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=pod
=head1 NAME
X509_get_extension_flags, X509_get_key_usage, X509_get_extended_key_usage -
retrieve certificate extension flags.
=head1 SYNOPSIS
#include <openssl/x509v3.h>
uint32_t X509_get_extension_flags(X509 *x);
uint32_t X509_get_key_usage(X509 *x);
uint32_t X509_get_extended_key_usage(X509 *x);
const ASN1_OCTET_STRING *X509_get0_subject_key_id(X509 *x);
=head1 DESCRIPTION
These functions retrieve flags related to commonly used certificate extensions.
X509_get_extension_flags() retrieves general information about a certificate,
it will return one or more of the following flags ored together.
=over 4
=item B<EXFLAG_V1>
The certificate is an obsolete version 1 certificate.
=item B<EXFLAG_BCONS>
The certificate contains a basic constraints extension.
=item B<EXFLAG_CA>
The certificate contains basic constraints and asserts the CA flag.
=item B<EXFLAG_PROXY>
The certificate is a valid proxy certificate.
=item B<EXFLAG_SI>
The certificate is self issued (that is subject and issuer names match).
=item B<EXFLAG_SS>
The subject and issuer names match and extension values imply it is self
signed.
=item B<EXFLAG_FRESHEST>
The freshest CRL extension is present in the certificate.
=item B<EXFLAG_CRITICAL>
The certificate contains an unhandled critical extension.
=item B<EXFLAG_INVALID>
Some certificate extension values are invalid or inconsistent. The
certificate should be rejected.
=item B<EXFLAG_KUSAGE>
The certificate contains a key usage extension. The value can be retrieved
using X509_get_key_usage().
=item B<EXFLAG_XKUSAGE>
The certificate contains an extended key usage extension. The value can be
retrieved using X509_get_extended_key_usage().
=back
X509_get_key_usage() returns the value of the key usage extension. If key
usage is present will return zero or more of the flags:
B<KU_DIGITAL_SIGNATURE>, B<KU_NON_REPUDIATION>, B<KU_KEY_ENCIPHERMENT>,
B<KU_DATA_ENCIPHERMENT>, B<KU_KEY_AGREEMENT>, B<KU_KEY_CERT_SIGN>,
B<KU_CRL_SIGN>, B<KU_ENCIPHER_ONLY> or B<KU_DECIPHER_ONLY> corresponding to
individual key usage bits. If key usage is absent then B<UINT32_MAX> is
returned.
X509_get_extended_key_usage() returns the value of the extended key usage
extension. If extended key usage is present it will return zero or more of the
flags: B<XKU_SSL_SERVER>, B<XKU_SSL_CLIENT>, B<XKU_SMIME>, B<XKU_CODE_SIGN>
B<XKU_OCSP_SIGN>, B<XKU_TIMESTAMP>, B<XKU_DVCS> or B<XKU_ANYEKU>. These
correspond to the OIDs B<id-kp-serverAuth>, B<id-kp-clientAuth>,
B<id-kp-emailProtection>, B<id-kp-codeSigning>, B<id-kp-OCSPSigning>,
B<id-kp-timeStamping>, B<id-kp-dvcs> and B<anyExtendedKeyUsage> respectively.
Additionally B<XKU_SGC> is set if either Netscape or Microsoft SGC OIDs are
present.
X509_get_extended_key_usage() return an internal pointer to the subject key
identifier of B<x> as an B<ASN1_OCTET_STRING> or B<NULL> if the extension
is not present or cannot be parsed.
=head1 NOTES
The value of the flags correspond to extension values which are cached
in the B<X509> structure. If the flags returned do not provide sufficient
information an application should examine extension values directly
for example using X509_get_ext_d2i().
If the key usage or extended key usage extension is absent then typically usage
is unrestricted. For this reason X509_get_key_usage() and
X509_get_extended_key_usage() return B<UINT32_MAX> when the corresponding
extension is absent. Applications can additionally check the return value of
X509_get_extension_flags() and take appropriate action is an extension is
absent.
If X509_get0_subject_key_id() returns B<NULL> then the extension may be
absent or malformed. Applications can determine the precise reason using
X509_get_ext_d2i().
=head1 RETURN VALUE
X509_get_extension_flags(), X509_get_key_usage() and
X509_get_extended_key_usage() return sets of flags corresponding to the
certificate extension values.
X509_get0_subject_key_id() returns the subject key identifier as a
pointer to an B<ASN1_OCTET_STRING> structure or B<NULL> if the extension
is absent or an error occurred during parsing.
=head1 SEE ALSO
L<X509_check_purpose(3)>
=cut
=head1 COPYRIGHT
Copyright 2015-2016 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (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>.
=cut
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