Ubuntu Manpage: ssl_ct_validation_cb, SSL_enable_ct, SSL_CTX_enable_ct, SSL_disable

Provided by: libssl-doc_3.0.13-0ubuntu3.4_all

NAME

       ssl_ct_validation_cb, SSL_enable_ct, SSL_CTX_enable_ct, SSL_disable_ct,
       SSL_CTX_disable_ct, SSL_set_ct_validation_callback, SSL_CTX_set_ct_validation_callback,
       SSL_ct_is_enabled, SSL_CTX_ct_is_enabled - control Certificate Transparency policy

SYNOPSIS

        #include <openssl/ssl.h>

        typedef int (*ssl_ct_validation_cb)(const CT_POLICY_EVAL_CTX *ctx,
                                           const STACK_OF(SCT) *scts, void *arg);

        int SSL_enable_ct(SSL *s, int validation_mode);
        int SSL_CTX_enable_ct(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_disable_ct(SSL *s);
        void SSL_CTX_disable_ct(SSL_CTX *ctx);
        int SSL_ct_is_enabled(const SSL *s);
        int SSL_CTX_ct_is_enabled(const SSL_CTX *ctx);

DESCRIPTION

SSL_enable_ct() and SSL_CTX_enable_ct() 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 validation_mode argument,
which can be either of SSL_CT_VALIDATION_PERMISSIVE or SSL_CT_VALIDATION_STRICT as
described below.

If validation_mode is equal to SSL_CT_VALIDATION_STRICT, then in a full TLS handshake with
the verification mode set to SSL_VERIFY_PEER, if the peer presents no valid SCTs the
handshake will be aborted. If the verification mode is SSL_VERIFY_NONE, the handshake
will continue despite lack of valid SCTs. However, in that case if the verification
status before the built-in callback was X509_V_OK it will be set to
X509_V_ERR_NO_VALID_SCTS after the callback. Applications can call
SSL_get_verify_result(3) to check the status at handshake completion, even after session
resumption since the verification status is part of the saved session state. See
SSL_set_verify(3), <SSL_get_verify_result(3)>, SSL_session_reused(3).

If validation_mode is equal to SSL_CT_VALIDATION_PERMISSIVE, then the handshake continues,
and the verification status is not modified, regardless of the validation status of any
SCTs. The application can still 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, such delayed SCT checks should only be performed when the session is
not resumed.

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
SSL_VERIFY_NONE and the callback returns a non-positive result.

An arbitrary callback data argument, 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) cipher suite. 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_disable_ct() and SSL_CTX_disable_ct() 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_enable_ct() or a non-null custom callback, and 0 otherwise.

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.

       The time returned by SSL_SESSION_get_time() will be used to evaluate whether any presented
       SCTs have timestamps that are in the future (and therefore invalid).

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
       (TLSEXT_TYPE_signed_certificate_timestamp).

RETURN VALUES

       SSL_enable_ct(), SSL_CTX_enable_ct(), SSL_CTX_set_ct_validation_callback() and
       SSL_set_ct_validation_callback() return 1 if the 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_disable_ct() and SSL_CTX_disable_ct() 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.

COPYRIGHT

       Copyright 2016-2020 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (the "License").  You may not use this file except
       in compliance with the License.  You can obtain a copy in the file LICENSE in the source
       distribution or at <https://www.openssl.org/source/license.html>.