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<a href="Crypto-module.html">Package Crypto</a> ::
<a href="Crypto.Cipher-module.html">Package Cipher</a> ::
Module ARC4
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<!-- ==================== MODULE DESCRIPTION ==================== -->
<h1 class="epydoc">Module ARC4</h1><p class="nomargin-top"></p>
<p>ARC4 symmetric cipher</p>
<p><a class="rst-reference external" href="http://en.wikipedia.org/wiki/RC4" target="_top">ARC4</a> (Alleged RC4) is an implementation of RC4 (Rivest's Cipher version 4),
a symmetric stream cipher designed by Ron Rivest in 1987.</p>
<p>The cipher started as a proprietary design, that was reverse engineered and
anonymously posted on Usenet in 1994. The company that owns RC4 (RSA Data
Inc.) never confirmed the correctness of the leaked algorithm.</p>
<p>Unlike RC2, the company has never published the full specification of RC4,
of whom it still holds the trademark.</p>
<p>ARC4 keys can vary in length from 40 to 2048 bits.</p>
<p>One problem of ARC4 is that it does not take a nonce or an IV. If it is required
to encrypt multiple messages with the same long-term key, a distinct
independent nonce must be created for each message, and a short-term key must
be derived from the combination of the long-term key and the nonce.
Due to the weak key scheduling algorithm of RC2, the combination must be carried
out with a complex function (e.g. a cryptographic hash) and not by simply
concatenating key and nonce.</p>
<p>New designs should not use ARC4. A good alternative is AES
(<a href="Crypto.Cipher.AES-module.html" class="link">Crypto.Cipher.AES</a>) in any of the modes that turn it into a stream cipher (OFB, CFB, or CTR).</p>
<p>As an example, encryption can be done as follows:</p>
<blockquote>
<pre class="py-doctest">
<span class="py-prompt">>>> </span><span class="py-keyword">from</span> Crypto.Cipher <span class="py-keyword">import</span> ARC4
<span class="py-prompt">>>> </span><span class="py-keyword">from</span> Crypto.Hash <span class="py-keyword">import</span> SHA
<span class="py-prompt">>>> </span><span class="py-keyword">from</span> Crypto <span class="py-keyword">import</span> Random
<span class="py-prompt">>>></span>
<span class="py-prompt">>>> </span>key = b<span class="py-string">'Very long and confidential key'</span>
<span class="py-prompt">>>> </span>nonce = Random.new().read(16)
<span class="py-prompt">>>> </span>tempkey = SHA.new(key+nonce).digest()
<span class="py-prompt">>>> </span>cipher = ARC4.new(tempkey)
<span class="py-prompt">>>> </span>msg = nonce + cipher.encrypt(b<span class="py-string">'Open the pod bay doors, HAL'</span>)</pre>
</blockquote>
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<span class="summary-type"> </span>
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<a href="Crypto.Cipher.ARC4.ARC4Cipher-class.html" class="summary-name">ARC4Cipher</a><br />
ARC4 cipher object
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<td><span class="summary-sig"><a href="Crypto.Cipher.ARC4-module.html#new" class="summary-sig-name">new</a>(<span class="summary-sig-arg">key</span>,
<span class="summary-sig-arg">*args</span>,
<span class="summary-sig-arg">**kwargs</span>)</span><br />
Create a new ARC4 cipher</td>
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<a name="block_size"></a><span class="summary-name">block_size</span> = <code title="1">1</code><br />
Size of a data block (in bytes)
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<a name="key_size"></a><span class="summary-name">key_size</span> = <code title="xrange(1, 257)">xrange(1, 257)</code><br />
Size of a key (in bytes)
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<h3 class="epydoc"><span class="sig"><span class="sig-name">new</span>(<span class="sig-arg">key</span>,
<span class="sig-arg">*args</span>,
<span class="sig-arg">**kwargs</span>)</span>
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Create a new ARC4 cipher
<dl class="fields">
<dt>Parameters:</dt>
<dd><ul class="nomargin-top">
<li><strong class="pname"><code>key</code></strong> (byte string) - The secret key to use in the symmetric cipher.
It can have any length, with a minimum of 40 bytes.
Its cryptograpic strength is always capped to 2048 bits (256 bytes).</li>
</ul></dd>
<dt>Returns:</dt>
<dd>an <a href="Crypto.Cipher.ARC4.ARC4Cipher-class.html" class="link">ARC4Cipher</a> object</dd>
</dl>
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