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=head1 NAME

X509_build_chain,
X509_verify_cert,
X509_STORE_CTX_verify - build and verify X509 certificate chain

=head1 SYNOPSIS

 #include <openssl/x509_vfy.h>

 STACK_OF(X509) *X509_build_chain(X509 *target, STACK_OF(X509) *certs,
                                  X509_STORE *store, int with_self_signed,
                                  OSSL_LIB_CTX *libctx, const char *propq);
 int X509_verify_cert(X509_STORE_CTX *ctx);
 int X509_STORE_CTX_verify(X509_STORE_CTX *ctx);

=head1 DESCRIPTION

X509_build_chain() builds a certificate chain starting from I<target>
using the optional list of intermediate CA certificates I<certs>.
If I<store> is NULL it builds the chain as far down as possible, ignoring errors.
Else the chain must reach a trust anchor contained in I<store>.
It internally uses a B<X509_STORE_CTX> structure associated with the library
context I<libctx> and property query string I<propq>, both of which may be NULL.
In case there is more than one possibility for the chain, only one is taken.

On success it returns a pointer to a new stack of (up_ref'ed) certificates
starting with I<target> and followed by all available intermediate certificates.
A self-signed trust anchor is included only if I<target> is the trust anchor
of I<with_self_signed> is 1.
If a non-NULL stack is returned the caller is responsible for freeing it.

The X509_verify_cert() function attempts to discover and validate a
certificate chain based on parameters in I<ctx>.
The verification context, of type B<X509_STORE_CTX>, can be constructed
using L<X509_STORE_CTX_new(3)> and L<X509_STORE_CTX_init(3)>.
It usually includes a target certificate to be verified,
a set of certificates serving as trust anchors,
a list of non-trusted certificates that may be helpful for chain construction,
flags such as X509_V_FLAG_X509_STRICT, and various other optional components
such as a callback function that allows customizing the verification outcome.
A complete description of the certificate verification process is contained in
the L<openssl-verification-options(1)> manual page.

Applications rarely call this function directly but it is used by
OpenSSL internally for certificate validation, in both the S/MIME and
SSL/TLS code.

A negative return value from X509_verify_cert() can occur if it is invoked
incorrectly, such as with no certificate set in I<ctx>, or when it is called
twice in succession without reinitialising I<ctx> for the second call.
A negative return value can also happen due to internal resource problems
or because an internal inconsistency has been detected
or if a retry operation is requested during internal lookups
(which never happens with standard lookup methods).
Applications must interpret any return value <= 0 as an error.

The X509_STORE_CTX_verify() behaves like X509_verify_cert() except that its
target certificate is the first element of the list of untrusted certificates
in I<ctx> unless a target certificate is set explicitly.

=head1 RETURN VALUES

X509_build_chain() returns NULL on error, else a stack of certificates.

Both X509_verify_cert() and X509_STORE_CTX_verify()
return 1 if a complete chain can be built and validated,
otherwise they return 0, and in exceptional circumstances (such as malloc
failure and internal errors) they can also return a negative code.

If a complete chain can be built and validated both functions return 1.
If the certificate must be rejected on the basis of the data available
or any required certificate status data is not available they return 0.
If no definite answer possible they usually return a negative code.

On error or failure additional error information can be obtained by
examining I<ctx> using, for example, L<X509_STORE_CTX_get_error(3)>.  Even if
verification indicated success, the stored error code may be different from
X509_V_OK, likely because a verification callback function has waived the error.

=head1 SEE ALSO

L<X509_STORE_CTX_new(3)>, L<X509_STORE_CTX_init(3)>,
L<X509_STORE_CTX_get_error(3)>

=head1 HISTORY

X509_build_chain() and X509_STORE_CTX_verify() were added in OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2009-2021 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
L<https://www.openssl.org/source/license.html>.

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