43341433a8
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>
120 lines
4.7 KiB
Text
120 lines
4.7 KiB
Text
=pod
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=head1 NAME
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SSL_ct_enable, SSL_CTX_ct_enable, SSL_ct_disable, SSL_CTX_ct_disable,
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SSL_set_ct_validation_callback, SSL_CTX_set_ct_validation_callback,
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SSL_ct_is_enabled, SSL_CTX_ct_is_enabled -
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control Certificate Transparency policy
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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int SSL_ct_enable(SSL *s, int validation_mode);
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int SSL_CTX_ct_enable(SSL_CTX *ctx, int validation_mode);
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int SSL_set_ct_validation_callback(SSL *s, ssl_ct_validation_cb callback,
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void *arg);
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int SSL_CTX_set_ct_validation_callback(SSL_CTX *ctx,
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ssl_ct_validation_cb callback,
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void *arg);
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void SSL_ct_disable(SSL *s);
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void SSL_CTX_ct_disable(SSL_CTX *ctx);
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int SSL_ct_is_enabled(const SSL *s);
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int SSL_CTX_ct_is_enabled(const SSL_CTX *ctx);
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=head1 DESCRIPTION
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SSL_ct_enable() and SSL_CTX_ct_enable() enable the processing of signed
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certificate timestamps (SCTs) either for a given SSL connection or for all
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connections that share the given SSL context, respectively.
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This is accomplished by setting a built-in CT validation callback.
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The behaviour of the callback is determined by the B<validation_mode> argument,
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which can be either of B<SSL_CT_VALIDATION_PERMISSIVE> or
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B<SSL_CT_VALIDATION_STRICT> as described below.
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If B<validation_mode> is equal to B<SSL_CT_VALIDATION_PERMISSIVE>, then the
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handshake continues regardless of the validation status of any SCTs.
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The application can inspect the validation status of the SCTs at handshake
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completion.
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Note that with session resumption there will not be any SCTs presented during
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the handshake.
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Therefore, in applications that delay SCT policy enforcement until after
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handshake completion, SCT checks should only be performed when the session is
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not reused.
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See L<SSL_session_reused(3)>.
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If B<validation_mode> is equal to B<SSL_CT_VALIDATION_STRICT>, then in a full
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TLS handshake with the verification mode set to B<SSL_VERIFY_PEER>, if the peer
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presents no valid SCTs the handshake will be aborted.
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See L<SSL_set_verify(3)>.
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SSL_set_ct_validation_callback() and SSL_CTX_set_ct_validation_callback()
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register a custom callback that may implement a different policy than either of
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the above.
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This callback can examine the peer's SCTs and determine whether they are
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sufficient to allow the connection to continue.
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The TLS handshake is aborted if the verification mode is not B<SSL_VERIFY_NONE>
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and the callback returns a non-positive result.
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An arbitrary callback context argument, B<arg>, can be passed in when setting
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the callback.
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This will be passed to the callback whenever it is invoked.
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Ownership of this context remains with the caller.
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If no callback is set, SCTs will not be requested and Certificate Transparency
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validation will not occur.
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No callback will be invoked when the peer presents no certificate, e.g. by
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employing an anonymous (aNULL) ciphersuite.
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In that case the handshake continues as it would had no callback been
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requested.
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Callbacks are also not invoked when the peer certificate chain is invalid or
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validated via DANE-TA(2) or DANE-EE(3) TLSA records which use a private X.509
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PKI, or no X.509 PKI at all, respectively.
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Clients that require SCTs are expected to not have enabled any aNULL ciphers
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nor to have specified server verification via DANE-TA(2) or DANE-EE(3) TLSA
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records.
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SSL_ct_disable() and SSL_CTX_ct_disable() turn off CT processing, whether
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enabled via the built-in or the custom callbacks, by setting a NULL callback.
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These may be implemented as macros.
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SSL_ct_is_enabled() and SSL_CTX_ct_is_enabled() return 1 if CT processing is
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enabled via either SSL_ct_enable() or a non-null custom callback, and 0
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otherwise.
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=head1 NOTES
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When SCT processing is enabled, OCSP stapling will be enabled. This is because
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one possible source of SCTs is the OCSP response from a server.
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=head1 RESTRICTIONS
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Certificate Transparency validation cannot be enabled and so a callback cannot
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be set if a custom client extension handler has been registered to handle SCT
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extensions (B<TLSEXT_TYPE_signed_certificate_timestamp>).
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=head1 RETURN VALUES
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SSL_ct_enable(), SSL_CTX_ct_enable(), SSL_CTX_set_ct_validation_callback() and
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SSL_set_ct_validation_callback() return 1 if the B<callback> is successfully
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set.
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They return 0 if an error occurs, e.g. a custom client extension handler has
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been setup to handle SCTs.
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SSL_ct_disable() and SSL_CTX_ct_disable() do not return a result.
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SSL_CTX_ct_is_enabled() and SSL_ct_is_enabled() return a 1 if a non-null CT
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validation callback is set, or 0 if no callback (or equivalently a NULL
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callback) is set.
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=head1 SEE ALSO
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L<ssl(3)>,
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L<SSL_session_reused(3)>,
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L<SSL_set_verify(3)>,
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L<SSL_CTX_set_verify(3)>,
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L<ssl_ct_validation_cb(3)>
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=cut
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