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<title>GnuTLS 3.4.8: Parameter generation</title>
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<a name="Parameter-generation"></a>
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<p>
Next: <a href="Deriving-keys-for-other-applications_002fprotocols.html#Deriving-keys-for-other-applications_002fprotocols" accesskey="n" rel="next">Deriving keys for other applications/protocols</a>, Previous: <a href="Re_002dauthentication.html#Re_002dauthentication" accesskey="p" rel="prev">Re-authentication</a>, Up: <a href="Advanced-topics.html#Advanced-topics" accesskey="u" rel="up">Advanced topics</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="Parameter-generation-1"></a>
<h4 class="subsection">6.12.4 Parameter generation</h4>
<a name="index-parameter-generation"></a>
<a name="index-generating-parameters"></a>
<p>Several TLS ciphersuites require additional parameters that
need to be generated or provided by the application. The
Diffie-Hellman based ciphersuites (ANON-DH or DHE), require
the group parameters to be provided. Those can either be
be generated on the fly using <a href="Core-TLS-API.html#gnutls_005fdh_005fparams_005fgenerate2">gnutls_dh_params_generate2</a>
or imported from pregenerated data using <a href="Core-TLS-API.html#gnutls_005fdh_005fparams_005fimport_005fpkcs3">gnutls_dh_params_import_pkcs3</a>.
The parameters can be used in a <acronym>TLS</acronym> session by calling
<a href="Core-TLS-API.html#gnutls_005fcertificate_005fset_005fdh_005fparams">gnutls_certificate_set_dh_params</a> or
<a href="Core-TLS-API.html#gnutls_005fanon_005fset_005fserver_005fdh_005fparams">gnutls_anon_set_server_dh_params</a> for anonymous sessions.
</p>
<dl compact="compact">
<dt><code><var>int</var> <a href="Core-TLS-API.html#gnutls_005fdh_005fparams_005fgenerate2">gnutls_dh_params_generate2</a> (gnutls_dh_params_t <var>dparams</var>, unsigned int <var>bits</var>)</code></dt>
<dt><code><var>int</var> <a href="Core-TLS-API.html#gnutls_005fdh_005fparams_005fimport_005fpkcs3">gnutls_dh_params_import_pkcs3</a> (gnutls_dh_params_t <var>params</var>, const gnutls_datum_t * <var>pkcs3_params</var>, gnutls_x509_crt_fmt_t <var>format</var>)</code></dt>
<dt><code><var>void</var> <a href="Core-TLS-API.html#gnutls_005fcertificate_005fset_005fdh_005fparams">gnutls_certificate_set_dh_params</a> (gnutls_certificate_credentials_t <var>res</var>, gnutls_dh_params_t <var>dh_params</var>)</code></dt>
<dt><code><var>void</var> <a href="Core-TLS-API.html#gnutls_005fanon_005fset_005fserver_005fdh_005fparams">gnutls_anon_set_server_dh_params</a> (gnutls_anon_server_credentials_t <var>res</var>, gnutls_dh_params_t <var>dh_params</var>)</code></dt>
</dl>
<p>Due to the time-consuming calculations required for the generation
of Diffie-Hellman parameters we suggest against performing generation
of them within an application. The <code>certtool</code> tool can be used to
generate or export known safe values that can be stored in code
or in a configuration file to provide the ability to replace. We also
recommend the usage of <a href="Core-TLS-API.html#gnutls_005fsec_005fparam_005fto_005fpk_005fbits">gnutls_sec_param_to_pk_bits</a>
(see <a href="Selecting-cryptographic-key-sizes.html#Selecting-cryptographic-key-sizes">Selecting cryptographic key sizes</a>) to determine
the bit size of the generated parameters.
</p>
<p>Note that the information stored in the generated PKCS #3 structure
changed with GnuTLS 3.0.9. Since that version the <code>privateValueLength</code>
member of the structure is set, allowing the server utilizing the
parameters to use keys of the size of the security parameter. This
provides better performance in key exchange.
</p>
<p>To allow renewal of the parameters within an application without
accessing the credentials, which are a shared structure,
an alternative interface is available using a callback function.
</p>
<dl>
<dt><a name="index-gnutls_005fcertificate_005fset_005fparams_005ffunction"></a>Function: <em>void</em> <strong>gnutls_certificate_set_params_function</strong> <em>(gnutls_certificate_credentials_t <var>res</var>, gnutls_params_function * <var>func</var>)</em></dt>
<dd><p><var>res</var>: is a gnutls_certificate_credentials_t type
</p>
<p><var>func</var>: is the function to be called
</p>
<p>This function will set a callback in order for the server to get
the Diffie-Hellman or RSA parameters for certificate
authentication. The callback should return <code>GNUTLS_E_SUCCESS</code> (0) on success.
</p></dd></dl>
<hr>
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<p>
Next: <a href="Deriving-keys-for-other-applications_002fprotocols.html#Deriving-keys-for-other-applications_002fprotocols" accesskey="n" rel="next">Deriving keys for other applications/protocols</a>, Previous: <a href="Re_002dauthentication.html#Re_002dauthentication" accesskey="p" rel="prev">Re-authentication</a>, Up: <a href="Advanced-topics.html#Advanced-topics" accesskey="u" rel="up">Advanced topics</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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