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<title>GnuTLS 3.4.8: X.509 extensions</title>
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<a name="X_002e509-extensions"></a>
<div class="header">
<p>
Next: <a href="X_002e509-public-and-private-keys.html#X_002e509-public-and-private-keys" accesskey="n" rel="next">X.509 public and private keys</a>, Previous: <a href="X_002e509-distinguished-names.html#X_002e509-distinguished-names" accesskey="p" rel="prev">X.509 distinguished names</a>, Up: <a href="X_002e509-certificates.html#X_002e509-certificates" accesskey="u" rel="up">X.509 certificates</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="X_002e509-extensions-1"></a>
<h4 class="subsubsection">4.1.1.5 X.509 extensions</h4>
<a name="index-X_002e509-extensions"></a>
<p>X.509 version 3 certificates include a list of extensions that can
be used to obtain additional information on the subject or the issuer
of the certificate. Those may be e-mail addresses, flags that indicate whether the
belongs to a CA etc. All the supported <acronym>X.509</acronym> version 3
extensions are shown in <a href="#tab_003ax509_002dext">Table 4.3</a>.
</p>
<p>The certificate extensions access is split into two parts. The first
requires to retrieve the extension, and the second is the parsing part.
</p>
<p>To enumerate and retrieve the DER-encoded extension data available in a certificate the following
two functions are available.
</p><dl compact="compact">
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fcrt_005fget_005fextension_005finfo">gnutls_x509_crt_get_extension_info</a> (gnutls_x509_crt_t <var>cert</var>, int <var>indx</var>, void * <var>oid</var>, size_t * <var>oid_size</var>, unsigned int * <var>critical</var>)</code></dt>
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fcrt_005fget_005fextension_005fdata2">gnutls_x509_crt_get_extension_data2</a> (gnutls_x509_crt_t <var>cert</var>, unsigned <var>indx</var>, gnutls_datum_t * <var>data</var>)</code></dt>
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fcrt_005fget_005fextension_005fby_005foid2">gnutls_x509_crt_get_extension_by_oid2</a> (gnutls_x509_crt_t <var>cert</var>, const char * <var>oid</var>, int <var>indx</var>, gnutls_datum_t * <var>output</var>, unsigned int * <var>critical</var>)</code></dt>
</dl>
<p>After a supported DER-encoded extension is retrieved it can be parsed using the APIs in <code>x509-ext.h</code>.
Complex extensions may require initializing an intermediate structure that holds the
parsed extension data. Examples of simple parsing functions are shown below.
</p><dl compact="compact">
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fext_005fimport_005fbasic_005fconstraints">gnutls_x509_ext_import_basic_constraints</a> (const gnutls_datum_t * <var>ext</var>, unsigned int * <var>ca</var>, int * <var>pathlen</var>)</code></dt>
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fext_005fexport_005fbasic_005fconstraints">gnutls_x509_ext_export_basic_constraints</a> (unsigned int <var>ca</var>, int <var>pathlen</var>, gnutls_datum_t * <var>ext</var>)</code></dt>
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fext_005fimport_005fkey_005fusage">gnutls_x509_ext_import_key_usage</a> (const gnutls_datum_t * <var>ext</var>, unsigned int * <var>key_usage</var>)</code></dt>
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fext_005fexport_005fkey_005fusage">gnutls_x509_ext_export_key_usage</a> (unsigned int <var>usage</var>, gnutls_datum_t * <var>ext</var>)</code></dt>
</dl>
<p>More complex extensions, such as Name Constraints, require an intermediate structure, in that
case <code>gnutls_x509_name_constraints_t</code> to be initialized in order to store the parsed
extension data.
</p><dl compact="compact">
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fext_005fimport_005fname_005fconstraints">gnutls_x509_ext_import_name_constraints</a> (const gnutls_datum_t * <var>ext</var>, gnutls_x509_name_constraints_t <var>nc</var>, unsigned int <var>flags</var>)</code></dt>
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fext_005fexport_005fname_005fconstraints">gnutls_x509_ext_export_name_constraints</a> (gnutls_x509_name_constraints_t <var>nc</var>, gnutls_datum_t * <var>ext</var>)</code></dt>
</dl>
<p>After the name constraints are extracted in the structure, the following functions
can be used to access them.
</p>
<dl compact="compact">
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fname_005fconstraints_005fget_005fpermitted">gnutls_x509_name_constraints_get_permitted</a> (gnutls_x509_name_constraints_t <var>nc</var>, unsigned <var>idx</var>, unsigned * <var>type</var>, gnutls_datum_t * <var>name</var>)</code></dt>
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fname_005fconstraints_005fget_005fexcluded">gnutls_x509_name_constraints_get_excluded</a> (gnutls_x509_name_constraints_t <var>nc</var>, unsigned <var>idx</var>, unsigned * <var>type</var>, gnutls_datum_t * <var>name</var>)</code></dt>
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fname_005fconstraints_005fadd_005fpermitted">gnutls_x509_name_constraints_add_permitted</a> (gnutls_x509_name_constraints_t <var>nc</var>, gnutls_x509_subject_alt_name_t <var>type</var>, const gnutls_datum_t * <var>name</var>)</code></dt>
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fname_005fconstraints_005fadd_005fexcluded">gnutls_x509_name_constraints_add_excluded</a> (gnutls_x509_name_constraints_t <var>nc</var>, gnutls_x509_subject_alt_name_t <var>type</var>, const gnutls_datum_t * <var>name</var>)</code></dt>
</dl>
<dl compact="compact">
<dt><code><var>unsigned</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fname_005fconstraints_005fcheck">gnutls_x509_name_constraints_check</a> (gnutls_x509_name_constraints_t <var>nc</var>, gnutls_x509_subject_alt_name_t <var>type</var>, const gnutls_datum_t * <var>name</var>)</code></dt>
<dt><code><var>unsigned</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fname_005fconstraints_005fcheck_005fcrt">gnutls_x509_name_constraints_check_crt</a> (gnutls_x509_name_constraints_t <var>nc</var>, gnutls_x509_subject_alt_name_t <var>type</var>, gnutls_x509_crt_t <var>cert</var>)</code></dt>
</dl>
<p>Other utility functions are listed below.
</p><dl compact="compact">
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fname_005fconstraints_005finit">gnutls_x509_name_constraints_init</a> (gnutls_x509_name_constraints_t * <var>nc</var>)</code></dt>
<dt><code><var>void</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fname_005fconstraints_005fdeinit">gnutls_x509_name_constraints_deinit</a> (gnutls_x509_name_constraints_t <var>nc</var>)</code></dt>
</dl>
<p>Similar functions exist for all of the other supported extensions, listed in <a href="#tab_003ax509_002dext">Table 4.3</a>.
</p>
<div class="float"><a name="tab_003ax509_002dext"></a>
<table>
<thead><tr><th width="30%">Extension</th><th width="20%">OID</th><th width="40%">Description</th></tr></thead>
<tr><td width="30%">Subject key id</td><td width="20%">2.5.29.14</td><td width="40%">An identifier of the key of the subject.</td></tr>
<tr><td width="30%">Key usage</td><td width="20%">2.5.29.15</td><td width="40%">Constraints the key’s usage of the certificate.</td></tr>
<tr><td width="30%">Private key usage period</td><td width="20%">2.5.29.16</td><td width="40%">Constraints the validity time of the private key.</td></tr>
<tr><td width="30%">Subject alternative name</td><td width="20%">2.5.29.17</td><td width="40%">Alternative names to subject’s distinguished name.</td></tr>
<tr><td width="30%">Issuer alternative name</td><td width="20%">2.5.29.18</td><td width="40%">Alternative names to the issuer’s distinguished name.</td></tr>
<tr><td width="30%">Basic constraints</td><td width="20%">2.5.29.19</td><td width="40%">Indicates whether this is a CA certificate or not, and specify the
maximum path lengths of certificate chains.</td></tr>
<tr><td width="30%">Name constraints</td><td width="20%">2.5.29.30</td><td width="40%">A field in CA certificates that restricts the scope of the name of
issued certificates.</td></tr>
<tr><td width="30%">CRL distribution points</td><td width="20%">2.5.29.31</td><td width="40%">This extension is set by the CA, in order to inform about the issued
CRLs.</td></tr>
<tr><td width="30%">Certificate policy</td><td width="20%">2.5.29.32</td><td width="40%">This extension is set to indicate the certificate policy as object
identifier and may contain a descriptive string or URL.</td></tr>
<tr><td width="30%">Authority key identifier</td><td width="20%">2.5.29.35</td><td width="40%">An identifier of the key of the issuer of the certificate. That is
used to distinguish between different keys of the same issuer.</td></tr>
<tr><td width="30%">Extended key usage</td><td width="20%">2.5.29.37</td><td width="40%">Constraints the purpose of the certificate.</td></tr>
<tr><td width="30%">Authority information access</td><td width="20%">1.3.6.1.5.5.7.1.1</td><td width="40%">Information on services by the issuer of the certificate.</td></tr>
<tr><td width="30%">Proxy Certification Information</td><td width="20%">1.3.6.1.5.5.7.1.14</td><td width="40%">Proxy Certificates includes this extension that contains the OID of
the proxy policy language used, and can specify limits on the maximum
lengths of proxy chains. Proxy Certificates are specified in
[<em>RFC3820</em>].</td></tr>
</table>
<div class="float-caption"><p><strong>Table 4.3: </strong>Supported X.509 certificate extensions.</p></div></div>
<p>Note, that there are also direct APIs to access extensions that may
be simpler to use for non-complex extensions. They are available
in <code>x509.h</code> and some examples are listed below.
</p><dl compact="compact">
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fcrt_005fget_005fbasic_005fconstraints">gnutls_x509_crt_get_basic_constraints</a> (gnutls_x509_crt_t <var>cert</var>, unsigned int * <var>critical</var>, unsigned int * <var>ca</var>, int * <var>pathlen</var>)</code></dt>
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fcrt_005fset_005fbasic_005fconstraints">gnutls_x509_crt_set_basic_constraints</a> (gnutls_x509_crt_t <var>crt</var>, unsigned int <var>ca</var>, int <var>pathLenConstraint</var>)</code></dt>
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fcrt_005fget_005fkey_005fusage">gnutls_x509_crt_get_key_usage</a> (gnutls_x509_crt_t <var>cert</var>, unsigned int * <var>key_usage</var>, unsigned int * <var>critical</var>)</code></dt>
<dt><code><var>int</var> <a href="X509-certificate-API.html#gnutls_005fx509_005fcrt_005fset_005fkey_005fusage">gnutls_x509_crt_set_key_usage</a> (gnutls_x509_crt_t <var>crt</var>, unsigned int <var>usage</var>)</code></dt>
</dl>
<hr>
<div class="header">
<p>
Next: <a href="X_002e509-public-and-private-keys.html#X_002e509-public-and-private-keys" accesskey="n" rel="next">X.509 public and private keys</a>, Previous: <a href="X_002e509-distinguished-names.html#X_002e509-distinguished-names" accesskey="p" rel="prev">X.509 distinguished names</a>, Up: <a href="X_002e509-certificates.html#X_002e509-certificates" accesskey="u" rel="up">X.509 certificates</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
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