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<a name="On-Record-Padding"></a>
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Previous: <a href="Weaknesses-and-countermeasures.html#Weaknesses-and-countermeasures" accesskey="p" rel="prev">Weaknesses and countermeasures</a>, Up: <a href="The-TLS-record-protocol.html#The-TLS-record-protocol" accesskey="u" rel="up">The TLS record protocol</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="On-record-padding"></a>
<h4 class="subsection">3.3.4 On record padding</h4>
<a name="index-record-padding"></a>
<a name="index-bad_005frecord_005fmac"></a>
<p>The TLS protocol allows for extra padding of records in CBC ciphers, to prevent
statistical analysis based on the length of exchanged messages (see [<em>RFC5246</em>] section 6.2.3.2).
GnuTLS appears to be one of few implementations that take advantage of this feature:
the user can provide some plaintext data with a range of lengths she wishes to hide,
and GnuTLS adds extra padding to make sure the attacker cannot tell the real plaintext
length is in a range smaller than the user-provided one.
Use <a href="Core-TLS-API.html#gnutls_005frecord_005fsend_005frange">gnutls_record_send_range</a> to send length-hidden messages and
<a href="Core-TLS-API.html#gnutls_005frecord_005fcan_005fuse_005flength_005fhiding">gnutls_record_can_use_length_hiding</a> to check whether the current
session supports length hiding. Using the standard <a href="Core-TLS-API.html#gnutls_005frecord_005fsend">gnutls_record_send</a>
will only add minimal padding.
</p>
<p>The TLS implementation in the Symbian operating system, frequently
used by Nokia and Sony-Ericsson mobile phones, cannot handle
non-minimal record padding. What happens when one of these clients
handshake with a GnuTLS server is that the client will fail to compute
the correct MAC for the record. The client sends a TLS alert
(<code>bad_record_mac</code>) and disconnects. Typically this will result
in error messages such as ’A TLS fatal alert has been received’, ’Bad
record MAC’, or both, on the GnuTLS server side.
</p>
<p>If compatibility with such devices is a concern, not sending length-hidden messages
solves the problem by using minimal padding.
</p>
<p>If you implement an application that has a configuration file, we
recommend that you make it possible for users or administrators to
specify a GnuTLS protocol priority string, which is used by your
application via <a href="Core-TLS-API.html#gnutls_005fpriority_005fset">gnutls_priority_set</a>. To allow the best
flexibility, make it possible to have a different priority string for
different incoming IP addresses.
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