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<a name="On-SSL-2-and-older-protocols"></a>
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<p>
Previous: <a href="How-to-use-TLS-in-application-protocols.html#How-to-use-TLS-in-application-protocols" accesskey="p" rel="prev">How to use TLS in application protocols</a>, Up: <a href="Introduction-to-TLS.html#Introduction-to-TLS" accesskey="u" rel="up">Introduction to TLS</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="On-SSL-2-and-older-protocols-1"></a>
<h3 class="section">3.8 On SSL 2 and older protocols</h3>
<a name="index-SSL-2"></a>
<p>One of the initial decisions in the <acronym>GnuTLS</acronym> development was
to implement the known security protocols for the transport layer.
Initially <acronym>TLS</acronym> 1.0 was implemented since it was the latest at
that time, and was considered to be the most advanced in security
properties. Later the <acronym>SSL</acronym> 3.0 protocol was implemented
since it is still the only protocol supported by several servers and
there are no serious security vulnerabilities known.
</p>
<p>One question that may arise is why we didn’t implement <acronym>SSL</acronym>
2.0 in the library. There are several reasons, most important being
that it has serious security flaws, unacceptable for a modern security
library. Other than that, this protocol is barely used by anyone
these days since it has been deprecated since 1996. The security
problems in <acronym>SSL</acronym> 2.0 include:
</p>
<ul>
<li> Message integrity compromised.
The <acronym>SSLv2</acronym> message authentication uses the MD5 function, and
is insecure.
</li><li> Man-in-the-middle attack.
There is no protection of the handshake in <acronym>SSLv2</acronym>, which
permits a man-in-the-middle attack.
</li><li> Truncation attack.
<acronym>SSLv2</acronym> relies on TCP FIN to close the session, so the
attacker can forge a TCP FIN, and the peer cannot tell if it was a
legitimate end of data or not.
</li><li> Weak message integrity for export ciphers.
The cryptographic keys in <acronym>SSLv2</acronym> are used for both message
authentication and encryption, so if weak encryption schemes are
negotiated (say 40-bit keys) the message authentication code uses the
same weak key, which isn’t necessary.
</li></ul>
<a name="index-PCT"></a>
<p>Other protocols such as Microsoft’s <acronym>PCT</acronym> 1 and <acronym>PCT</acronym>
2 were not implemented because they were also abandoned and deprecated
by <acronym>SSL</acronym> 3.0 and later <acronym>TLS</acronym> 1.0.
</p>
<hr>
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<p>
Previous: <a href="How-to-use-TLS-in-application-protocols.html#How-to-use-TLS-in-application-protocols" accesskey="p" rel="prev">How to use TLS in application protocols</a>, Up: <a href="Introduction-to-TLS.html#Introduction-to-TLS" accesskey="u" rel="up">Introduction to TLS</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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