checking or translation of the command value. For example if the return
551 value is B<SSL_CONF_TYPE_FILE> an application could translate a relative
552 pathname to an absolute pathname.
556 Set supported signature algorithms:
558 SSL_CONF_cmd(ctx, "SignatureAlgorithms", "ECDSA+SHA256:RSA+SHA256:DSA+SHA256");
560 There are various ways to select the supported protocols.
562 This set the minimum protocol version to TLSv1, and so disables SSLv3.
563 This is the recommended way to disable protocols.
565 SSL_CONF_cmd(ctx, "MinProtocol", "TLSv1");
567 The following also disables SSLv3:
569 SSL_CONF_cmd(ctx, "Protocol", "-SSLv3");
571 The following will first enable all protocols, and then disable
573 If no protocol versions were disabled before this has the same effect as
574 "-SSLv3", but if some versions were disables this will re-enable them before
577 SSL_CONF_cmd(ctx, "Protocol", "ALL,-SSLv3");
581 SSL_CONF_cmd(ctx, "MinProtocol", "TLSv1.2");
582 SSL_CONF_cmd(ctx, "MaxProtocol", "TLSv1.2");
584 This also only enables TLSv1.2:
586 SSL_CONF_cmd(ctx, "Protocol", "-ALL,TLSv1.2");
588 Disable TLS session tickets:
590 SSL_CONF_cmd(ctx, "Options", "-SessionTicket");
594 SSL_CONF_cmd(ctx, "Options", "Compression");
596 Set supported curves to P-256, P-384:
598 SSL_CONF_cmd(ctx, "Curves", "P-256:P-384");
602 SSL_CONF_cmd() returns 1 if the value of B<cmd> is recognised and B<value> is
603 B<NOT> used and 2 if both B<cmd> and B<value> are used. In other words it
604 returns the number of arguments processed. This is useful when processing
607 A return value of -2 means B<cmd> is not recognised.
609 A return value of -3 means B<cmd> is recognised and the command requires a
610 value but B<value> is NULL.
612 A return code of 0 indicates that both B<cmd> and B<value> are valid but an
613 error occurred attempting to perform the operation: for example due to an
614 error in the syntax of B<value> in this case the error queue may provide
615 additional information.
619 L<SSL_CONF_CTX_new(3)>,
620 L<SSL_CONF_CTX_set_flags(3)>,
621 L<SSL_CONF_CTX_set1_prefix(3)>,
622 L<SSL_CONF_CTX_set_ssl_ctx(3)>,
623 L<SSL_CONF_cmd_argv(3)>,
624 L<SSL_CTX_set_options(3)>
628 SSL_CONF_cmd() was first added to OpenSSL 1.0.2
630 B<SSL_OP_NO_SSL2> doesn't have effect since 1.1.0, but the macro is retained
631 for backwards compatibility.
633 B<SSL_CONF_TYPE_NONE> was first added to OpenSSL 1.1.0. In earlier versions of
634 OpenSSL passing a command which didn't take an argument would return
635 B<SSL_CONF_TYPE_UNKNOWN>.
637 B<MinProtocol> and B<MaxProtocol> where added in OpenSSL 1.1.0.
639 B<AllowNoDHEKEX> and B<PrioritizeChaCha> were added in OpenSSL 1.1.1.
643 Copyright 2012-2018 The OpenSSL Project Authors. All Rights Reserved.
645 Licensed under the OpenSSL license (the "License"). You may not use
646 this file except in compliance with the License. You can obtain a copy
647 in the file LICENSE in the source distribution or at
648 L<https://www.openssl.org/source/license.html>.