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=pod
=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_ct_is_enabled, SSL_CTX_ct_is_enabled -
control Certificate Transparency policy
=head1 SYNOPSIS
#include <openssl/ssl.h>
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_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
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
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>).
=head1 RETURN VALUES
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_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<SSL_session_reused(3)>,
L<SSL_set_verify(3)>,
L<SSL_CTX_set_verify(3)>,
L<ssl_ct_validation_cb(3)>
=cut
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