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<a name="Anonymous-authentication"></a>
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<p>
Previous: <a href="PSK-authentication.html#PSK-authentication" accesskey="p" rel="prev">PSK authentication</a>, Up: <a href="Shared_002dkey-and-anonymous-authentication.html#Shared_002dkey-and-anonymous-authentication" accesskey="u" rel="up">Shared-key and anonymous authentication</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="Anonymous-authentication-1"></a>
<h4 class="subsection">4.3.3 Anonymous authentication</h4>
<a name="index-anonymous-authentication"></a>
<p>The anonymous key exchange offers encryption without any
indication of the peer’s identity. This kind of authentication
is vulnerable to a man in the middle attack, but can be
used even if there is no prior communication or shared trusted parties
with the peer. It is useful to establish a session over which certificate
authentication will occur in order to hide the indentities of the participants
from passive eavesdroppers.
</p>
<p>Unless in the above case, it is not recommended to use anonymous authentication.
In the cases where there is no prior communication with the peers,
an alternative with better properties, such as key continuity, is trust on first use
(see <a href="Verifying-a-certificate-using-trust-on-first-use-authentication.html#Verifying-a-certificate-using-trust-on-first-use-authentication">Verifying a certificate using trust on first use authentication</a>).
</p>
<p>The available key exchange algorithms for anonymous authentication are
shown below, but note that few public servers support them, and they
have to be explicitly enabled.
</p>
<dl compact="compact">
<dt><code>ANON_DH:</code></dt>
<dd><p>This algorithm exchanges Diffie-Hellman parameters.
</p>
</dd>
<dt><code>ANON_ECDH:</code></dt>
<dd><p>This algorithm exchanges elliptic curve Diffie-Hellman parameters. It is more
efficient than ANON_DH on equivalent security levels.
</p>
</dd>
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