NAME

       SSL_CTX_set_cert_verify_callback - set peer certificate verification procedure

SYNOPSIS

        #include <openssl/ssl.h>

        void SSL_CTX_set_cert_verify_callback(SSL_CTX *ctx,
                                              int (*callback)(X509_STORE_CTX *, void *),
                                              void *arg);

DESCRIPTION

       SSL_CTX_set_cert_verify_callback() sets the verification callback function for ctx. SSL objects that are
       created from ctx inherit the setting valid at the time when SSL_new(3) is called.

NOTES

       When a peer certificate has been received during a SSL/TLS handshake, a verification function is called
       regardless of the verification mode.  If the application does not explicitly specify a verification
       callback function, the built-in verification function is used.  If a verification callback callback is
       specified via SSL_CTX_set_cert_verify_callback(), the supplied callback function is called instead with
       the arguments callback(X509_STORE_CTX *x509_store_ctx, void *arg).  The argument arg is specified by the
       application when setting callback.  By setting callback to NULL, the default behaviour is restored.

       callback should return 1 to indicate verification success and 0 to indicate verification failure.  In
       server mode, a return value of 0 leads to handshake failure.  In client mode, the behaviour is as
       follows.  All values, including 0, are ignored if the verification mode is SSL_VERIFY_NONE.  Otherwise,
       when the return value is less than or equal to 0, the handshake will fail.

       In client mode callback may also call the SSL_set_retry_verify(3) function on the SSL object set in the
       x509_store_ctx ex data (see SSL_get_ex_data_X509_STORE_CTX_idx(3)) and return 1. This would be typically
       done in case the certificate verification was not yet able to succeed. This makes the handshake suspend
       and return control to the calling application with SSL_ERROR_WANT_RETRY_VERIFY. The app can for instance
       fetch further certificates or cert status information needed for the verification. Calling SSL_connect(3)
       again resumes the connection attempt by retrying the server certificate verification step.  This process
       may even be repeated if need be.

       In any case a viable verification result value must be reflected in the error member of x509_store_ctx,
       which can be done using X509_STORE_CTX_set_error(3).  This is particularly important in case the callback
       allows the connection to continue (by returning 1).  Note that the verification status in the store
       context is a possibly durable indication of the chain's validity!  This gets recorded in the SSL session
       (and thus also in session tickets) and the validity of the originally presented chain is then visible on
       resumption, even though no chain is presented int that case.  Moreover, the calling application will be
       informed about the detailed result of the verification procedure and may elect to base further decisions
       on it.

       Within x509_store_ctx, callback has access to the verify_callback function set using
       SSL_CTX_set_verify(3).

RETURN VALUES

       SSL_CTX_set_cert_verify_callback() does not return a value.

WARNINGS

       Do not mix the verification callback described in this function with the verify_callback function called
       during the verification process. The latter is set using the SSL_CTX_set_verify(3) family of functions.

       Providing a complete verification procedure including certificate purpose settings etc is a complex task.
       The built-in procedure is quite powerful and in most cases it should be sufficient to modify its
       behaviour using the verify_callback function.

BUGS

       SSL_CTX_set_cert_verify_callback() does not provide diagnostic information.

SEE ALSO

       ssl(7), SSL_CTX_set_verify(3), X509_STORE_CTX_set_error(3), SSL_get_verify_result(3),
       SSL_set_retry_verify(3), SSL_CTX_load_verify_locations(3)

COPYRIGHT

       Copyright 2001-2022 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>.

3.0.13                                             2025-09-18             SSL_CTX_SET_CERT_VERIFY_CALLBACK(3SSL)