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<a href="Crypto-module.html">Package Crypto</a> ::
<a href="Crypto.Cipher-module.html">Package Cipher</a> ::
Module DES3
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<!-- ==================== MODULE DESCRIPTION ==================== -->
<h1 class="epydoc">Module DES3</h1><p class="nomargin-top"></p>
<p>Triple DES symmetric cipher</p>
<p><a class="rst-reference external" href="http://en.wikipedia.org/wiki/Triple_DES" target="_top">Triple DES</a> (or TDES or TDEA or 3DES) is a symmetric block cipher standardized by <a class="rst-reference external" href="http://csrc.nist.gov/publications/nistpubs/800-67/SP800-67.pdf" target="_top">NIST</a>.
It has a fixed data block size of 8 bytes. Its keys are 128 (<em>Option 1</em>) or 192
bits (<em>Option 2</em>) long.
However, 1 out of 8 bits is used for redundancy and do not contribute to
security. The effective key length is respectively 112 or 168 bits.</p>
<p>TDES consists of the concatenation of 3 simple <a href="Crypto.Cipher.DES-module.html" class="link">DES</a> ciphers.</p>
<p>The plaintext is first DES encrypted with <em>K1</em>, then decrypted with <em>K2</em>,
and finally encrypted again with <em>K3</em>. The ciphertext is decrypted in the reverse manner.</p>
<p>The 192 bit key is a bundle of three 64 bit independent subkeys: <em>K1</em>, <em>K2</em>, and <em>K3</em>.</p>
<p>The 128 bit key is split into <em>K1</em> and <em>K2</em>, whereas <em>K1=K3</em>.</p>
<p>It is important that all subkeys are different, otherwise TDES would degrade to
single <a href="Crypto.Cipher.DES-module.html" class="link">DES</a>.</p>
<p>TDES is cryptographically secure, even though it is neither as secure nor as fast
as <a href="Crypto.Cipher.AES-module.html" class="link">AES</a>.</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> DES
<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-keyword">from</span> Crypto.Util <span class="py-keyword">import</span> Counter
<span class="py-prompt">>>></span>
<span class="py-prompt">>>> </span>key = b<span class="py-string">'-8B key-'</span>
<span class="py-prompt">>>> </span>nonce = Random.new().read(DES.block_size/2)
<span class="py-prompt">>>> </span>ctr = Counter.new(DES.block_size*8/2, prefix=nonce)
<span class="py-prompt">>>> </span>cipher = DES.new(key, DES.MODE_CTR, counter=ctr)
<span class="py-prompt">>>> </span>plaintext = b<span class="py-string">'We are no longer the knights who say ni!'</span>
<span class="py-prompt">>>> </span>msg = nonce + cipher.encrypt(plaintext)</pre>
</blockquote>
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<span class="summary-type"> </span>
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<a href="Crypto.Cipher.DES3.DES3Cipher-class.html" class="summary-name">DES3Cipher</a><br />
TDES cipher object
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<!-- ==================== FUNCTIONS ==================== -->
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<span class="table-header">Functions</span></td>
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<span class="summary-type"> </span>
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<td><span class="summary-sig"><a href="Crypto.Cipher.DES3-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 TDES cipher</td>
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<!-- ==================== VARIABLES ==================== -->
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<span class="table-header">Variables</span></td>
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<span class="summary-type"> </span>
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<a name="MODE_ECB"></a><span class="summary-name">MODE_ECB</span> = <code title="1">1</code><br />
Electronic Code Book (ECB). See <a href="Crypto.Cipher.blockalgo-module.html#MODE_ECB" class="link">blockalgo.MODE_ECB</a>.
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<span class="summary-type"> </span>
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<a name="MODE_CBC"></a><span class="summary-name">MODE_CBC</span> = <code title="2">2</code><br />
Cipher-Block Chaining (CBC). See <a href="Crypto.Cipher.blockalgo-module.html#MODE_CBC" class="link">blockalgo.MODE_CBC</a>.
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<span class="summary-type"> </span>
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<a name="MODE_CFB"></a><span class="summary-name">MODE_CFB</span> = <code title="3">3</code><br />
Cipher FeedBack (CFB). See <a href="Crypto.Cipher.blockalgo-module.html#MODE_CFB" class="link">blockalgo.MODE_CFB</a>.
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<span class="summary-type"> </span>
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<a name="MODE_PGP"></a><span class="summary-name">MODE_PGP</span> = <code title="4">4</code><br />
This mode should not be used.
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<span class="summary-type"> </span>
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<a name="MODE_OFB"></a><span class="summary-name">MODE_OFB</span> = <code title="5">5</code><br />
Output FeedBack (OFB). See <a href="Crypto.Cipher.blockalgo-module.html#MODE_OFB" class="link">blockalgo.MODE_OFB</a>.
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<span class="summary-type"> </span>
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<a name="MODE_CTR"></a><span class="summary-name">MODE_CTR</span> = <code title="6">6</code><br />
CounTer Mode (CTR). See <a href="Crypto.Cipher.blockalgo-module.html#MODE_CTR" class="link">blockalgo.MODE_CTR</a>.
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<span class="summary-type"> </span>
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<a name="MODE_OPENPGP"></a><span class="summary-name">MODE_OPENPGP</span> = <code title="7">7</code><br />
OpenPGP Mode. See <a href="Crypto.Cipher.blockalgo-module.html#MODE_OPENPGP" class="link">blockalgo.MODE_OPENPGP</a>.
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<span class="summary-type"> </span>
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<a name="block_size"></a><span class="summary-name">block_size</span> = <code title="8">8</code><br />
Size of a data block (in bytes)
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<span class="summary-type"> </span>
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<a name="key_size"></a><span class="summary-name">key_size</span> = <code title="(16, 24)"><code class="variable-group">(</code>16<code class="variable-op">, </code>24<code class="variable-group">)</code></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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>
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Create a new TDES 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 must be 16 or 24 bytes long. The parity bits will be ignored.</li>
<li><strong class="pname"><code>mode</code></strong> (a <em>MODE_*</em> constant) - The chaining mode to use for encryption or decryption.
Default is <a href="Crypto.Cipher.DES3-module.html#MODE_ECB" class="link">MODE_ECB</a>.</li>
<li><p><strong class="pname"><code>IV</code></strong> (byte string) - The initialization vector to use for encryption or decryption.</p>
<p>It is ignored for <a href="Crypto.Cipher.DES3-module.html#MODE_ECB" class="link">MODE_ECB</a> and <a href="Crypto.Cipher.DES3-module.html#MODE_CTR" class="link">MODE_CTR</a>.</p>
<p>For <a href="Crypto.Cipher.DES3-module.html#MODE_OPENPGP" class="link">MODE_OPENPGP</a>, IV must be <a href="Crypto.Cipher.DES3-module.html#block_size" class="link">block_size</a> bytes long for encryption
and <a href="Crypto.Cipher.DES3-module.html#block_size" class="link">block_size</a> +2 bytes for decryption (in the latter case, it is
actually the <em>encrypted</em> IV which was prefixed to the ciphertext).
It is mandatory.</p>
<p>For all other modes, it must be <a href="Crypto.Cipher.DES3-module.html#block_size" class="link">block_size</a> bytes longs.</p></li>
<li><strong class="pname"><code>counter</code></strong> (callable) - (<em>Only</em> <a href="Crypto.Cipher.DES3-module.html#MODE_CTR" class="link">MODE_CTR</a>). A stateful function that returns the next
<em>counter block</em>, which is a byte string of <a href="Crypto.Cipher.DES3-module.html#block_size" class="link">block_size</a> bytes.
For better performance, use <a href="Crypto.Util.Counter-module.html" class="link" onclick="show_private();">Crypto.Util.Counter</a>.</li>
<li><strong class="pname"><code>segment_size</code></strong> (integer) - (<em>Only</em> <a href="Crypto.Cipher.DES3-module.html#MODE_CFB" class="link">MODE_CFB</a>).The number of bits the plaintext and ciphertext
are segmented in.
It must be a multiple of 8. If 0 or not specified, it will be assumed to be 8.</li>
</ul></dd>
<dt>Returns:</dt>
<dd>an <a href="Crypto.Cipher.DES3.DES3Cipher-class.html" class="link">DES3Cipher</a> object</dd>
</dl>
<div class="fields"> <p><strong>Attention:</strong>
it is important that all 8 byte subkeys are different,
otherwise TDES would degrade to single <a href="Crypto.Cipher.DES-module.html" class="link">DES</a>.
</p>
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