1SSL_CTX_SET_CT_VALIDATION_CALLBACK(3)OpenSSSLSL_CTX_SET_CT_VALIDATION_CALLBACK(3)
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NAME

6       ssl_ct_validation_cb, SSL_enable_ct, SSL_CTX_enable_ct, SSL_disable_ct,
7       SSL_CTX_disable_ct, SSL_set_ct_validation_callback,
8       SSL_CTX_set_ct_validation_callback, SSL_ct_is_enabled,
9       SSL_CTX_ct_is_enabled - control Certificate Transparency policy
10

SYNOPSIS

12        #include <openssl/ssl.h>
13
14        typedef int (*ssl_ct_validation_cb)(const CT_POLICY_EVAL_CTX *ctx,
15                                           const STACK_OF(SCT) *scts, void *arg);
16
17        int SSL_enable_ct(SSL *s, int validation_mode);
18        int SSL_CTX_enable_ct(SSL_CTX *ctx, int validation_mode);
19        int SSL_set_ct_validation_callback(SSL *s, ssl_ct_validation_cb callback,
20                                           void *arg);
21        int SSL_CTX_set_ct_validation_callback(SSL_CTX *ctx,
22                                               ssl_ct_validation_cb callback,
23                                               void *arg);
24        void SSL_disable_ct(SSL *s);
25        void SSL_CTX_disable_ct(SSL_CTX *ctx);
26        int SSL_ct_is_enabled(const SSL *s);
27        int SSL_CTX_ct_is_enabled(const SSL_CTX *ctx);
28

DESCRIPTION

30       SSL_enable_ct() and SSL_CTX_enable_ct() enable the processing of signed
31       certificate timestamps (SCTs) either for a given SSL connection or for
32       all connections that share the given SSL context, respectively.  This
33       is accomplished by setting a built-in CT validation callback.  The
34       behaviour of the callback is determined by the validation_mode
35       argument, which can be either of SSL_CT_VALIDATION_PERMISSIVE or
36       SSL_CT_VALIDATION_STRICT as described below.
37
38       If validation_mode is equal to SSL_CT_VALIDATION_STRICT, then in a full
39       TLS handshake with the verification mode set to SSL_VERIFY_PEER, if the
40       peer presents no valid SCTs the handshake will be aborted.  If the
41       verification mode is SSL_VERIFY_NONE, the handshake will continue
42       despite lack of valid SCTs.  However, in that case if the verification
43       status before the built-in callback was X509_V_OK it will be set to
44       X509_V_ERR_NO_VALID_SCTS after the callback.  Applications can call
45       SSL_get_verify_result(3) to check the status at handshake completion,
46       even after session resumption since the verification status is part of
47       the saved session state.  See SSL_set_verify(3),
48       <SSL_get_verify_result(3)>, SSL_session_reused(3).
49
50       If validation_mode is equal to SSL_CT_VALIDATION_PERMISSIVE, then the
51       handshake continues, and the verification status is not modified,
52       regardless of the validation status of any SCTs.  The application can
53       still inspect the validation status of the SCTs at handshake
54       completion.  Note that with session resumption there will not be any
55       SCTs presented during the handshake.  Therefore, in applications that
56       delay SCT policy enforcement until after handshake completion, such
57       delayed SCT checks should only be performed when the session is not
58       resumed.
59
60       SSL_set_ct_validation_callback() and
61       SSL_CTX_set_ct_validation_callback() register a custom callback that
62       may implement a different policy than either of the above.  This
63       callback can examine the peer's SCTs and determine whether they are
64       sufficient to allow the connection to continue.  The TLS handshake is
65       aborted if the verification mode is not SSL_VERIFY_NONE and the
66       callback returns a non-positive result.
67
68       An arbitrary callback context argument, arg, can be passed in when
69       setting the callback.  This will be passed to the callback whenever it
70       is invoked.  Ownership of this context remains with the caller.
71
72       If no callback is set, SCTs will not be requested and Certificate
73       Transparency validation will not occur.
74
75       No callback will be invoked when the peer presents no certificate, e.g.
76       by employing an anonymous (aNULL) cipher suite.  In that case the
77       handshake continues as it would had no callback been requested.
78       Callbacks are also not invoked when the peer certificate chain is
79       invalid or validated via DANE-TA(2) or DANE-EE(3) TLSA records which
80       use a private X.509 PKI, or no X.509 PKI at all, respectively.  Clients
81       that require SCTs are expected to not have enabled any aNULL ciphers
82       nor to have specified server verification via DANE-TA(2) or DANE-EE(3)
83       TLSA records.
84
85       SSL_disable_ct() and SSL_CTX_disable_ct() turn off CT processing,
86       whether enabled via the built-in or the custom callbacks, by setting a
87       NULL callback.  These may be implemented as macros.
88
89       SSL_ct_is_enabled() and SSL_CTX_ct_is_enabled() return 1 if CT
90       processing is enabled via either SSL_enable_ct() or a non-null custom
91       callback, and 0 otherwise.
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NOTES

94       When SCT processing is enabled, OCSP stapling will be enabled. This is
95       because one possible source of SCTs is the OCSP response from a server.
96
97       The time returned by SSL_SESSION_get_time() will be used to evaluate
98       whether any presented SCTs have timestamps that are in the future (and
99       therefore invalid).
100

RESTRICTIONS

102       Certificate Transparency validation cannot be enabled and so a callback
103       cannot be set if a custom client extension handler has been registered
104       to handle SCT extensions (TLSEXT_TYPE_signed_certificate_timestamp).
105

RETURN VALUES

107       SSL_enable_ct(), SSL_CTX_enable_ct(),
108       SSL_CTX_set_ct_validation_callback() and
109       SSL_set_ct_validation_callback() return 1 if the callback is
110       successfully set.  They return 0 if an error occurs, e.g. a custom
111       client extension handler has been setup to handle SCTs.
112
113       SSL_disable_ct() and SSL_CTX_disable_ct() do not return a result.
114
115       SSL_CTX_ct_is_enabled() and SSL_ct_is_enabled() return a 1 if a non-
116       null CT validation callback is set, or 0 if no callback (or
117       equivalently a NULL callback) is set.
118

SEE ALSO

120       ssl(7), <SSL_get_verify_result(3)>, SSL_session_reused(3),
121       SSL_set_verify(3), SSL_CTX_set_verify(3), SSL_SESSION_get_time(3)
122
124       Copyright 2016-2017 The OpenSSL Project Authors. All Rights Reserved.
125
126       Licensed under the OpenSSL license (the "License").  You may not use
127       this file except in compliance with the License.  You can obtain a copy
128       in the file LICENSE in the source distribution or at
129       <https://www.openssl.org/source/license.html>.
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1331.1.1l                            2021-09-S1S5L_CTX_SET_CT_VALIDATION_CALLBACK(3)
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