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<title>GnuTLS 3.5.3: Deriving keys for other applications/protocols</title>
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<a name="Deriving-keys-for-other-applications_002fprotocols"></a>
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Next: <a href="Channel-Bindings.html#Channel-Bindings" accesskey="n" rel="next">Channel Bindings</a>, Previous: <a href="Parameter-generation.html#Parameter-generation" accesskey="p" rel="prev">Parameter generation</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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<hr>
<a name="Deriving-keys-for-other-applications_002fprotocols-1"></a>
<h4 class="subsection">6.12.5 Deriving keys for other applications/protocols</h4>
<a name="index-keying-material-exporters"></a>
<a name="index-exporting-keying-material"></a>
<a name="index-deriving-keys"></a>
<a name="index-key-extraction"></a>
<p>In several cases, after a TLS connection is established, it is desirable
to derive keys to be used in another application or protocol (e.g., in an
other TLS session using pre-shared keys). The following describe GnuTLS’
implementation of RFC5705 to extract keys based on a session’s master secret.
</p>
<p>The API to use is <a href="Core-TLS-API.html#gnutls_005fprf">gnutls_prf</a>. The
function needs to be provided with a label,
and additional context data to mix in the <code>extra</code> parameter.
Moreover, the API allows to switch the mix of the
client and server random nonces, using the <code>server_random_first</code> parameter.
In typical uses you don’t need it, so a zero value should be provided in <code>server_random_first</code>.
</p>
<p>For example, after establishing a TLS session using
<a href="Core-TLS-API.html#gnutls_005fhandshake">gnutls_handshake</a>, you can obtain 32-bytes to be used as key, using this call:
</p>
<div class="example">
<pre class="example">#define MYLABEL "EXPORTER-My-protocol-name"
#define MYCONTEXT "my-protocol's-1st-session"
char out[32];
rc = gnutls_prf (session, sizeof(MYLABEL)-1, MYLABEL, 0,
sizeof(MYCONTEXT)-1, MYCONTEXT, 32, out);
</pre></div>
<p>The output key depends on TLS’ master secret, and is the same on both client
and server.
</p>
<p>If you don’t want to use the RFC5705 interface and not mix in the client and server random
nonces, there is a low-level TLS PRF interface called <a href="Core-TLS-API.html#gnutls_005fprf_005fraw">gnutls_prf_raw</a>.
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