Suppress CT callback as appropriate

Suppress CT callbacks with aNULL or PSK ciphersuites that involve
no certificates.  Ditto when the certificate chain is validated via
DANE-TA(2) or DANE-EE(3) TLSA records.  Also skip SCT processing
when the chain is fails verification.

Move and consolidate CT callbacks from libcrypto to libssl.  We
also simplify the interface to SSL_{,CTX_}_enable_ct() which can
specify either a permissive mode that just collects information or
a strict mode that requires at least one valid SCT or else asks to
abort the connection.

Simplified SCT processing and options in s_client(1) which now has
just a simple pair of "-noct" vs. "-ct" options, the latter enables
the permissive callback so that we can complete the handshake and
report all relevant information.  When printing SCTs, print the
validation status if set and not valid.

Signed-off-by: Rob Percival <robpercival@google.com>
Reviewed-by: Emilia Käsper <emilia@openssl.org>

3 files changed, 91 insertions, 38 deletions

diff --git a/doc/apps/s_client.pod b/doc/apps/s_client.pod
index 881fbcfefe..e06af14ec9 100644
--- a/doc/apps/s_client.pod
+++ b/doc/apps/s_client.pod
@@ -95,7 +95,7 @@ B<openssl> B<s_client>
 [B<-serverinfo types>]
 [B<-status>]
 [B<-nextprotoneg protocols>]
-[B<-noct|requestct|requirect>]
+[B<-ct|noct>]
 [B<-ctlogfile>]
 
 =head1 DESCRIPTION
@@ -464,14 +464,12 @@ Empty list of protocols is treated specially and will cause the client to
 advertise support for the TLS extension but disconnect just after
 receiving ServerHello with a list of server supported protocols.
 
-=item B<-noct|requestct|requirect>
+=item B<-ct|noct>
 
-Use one of these three options to control whether Certificate Transparency (CT)
-is disabled (-noct), enabled but not enforced (-requestct), or enabled and
-enforced (-requirect). If CT is enabled, signed certificate timestamps (SCTs)
-will be requested from the server and invalid SCTs will cause the connection to
-be aborted. If CT is enforced, at least one valid SCT from a recognised CT log
-(see B<-ctlogfile>) will be required or the connection will be aborted.
+Use one of these two options to control whether Certificate Transparency (CT)
+is enabled (B<-ct>) or disabled (B<-noct>).
+If CT is enabled, signed certificate timestamps (SCTs) will be requested from
+the server and reported at handshake completion.
 
 Enabling CT also enables OCSP stapling, as this is one possible delivery method
 for SCTs.
diff --git a/doc/ssl/SSL_CTX_set_ct_validation_callback.pod b/doc/ssl/SSL_CTX_set_ct_validation_callback.pod
index 167a044536..ec51c75eb4 100644
--- a/doc/ssl/SSL_CTX_set_ct_validation_callback.pod
+++ b/doc/ssl/SSL_CTX_set_ct_validation_callback.pod
@@ -2,39 +2,92 @@
 
 =head1 NAME
 
+SSL_ct_enable, SSL_CTX_ct_enable, SSL_ct_disable, SSL_CTX_ct_disable,
 SSL_set_ct_validation_callback, SSL_CTX_set_ct_validation_callback,
-SSL_get_ct_validation_callback, SSL_CTX_get_ct_validation_callback -
+SSL_ct_is_enabled, SSL_CTX_ct_is_enabled -
 control Certificate Transparency policy
 
 =head1 SYNOPSIS
 
  #include <openssl/ssl.h>
 
- int SSL_set_ct_validation_callback(SSL *s, ct_validation_cb callback, void *arg);
- int SSL_CTX_set_ct_validation_callback(SSL_CTX *ctx, ct_validation_cb callback, void *arg);
- ct_validation_cb SSL_get_ct_validation_callback(const SSL *s);
- ct_validation_cb SSL_CTX_get_ct_validation_callback(const SSL_CTX *ctx);
+ int SSL_ct_enable(SSL *s, int validation_mode);
+ int SSL_CTX_ct_enable(SSL_CTX *ctx, int validation_mode);
+ int SSL_set_ct_validation_callback(SSL *s, ssl_ct_validation_cb callback,
+                                    void *arg);
+ int SSL_CTX_set_ct_validation_callback(SSL_CTX *ctx,
+                                        ssl_ct_validation_cb callback,
+                                        void *arg);
+ void SSL_ct_disable(SSL *s);
+ void SSL_CTX_ct_disable(SSL_CTX *ctx);
+ int SSL_ct_is_enabled(const SSL *s);
+ int SSL_CTX_ct_is_enabled(const SSL_CTX *ctx);
 
 =head1 DESCRIPTION
 
-SSL_set_ct_validation_callback() and SSL_CTX_set_ct_validation_callback() set
-the function that is called when Certificate Transparency validation needs to
-occur. It is the responsibility of this function to examine the signed
-certificate timestamps (SCTs) that are passed to it and determine whether they
-are sufficient to allow the connection to continue. If they are, the function
-must return 1, otherwise it must return 0.
-
-An arbitrary piece of user data, B<arg>, can be passed in when setting the
-callback. This will be passed to the callback whenever it is invoked. Ownership
-of this userdata remains with the caller.
+SSL_ct_enable() and SSL_CTX_ct_enable() enable the processing of signed
+certificate timestamps (SCTs) either for a given SSL connection or for all
+connections that share the given SSL context, respectively.
+This is accomplished by setting a built-in CT validation callback.
+The behaviour of the callback is determined by the B<validation_mode> argument,
+which can be either of B<SSL_CT_VALIDATION_PERMISSIVE> or
+B<SSL_CT_VALIDATION_STRICT> as described below.
+
+If B<validation_mode> is equal to B<SSL_CT_VALIDATION_PERMISSIVE>, then the
+handshake continues regardless of the validation status of any SCTs.
+The application can inspect the validation status of the SCTs at handshake
+completion.
+Note that with session resumption there will not be any SCTs presented during
+the handshake.
+Therefore, in applications that delay SCT policy enforcement until after
+handshake completion, SCT checks should only be performed when the session is
+not reused.
+See L<SSL_session_reused(3)>.
+
+If B<validation_mode> is equal to B<SSL_CT_VALIDATION_STRICT>, then in a full
+TLS handshake with the verification mode set to B<SSL_VERIFY_PEER>, if the peer
+presents no valid SCTs the handshake will be aborted.
+See L<SSL_set_verify(3)>.
+
+SSL_set_ct_validation_callback() and SSL_CTX_set_ct_validation_callback()
+register a custom callback that may implement a different policy than either of
+the above.
+This callback can examine the peer's SCTs and determine whether they are
+sufficient to allow the connection to continue.
+The TLS handshake is aborted if the verification mode is not B<SSL_VERIFY_NONE>
+and the callback returns a non-positive result.
+
+An arbitrary callback context argument, B<arg>, can be passed in when setting
+the callback.
+This will be passed to the callback whenever it is invoked.
+Ownership of this context remains with the caller.
 
 If no callback is set, SCTs will not be requested and Certificate Transparency
 validation will not occur.
 
+No callback will be invoked when the peer presents no certificate, e.g. by
+employing an anonymous (aNULL) ciphersuite.
+In that case the handshake continues as it would had no callback been
+requested.
+Callbacks are also not invoked when the peer certificate chain is invalid or
+validated via DANE-TA(2) or DANE-EE(3) TLSA records which use a private X.509
+PKI, or no X.509 PKI at all, respectively.
+Clients that require SCTs are expected to not have enabled any aNULL ciphers
+nor to have specified server verification via DANE-TA(2) or DANE-EE(3) TLSA
+records.
+
+SSL_ct_disable() and SSL_CTX_ct_disable() turn off CT processing, whether
+enabled via the built-in or the custom callbacks, by setting a NULL callback.
+These may be implemented as macros.
+
+SSL_ct_is_enabled() and SSL_CTX_ct_is_enabled() return 1 if CT processing is
+enabled via either SSL_ct_enable() or a non-null custom callback, and 0
+otherwise.
+
 =head1 NOTES
 
-If a callback is set, OCSP stapling will be enabled. This is because one
-possible source of SCTs is the OCSP response from a server.
+When SCT processing is enabled, OCSP stapling will be enabled. This is because
+one possible source of SCTs is the OCSP response from a server.
 
 =head1 RESTRICTIONS
 
@@ -42,24 +95,26 @@ Certificate Transparency validation cannot be enabled and so a callback cannot
 be set if a custom client extension handler has been registered to handle SCT
 extensions (B<TLSEXT_TYPE_signed_certificate_timestamp>).
 
-If an SCT callback is enabled, a handshake may fail if the peer does
-not provide a certificate, which can happen when using opportunistic
-encryption with anonymous (B<aNULL>) cipher-suites enabled on both ends.
-SCTs should only be used when the application requires an authenticated
-connection, and wishes to perform additional validation on that identity.
-
 =head1 RETURN VALUES
 
-SSL_CTX_set_ct_validation_callback() and SSL_set_ct_validation_callback()
-return 1 if the B<callback> is successfully set. They return 0 if an error
-occurs, e.g. a custom client extension handler has been setup to handle SCTs.
+SSL_ct_enable(), SSL_CTX_ct_enable(), SSL_CTX_set_ct_validation_callback() and
+SSL_set_ct_validation_callback() return 1 if the B<callback> is successfully
+set.
+They return 0 if an error occurs, e.g. a custom client extension handler has
+been setup to handle SCTs.
+
+SSL_ct_disable() and SSL_CTX_ct_disable() do not return a result.
 
-SSL_CTX_get_ct_validation_callback() and SSL_get_ct_validation_callback()
-return the current callback, or NULL if no callback is set.
+SSL_CTX_ct_is_enabled() and SSL_ct_is_enabled() return a 1 if a non-null CT
+validation callback is set, or 0 if no callback (or equivalently a NULL
+callback) is set.
 
 =head1 SEE ALSO
 
 L<ssl(3)>,
-L<ct_validation_cb(3)>
+L<SSL_session_reused(3)>,
+L<SSL_set_verify(3)>,
+L<SSL_CTX_set_verify(3)>,
+L<ssl_ct_validation_cb(3)>
 
 =cut
diff --git a/doc/ssl/SSL_CTX_set_ctlog_list_file.pod b/doc/ssl/SSL_CTX_set_ctlog_list_file.pod
index 9ef15adb90..9e5798f04c 100644
--- a/doc/ssl/SSL_CTX_set_ctlog_list_file.pod
+++ b/doc/ssl/SSL_CTX_set_ctlog_list_file.pod
@@ -49,6 +49,6 @@ the case of an error, the log list may have been partially loaded.
 =head1 SEE ALSO
 
 L<ssl(3)>,
-L<ct_validation_cb(3)>
+L<ssl_ct_validation_cb(3)>
 
 =cut