SSL_CTX_set_cert_verify_callback - set peer certificate verification procedure
#include <openssl/ssl.h>
void SSL_CTX_set_cert_verify_callback(SSL_CTX *ctx,
int (*callback)(X509_STORE_CTX *, void *),
void *arg);
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.
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).
SSL_CTX_set_cert_verify_callback() does not return a value.
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.
SSL_CTX_set_cert_verify_callback() does not provide diagnostic
information.
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 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>.