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</div><hr /><div id="mcrypt.constants" class="appendix">
<h1>Predefined Constants</h1>
<p class="simpara">
The constants below are defined by this extension, and
will only be available when the extension has either
been compiled into PHP or dynamically loaded at runtime.
</p>
<p class="para">
Mcrypt can operate in four block cipher modes (<em>CBC</em>,
<em>OFB</em>, <em>CFB</em>, and
<em>ECB</em>). If linked against libmcrypt-2.4.x or higher the
functions can also operate in the block cipher mode <em>nOFB</em>
and in <em>STREAM</em> mode. Below you find a list with all
supported encryption modes together with the constants that are
defined for the encryption mode. For a more complete reference and
discussion see Applied Cryptography by Schneier (ISBN 0-471-11709-9).
<ul class="itemizedlist">
<li class="listitem">
<span class="simpara">
<strong><code>MCRYPT_MODE_ECB</code></strong> (<em>electronic
codebook</em>) is a block cipher mode that is generally
unsuitable for most purposes. The use of this mode is not recommended.
</span>
</li>
<li class="listitem">
<span class="simpara">
<strong><code>MCRYPT_MODE_CBC</code></strong> (<em>cipher block
chaining</em>) is a block cipher mode that is significantly
more secure than <em>ECB</em> mode.
</span>
</li>
<li class="listitem">
<span class="simpara">
<strong><code>MCRYPT_MODE_CFB</code></strong> (<em>cipher
feedback, in 8-bit mode</em>) is a stream cipher mode.
It is recommended to use <em>NCFB</em> mode rather
than <em>CFB</em> mode.
</span>
</li>
<li class="listitem">
<span class="simpara">
<strong><code>MCRYPT_MODE_OFB</code></strong> (<em>output feedback, in
8-bit mode</em>) is a stream cipher mode comparable to <em>CFB</em>, but
can be used in applications where error propagation cannot
be tolerated.
It is recommended to use <em>NOFB</em> mode rather
than <em>OFB</em> mode.
</span>
</li>
<li class="listitem">
<span class="simpara">
<strong><code>MCRYPT_MODE_NOFB</code></strong> (<em>output feedback,
in n-bit mode</em>) is comparable to <em>OFB</em> mode,
but operates on the full block size of the algorithm.
</span>
</li>
<li class="listitem">
<span class="simpara">
<strong><code>MCRYPT_MODE_STREAM</code></strong> is an extra mode to include
some stream algorithms like <em>"WAKE"</em> or <em>"RC4"</em>.
</span>
</li>
</ul>
</p>
<p class="para">
Mcrypt supports some other modes of operation for which there are no predefined constants.
They can be utilised by passing a string in place of the missing constants.
<ul class="itemizedlist">
<li class="listitem">
<span class="simpara">
<strong><code>"ctr"</code></strong> (<em>counter mode</em>) is a stream cipher mode.
</span>
</li>
<li class="listitem">
<span class="simpara">
<strong><code>"ncfb"</code></strong> (<em>cipher feedback,
in n-bit mode</em>) is comparable to <em>CFB</em> mode,
but operates on the full block size of the algorithm.
</span>
</li>
</ul>
</p>
<p class="para">
Some other mode and random device constants:
<dl>
<dt id="constant.mcrypt-encrypt">
<strong><code>MCRYPT_ENCRYPT</code></strong>
(<span class="type"><a href="language.types.integer.html" class="type integer">integer</a></span>)
</dt>
<dd>
<span class="simpara">
</span>
</dd>
<dt id="constant.mcrypt-decrypt">
<strong><code>MCRYPT_DECRYPT</code></strong>
(<span class="type"><a href="language.types.integer.html" class="type integer">integer</a></span>)
</dt>
<dd>
<span class="simpara">
</span>
</dd>
<dt id="constant.mcrypt-dev-random">
<strong><code>MCRYPT_DEV_RANDOM</code></strong>
(<span class="type"><a href="language.types.integer.html" class="type integer">integer</a></span>)
</dt>
<dd>
<span class="simpara">
</span>
</dd>
<dt id="constant.mcrypt-dev-urandom">
<strong><code>MCRYPT_DEV_URANDOM</code></strong>
(<span class="type"><a href="language.types.integer.html" class="type integer">integer</a></span>)
</dt>
<dd>
<span class="simpara">
</span>
</dd>
<dt id="constant.mcrypt-rand">
<strong><code>MCRYPT_RAND</code></strong>
(<span class="type"><a href="language.types.integer.html" class="type integer">integer</a></span>)
</dt>
<dd>
<span class="simpara">
</span>
</dd>
</dl>
</p>
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