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<title>GnuTLS 3.4.9: Verifying a certificate using trust on first use authentication</title>
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<a name="Verifying-a-certificate-using-trust-on-first-use-authentication"></a>
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<p>
Next: <a href="Verifying-a-certificate-using-DANE.html#Verifying-a-certificate-using-DANE" accesskey="n" rel="next">Verifying a certificate using DANE</a>, Up: <a href="Advanced-certificate-verification.html#Advanced-certificate-verification" accesskey="u" rel="up">Advanced certificate verification</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="Verifying-a-certificate-using-trust-on-first-use-authentication-1"></a>
<h4 class="subsubsection">4.1.3.1 Verifying a certificate using trust on first use authentication</h4>
<a name="index-verifying-certificate-paths-2"></a>
<a name="index-SSH_002dstyle-authentication"></a>
<a name="index-Trust-on-first-use"></a>
<a name="index-Key-pinning"></a>
<p>It is possible to use a trust on first use (TOFU) authentication
method in GnuTLS. That is the concept used by the SSH programs, where the
public key of the peer is not verified, or verified in an out-of-bound way,
but subsequent connections to the same peer require the public key to
remain the same. Such a system in combination with the typical CA
verification of a certificate, and OCSP revocation checks,
can help to provide multiple factor verification, where a single point of
failure is not enough to compromise the system. For example a server compromise
may be detected using OCSP, and a CA compromise can be detected using
the trust on first use method.
Such a hybrid system with X.509 and trust on first use authentication is
shown in <a href="Simple-client-example-with-SSH_002dstyle-certificate-verification.html#Simple-client-example-with-SSH_002dstyle-certificate-verification">Simple client example with SSH-style certificate verification</a>.
</p>
<p>See <a href="Certificate-verification.html#Certificate-verification">Certificate verification</a> on how to use the available functionality.
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