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✏️ Editing: SSL_dane_enable.3

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.\" ========================================================================
.\"
.IX Title "SSL_CTX_DANE_ENABLE 3"
.TH SSL_CTX_DANE_ENABLE 3 "2023-09-11" "1.1.1w" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
SSL_CTX_dane_enable, SSL_CTX_dane_mtype_set, SSL_dane_enable, SSL_dane_tlsa_add, SSL_get0_dane_authority, SSL_get0_dane_tlsa, SSL_CTX_dane_set_flags, SSL_CTX_dane_clear_flags, SSL_dane_set_flags, SSL_dane_clear_flags \&\- enable DANE TLS authentication of the remote TLS server in the local TLS client
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/ssl.h>
\&
\& int SSL_CTX_dane_enable(SSL_CTX *ctx);
\& int SSL_CTX_dane_mtype_set(SSL_CTX *ctx, const EVP_MD *md,
\&                            uint8_t mtype, uint8_t ord);
\& int SSL_dane_enable(SSL *s, const char *basedomain);
\& int SSL_dane_tlsa_add(SSL *s, uint8_t usage, uint8_t selector,
\&                       uint8_t mtype, unsigned const char *data, size_t dlen);
\& int SSL_get0_dane_authority(SSL *s, X509 **mcert, EVP_PKEY **mspki);
\& int SSL_get0_dane_tlsa(SSL *s, uint8_t *usage, uint8_t *selector,
\&                        uint8_t *mtype, unsigned const char **data,
\&                        size_t *dlen);
\& unsigned long SSL_CTX_dane_set_flags(SSL_CTX *ctx, unsigned long flags);
\& unsigned long SSL_CTX_dane_clear_flags(SSL_CTX *ctx, unsigned long flags);
\& unsigned long SSL_dane_set_flags(SSL *ssl, unsigned long flags);
\& unsigned long SSL_dane_clear_flags(SSL *ssl, unsigned long flags);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
These functions implement support for \s-1DANE TLSA\s0 (\s-1RFC6698\s0 and \s-1RFC7671\s0)
peer authentication.
.PP
\&\fBSSL_CTX_dane_enable()\fR must be called first to initialize the shared state
required for \s-1DANE\s0 support.
Individual connections associated with the context can then enable
per-connection \s-1DANE\s0 support as appropriate.
\&\s-1DANE\s0 authentication is implemented in the \fBX509_verify_cert\fR\|(3) function, and
applications that override \fBX509_verify_cert\fR\|(3) via
\&\fBSSL_CTX_set_cert_verify_callback\fR\|(3) are responsible to authenticate the peer
chain in whatever manner they see fit.
.PP
\&\fBSSL_CTX_dane_mtype_set()\fR may then be called zero or more times to adjust the
supported digest algorithms.
This must be done before any \s-1SSL\s0 handles are created for the context.
.PP
The \fBmtype\fR argument specifies a \s-1DANE TLSA\s0 matching type and the \fBmd\fR
argument specifies the associated digest algorithm handle.
The \fBord\fR argument specifies a strength ordinal.
Algorithms with a larger strength ordinal are considered more secure.
Strength ordinals are used to implement \s-1RFC7671\s0 digest algorithm agility.
Specifying a \fB\s-1NULL\s0\fR digest algorithm for a matching type disables
support for that matching type.
Matching type \fBFull\fR\|(0) cannot be modified or disabled.
.PP
By default, matching type \f(CW\*(C`SHA2\-256(1)\*(C'\fR (see \s-1RFC7218\s0 for definitions
of the \s-1DANE TLSA\s0 parameter acronyms) is mapped to \f(CW\*(C`EVP_sha256()\*(C'\fR
with a strength ordinal of \f(CW1\fR and matching type \f(CW\*(C`SHA2\-512(2)\*(C'\fR
is mapped to \f(CW\*(C`EVP_sha512()\*(C'\fR with a strength ordinal of \f(CW2\fR.
.PP
\&\fBSSL_dane_enable()\fR must be called before the \s-1SSL\s0 handshake is initiated with
\&\fBSSL_connect\fR\|(3) if (and only if) you want to enable \s-1DANE\s0 for that connection.
(The connection must be associated with a DANE-enabled \s-1SSL\s0 context).
The \fBbasedomain\fR argument specifies the \s-1RFC7671 TLSA\s0 base domain,
which will be the primary peer reference identifier for certificate
name checks.
Additional server names can be specified via \fBSSL_add1_host\fR\|(3).
The \fBbasedomain\fR is used as the default \s-1SNI\s0 hint if none has yet been
specified via \fBSSL_set_tlsext_host_name\fR\|(3).
.PP
\&\fBSSL_dane_tlsa_add()\fR may then be called one or more times, to load each of the
\&\s-1TLSA\s0 records that apply to the remote \s-1TLS\s0 peer.
(This too must be done prior to the beginning of the \s-1SSL\s0 handshake).
The arguments specify the fields of the \s-1TLSA\s0 record.
The \fBdata\fR field is provided in binary (wire \s-1RDATA\s0) form, not the hexadecimal
\&\s-1ASCII\s0 presentation form, with an explicit length passed via \fBdlen\fR.
The library takes a copy of the \fBdata\fR buffer contents and the caller may
free the original \fBdata\fR buffer when convenient.
A return value of 0 indicates that \*(L"unusable\*(R" \s-1TLSA\s0 records (with invalid or
unsupported parameters) were provided.
A negative return value indicates an internal error in processing the record.
.PP
The caller is expected to check the return value of each \fBSSL_dane_tlsa_add()\fR
call and take appropriate action if none are usable or an internal error
is encountered in processing some records.
.PP
If no \s-1TLSA\s0 records are added successfully, \s-1DANE\s0 authentication is not enabled,
and authentication will be based on any configured traditional trust-anchors;
authentication success in this case does not mean that the peer was
DANE-authenticated.
.PP
\&\fBSSL_get0_dane_authority()\fR can be used to get more detailed information about
the matched \s-1DANE\s0 trust-anchor after successful connection completion.
The return value is negative if \s-1DANE\s0 verification failed (or was not enabled),
0 if an \s-1EE TLSA\s0 record directly matched the leaf certificate, or a positive
number indicating the depth at which a \s-1TA\s0 record matched an issuer certificate.
The complete verified chain can be retrieved via \fBSSL_get0_verified_chain\fR\|(3).
The return value is an index into this verified chain, rather than the list of
certificates sent by the peer as returned by \fBSSL_get_peer_cert_chain\fR\|(3).
.PP
If the \fBmcert\fR argument is not \fB\s-1NULL\s0\fR and a \s-1TLSA\s0 record matched a chain
certificate, a pointer to the matching certificate is returned via \fBmcert\fR.
The returned address is a short-term internal reference to the certificate and
must not be freed by the application.
Applications that want to retain access to the certificate can call
\&\fBX509_up_ref\fR\|(3) to obtain a long-term reference which must then be freed via
\&\fBX509_free\fR\|(3) once no longer needed.
.PP
If no \s-1TLSA\s0 records directly matched any elements of the certificate chain, but
a \s-1\fBDANE\-TA\s0\fR\|(2) \s-1\fBSPKI\s0\fR\|(1) \fBFull\fR\|(0) record provided the public key that signed an
element of the chain, then that key is returned via \fBmspki\fR argument (if not
\&\s-1NULL\s0).
In this case the return value is the depth of the top-most element of the
validated certificate chain.
As with \fBmcert\fR this is a short-term internal reference, and
\&\fBEVP_PKEY_up_ref\fR\|(3) and \fBEVP_PKEY_free\fR\|(3) can be used to acquire and
release long-term references respectively.
.PP
\&\fBSSL_get0_dane_tlsa()\fR can be used to retrieve the fields of the \s-1TLSA\s0 record that
matched the peer certificate chain.
The return value indicates the match depth or failure to match just as with
\&\fBSSL_get0_dane_authority()\fR.
When the return value is nonnegative, the storage pointed to by the \fBusage\fR,
\&\fBselector\fR, \fBmtype\fR and \fBdata\fR parameters is updated to the corresponding
\&\s-1TLSA\s0 record fields.
The \fBdata\fR field is in binary wire form, and is therefore not NUL-terminated,
its length is returned via the \fBdlen\fR parameter.
If any of these parameters is \s-1NULL,\s0 the corresponding field is not returned.
The \fBdata\fR parameter is set to a short-term internal-copy of the associated
data field and must not be freed by the application.
Applications that need long-term access to this field need to copy the content.
.PP
\&\fBSSL_CTX_dane_set_flags()\fR and \fBSSL_dane_set_flags()\fR can be used to enable
optional \s-1DANE\s0 verification features.
\&\fBSSL_CTX_dane_clear_flags()\fR and \fBSSL_dane_clear_flags()\fR can be used to disable
the same features.
The \fBflags\fR argument is a bit mask of the features to enable or disable.
The \fBflags\fR set for an \fB\s-1SSL_CTX\s0\fR context are copied to each \fB\s-1SSL\s0\fR handle
associated with that context at the time the handle is created.
Subsequent changes in the context's \fBflags\fR have no effect on the \fBflags\fR set
for the handle.
.PP
At present, the only available option is \fB\s-1DANE_FLAG_NO_DANE_EE_NAMECHECKS\s0\fR
which can be used to disable server name checks when authenticating via
\&\s-1\fBDANE\-EE\s0\fR\|(3) \s-1TLSA\s0 records.
For some applications, primarily web browsers, it is not safe to disable name
checks due to \*(L"unknown key share\*(R" attacks, in which a malicious server can
convince a client that a connection to a victim server is instead a secure
connection to the malicious server.
The malicious server may then be able to violate cross-origin scripting
restrictions.
Thus, despite the text of \s-1RFC7671,\s0 name checks are by default enabled for
\&\s-1\fBDANE\-EE\s0\fR\|(3) \s-1TLSA\s0 records, and can be disabled in applications where it is safe
to do so.
In particular, \s-1SMTP\s0 and \s-1XMPP\s0 clients should set this option as \s-1SRV\s0 and \s-1MX\s0
records already make it possible for a remote domain to redirect client
connections to any server of its choice, and in any case \s-1SMTP\s0 and \s-1XMPP\s0 clients
do not execute scripts downloaded from remote servers.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
The functions \fBSSL_CTX_dane_enable()\fR, \fBSSL_CTX_dane_mtype_set()\fR,
\&\fBSSL_dane_enable()\fR and \fBSSL_dane_tlsa_add()\fR return a positive value on success.
Negative return values indicate resource problems (out of memory, etc.) in the
\&\s-1SSL\s0 library, while a return value of \fB0\fR indicates incorrect usage or invalid
input, such as an unsupported \s-1TLSA\s0 record certificate usage, selector or
matching type.
Invalid input also includes malformed data, either a digest length that does
not match the digest algorithm, or a \f(CWFull(0)\fR (binary \s-1ASN.1 DER\s0 form)
certificate or a public key that fails to parse.
.PP
The functions \fBSSL_get0_dane_authority()\fR and \fBSSL_get0_dane_tlsa()\fR return a
negative value when \s-1DANE\s0 authentication failed or was not enabled, a
nonnegative value indicates the chain depth at which the \s-1TLSA\s0 record matched a
chain certificate, or the depth of the top-most certificate, when the \s-1TLSA\s0
record is a full public key that is its signer.
.PP
The functions \fBSSL_CTX_dane_set_flags()\fR, \fBSSL_CTX_dane_clear_flags()\fR,
\&\fBSSL_dane_set_flags()\fR and \fBSSL_dane_clear_flags()\fR return the \fBflags\fR in effect
before they were called.
.SH "EXAMPLES"
.IX Header "EXAMPLES"
Suppose \*(L"smtp.example.com\*(R" is the \s-1MX\s0 host of the domain \*(L"example.com\*(R", and has
DNSSEC-validated \s-1TLSA\s0 records.
The calls below will perform \s-1DANE\s0 authentication and arrange to match either
the \s-1MX\s0 hostname or the destination domain name in the \s-1SMTP\s0 server certificate.
Wildcards are supported, but must match the entire label.
The actual name matched in the certificate (which might be a wildcard) is
retrieved, and must be copied by the application if it is to be retained beyond
the lifetime of the \s-1SSL\s0 connection.
.PP
.Vb 7
\& SSL_CTX *ctx;
\& SSL *ssl;
\& int (*verify_cb)(int ok, X509_STORE_CTX *sctx) = NULL;
\& int num_usable = 0;
\& const char *nexthop_domain = "example.com";
\& const char *dane_tlsa_domain = "smtp.example.com";
\& uint8_t usage, selector, mtype;
\&
\& if ((ctx = SSL_CTX_new(TLS_client_method())) == NULL)
\&     /* error */
\& if (SSL_CTX_dane_enable(ctx) <= 0)
\&     /* error */
\& if ((ssl = SSL_new(ctx)) == NULL)
\&     /* error */
\& if (SSL_dane_enable(ssl, dane_tlsa_domain) <= 0)
\&     /* error */
\&
\& /*
\&  * For many applications it is safe to skip DANE\-EE(3) namechecks.  Do not
\&  * disable the checks unless "unknown key share" attacks pose no risk for
\&  * your application.
\&  */
\& SSL_dane_set_flags(ssl, DANE_FLAG_NO_DANE_EE_NAMECHECKS);
\&
\& if (!SSL_add1_host(ssl, nexthop_domain))
\&     /* error */
\& SSL_set_hostflags(ssl, X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS);
\&
\& for (... each TLSA record ...) {
\&     unsigned char *data;
\&     size_t len;
\&     int ret;
\&
\&     /* set usage, selector, mtype, data, len */
\&
\&     /*
\&      * Opportunistic DANE TLS clients support only DANE\-TA(2) or DANE\-EE(3).
\&      * They treat all other certificate usages, and in particular PKIX\-TA(0)
\&      * and PKIX\-EE(1), as unusable.
\&      */
\&     switch (usage) {
\&     default:
\&     case 0:     /* PKIX\-TA(0) */
\&     case 1:     /* PKIX\-EE(1) */
\&         continue;
\&     case 2:     /* DANE\-TA(2) */
\&     case 3:     /* DANE\-EE(3) */
\&         break;
\&     }
\&
\&     ret = SSL_dane_tlsa_add(ssl, usage, selector, mtype, data, len);
\&     /* free data as appropriate */
\&
\&     if (ret < 0)
\&         /* handle SSL library internal error */
\&     else if (ret == 0)
\&         /* handle unusable TLSA record */
\&     else
\&         ++num_usable;
\& }
\&
\& /*
\&  * At this point, the verification mode is still the default SSL_VERIFY_NONE.
\&  * Opportunistic DANE clients use unauthenticated TLS when all TLSA records
\&  * are unusable, so continue the handshake even if authentication fails.
\&  */
\& if (num_usable == 0) {
\&     /* Log all records unusable? */
\&
\&     /* Optionally set verify_cb to a suitable non\-NULL callback. */
\&     SSL_set_verify(ssl, SSL_VERIFY_NONE, verify_cb);
\& } else {
\&     /* At least one usable record.  We expect to verify the peer */
\&
\&     /* Optionally set verify_cb to a suitable non\-NULL callback. */
\&
\&     /*
\&      * Below we elect to fail the handshake when peer verification fails.
\&      * Alternatively, use the permissive SSL_VERIFY_NONE verification mode,
\&      * complete the handshake, check the verification status, and if not
\&      * verified disconnect gracefully at the application layer, especially if
\&      * application protocol supports informing the server that authentication
\&      * failed.
\&      */
\&     SSL_set_verify(ssl, SSL_VERIFY_PEER, verify_cb);
\& }
\&
\& /*
\&  * Load any saved session for resumption, making sure that the previous
\&  * session applied the same security and authentication requirements that
\&  * would be expected of a fresh connection.
\&  */
\&
\& /* Perform SSL_connect() handshake and handle errors here */
\&
\& if (SSL_session_reused(ssl)) {
\&     if (SSL_get_verify_result(ssl) == X509_V_OK) {
\&         /*
\&          * Resumed session was originally verified, this connection is
\&          * authenticated.
\&          */
\&     } else {
\&         /*
\&          * Resumed session was not originally verified, this connection is not
\&          * authenticated.
\&          */
\&     }
\& } else if (SSL_get_verify_result(ssl) == X509_V_OK) {
\&     const char *peername = SSL_get0_peername(ssl);
\&     EVP_PKEY *mspki = NULL;
\&
\&     int depth = SSL_get0_dane_authority(ssl, NULL, &mspki);
\&     if (depth >= 0) {
\&         (void) SSL_get0_dane_tlsa(ssl, &usage, &selector, &mtype, NULL, NULL);
\&         printf("DANE TLSA %d %d %d %s at depth %d\en", usage, selector, mtype,
\&                (mspki != NULL) ? "TA public key verified certificate" :
\&                depth ? "matched TA certificate" : "matched EE certificate",
\&                depth);
\&     }
\&     if (peername != NULL) {
\&         /* Name checks were in scope and matched the peername */
\&         printf("Verified peername: %s\en", peername);
\&     }
\& } else {
\&     /*
\&      * Not authenticated, presumably all TLSA rrs unusable, but possibly a
\&      * callback suppressed connection termination despite the presence of
\&      * usable TLSA RRs none of which matched.  Do whatever is appropriate for
\&      * fresh unauthenticated connections.
\&      */
\& }
.Ve
.SH "NOTES"
.IX Header "NOTES"
It is expected that the majority of clients employing \s-1DANE TLS\s0 will be doing
\&\*(L"opportunistic \s-1DANE TLS\*(R"\s0 in the sense of \s-1RFC7672\s0 and \s-1RFC7435.\s0
That is, they will use \s-1DANE\s0 authentication when DNSSEC-validated \s-1TLSA\s0 records
are published for a given peer, and otherwise will use unauthenticated \s-1TLS\s0 or
even cleartext.
.PP
Such applications should generally treat any \s-1TLSA\s0 records published by the peer
with usages \s-1\fBPKIX\-TA\s0\fR\|(0) and \s-1\fBPKIX\-EE\s0\fR\|(1) as \*(L"unusable\*(R", and should not include
them among the \s-1TLSA\s0 records used to authenticate peer connections.
In addition, some \s-1TLSA\s0 records with supported usages may be \*(L"unusable\*(R" as a
result of invalid or unsupported parameters.
.PP
When a peer has \s-1TLSA\s0 records, but none are \*(L"usable\*(R", an opportunistic
application must avoid cleartext, but cannot authenticate the peer,
and so should generally proceed with an unauthenticated connection.
Opportunistic applications need to note the return value of each
call to \fBSSL_dane_tlsa_add()\fR, and if all return 0 (due to invalid
or unsupported parameters) disable peer authentication by calling
\&\fBSSL_set_verify\fR\|(3) with \fBmode\fR equal to \fB\s-1SSL_VERIFY_NONE\s0\fR.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBSSL_new\fR\|(3),
\&\fBSSL_add1_host\fR\|(3),
\&\fBSSL_set_hostflags\fR\|(3),
\&\fBSSL_set_tlsext_host_name\fR\|(3),
\&\fBSSL_set_verify\fR\|(3),
\&\fBSSL_CTX_set_cert_verify_callback\fR\|(3),
\&\fBSSL_get0_verified_chain\fR\|(3),
\&\fBSSL_get_peer_cert_chain\fR\|(3),
\&\fBSSL_get_verify_result\fR\|(3),
\&\fBSSL_connect\fR\|(3),
\&\fBSSL_get0_peername\fR\|(3),
\&\fBX509_verify_cert\fR\|(3),
\&\fBX509_up_ref\fR\|(3),
\&\fBX509_free\fR\|(3),
\&\fBEVP_get_digestbyname\fR\|(3),
\&\fBEVP_PKEY_up_ref\fR\|(3),
\&\fBEVP_PKEY_free\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
These functions were added in OpenSSL 1.1.0.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2016\-2020 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.

MR SKB