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.\" $OpenBSD: SSL_CTX_set_cert_verify_callback.3,v 1.5 2019/06/08 15:25:43 schwarze Exp $
.\" OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
.\"
.\" This file was written by Lutz Jaenicke <jaenicke@openssl.org>.
.\" Copyright (c) 2001, 2002 The OpenSSL Project. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\"
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\"
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in
.\" the documentation and/or other materials provided with the
.\" distribution.
.\"
.\" 3. All advertising materials mentioning features or use of this
.\" software must display the following acknowledgment:
.\" "This product includes software developed by the OpenSSL Project
.\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
.\"
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.\" endorse or promote products derived from this software without
.\" prior written permission. For written permission, please contact
.\" openssl-core@openssl.org.
.\"
.\" 5. Products derived from this software may not be called "OpenSSL"
.\" nor may "OpenSSL" appear in their names without prior written
.\" permission of the OpenSSL Project.
.\"
.\" 6. Redistributions of any form whatsoever must retain the following
.\" acknowledgment:
.\" "This product includes software developed by the OpenSSL Project
.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)"
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR
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.\"
.Dd $Mdocdate: June 8 2019 $
.Dt SSL_CTX_SET_CERT_VERIFY_CALLBACK 3
.Os
.Sh NAME
.Nm SSL_CTX_set_cert_verify_callback
.Nd set peer certificate verification procedure
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft void
.Fo SSL_CTX_set_cert_verify_callback
.Fa "SSL_CTX *ctx"
.Fa "int (*callback)(X509_STORE_CTX *, void *)"
.Fa "void *arg"
.Fc
.Sh DESCRIPTION
.Fn SSL_CTX_set_cert_verify_callback
sets the verification callback function for
.Fa ctx .
.Vt SSL
objects that are created from
.Fa ctx
inherit the setting valid at the time when
.Xr SSL_new 3
is called.
.Pp
Whenever a certificate is verified during a SSL/TLS handshake,
a verification function is called.
If the application does not explicitly specify a verification callback
function, the built-in verification function is used.
If a verification callback
.Fa callback
is specified via
.Fn SSL_CTX_set_cert_verify_callback ,
the supplied callback function is called instead.
By setting
.Fa callback
to
.Dv NULL ,
the default behaviour is restored.
.Pp
When the verification must be performed,
.Fa callback
will be called with the arguments
.Fn callback "X509_STORE_CTX *x509_store_ctx" "void *arg" .
The argument
.Fa arg
is specified by the application when setting
.Fa callback .
.Pp
.Fa callback
should return 1 to indicate verification success and 0 to indicate verification
failure.
If
.Dv SSL_VERIFY_PEER
is set and
.Fa callback
returns 0, the handshake will fail.
As the verification procedure may allow the connection to continue in case of
failure (by always returning 1) the verification result must be set in any case
using the
.Fa error
member of
.Fa x509_store_ctx
so that the calling application will be informed about the detailed result of
the verification procedure!
.Pp
Within
.Fa x509_store_ctx ,
.Fa callback
has access to the
.Fa verify_callback
function set using
.Xr SSL_CTX_set_verify 3 .
.Sh SEE ALSO
.Xr ssl 3 ,
.Xr SSL_CTX_load_verify_locations 3 ,
.Xr SSL_CTX_set_verify 3 ,
.Xr SSL_get_verify_result 3
.Sh HISTORY
.Fn SSL_CTX_set_cert_verify_callback
first appeared in SSLeay 0.6.1 and has been available since
.Ox 2.4 .
.Pp
Previous to OpenSSL 0.9.7, the
.Fa arg
argument to
.Fn SSL_CTX_set_cert_verify_callback
was ignored, and
.Fa callback
was called
simply as
.Ft int
.Fn (*callback) "X509_STORE_CTX *" .
To compile software written for previous versions of OpenSSL,
a dummy argument will have to be added to
.Fa callback .
.Sh CAVEATS
Do not mix the verification callback described in this function with the
.Fa verify_callback
function called during the verification process.
The latter is set using the
.Xr SSL_CTX_set_verify 3
family of functions.
.Pp
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
.Fa verify_callback
function.
.Sh BUGS
.Fn SSL_CTX_set_cert_verify_callback
does not provide diagnostic information.
|
