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<?xml version="1.0" encoding="ascii"?>
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<a href="Crypto-module.html">Package Crypto</a> ::
<a href="Crypto.Cipher-module.html">Package Cipher</a> ::
Module blockalgo
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<!-- ==================== MODULE DESCRIPTION ==================== -->
<h1 class="epydoc">Module blockalgo</h1><p class="nomargin-top"></p>
Module with definitions common to all block ciphers.
<!-- ==================== CLASSES ==================== -->
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<span class="summary-type"> </span>
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<a href="Crypto.Cipher.blockalgo.BlockAlgo-class.html" class="summary-name">BlockAlgo</a><br />
Class modelling an abstract block cipher.
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<!-- ==================== VARIABLES ==================== -->
<a name="section-Variables"></a>
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<span class="table-header">Variables</span></td>
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<a href="Crypto.Cipher.blockalgo-module.html#MODE_ECB" class="summary-name">MODE_ECB</a> = <code title="1">1</code><br />
<em>Electronic Code Book (ECB)</em>.
This is the simplest encryption mode. Each of the plaintext blocks
is directly encrypted into a ciphertext block, independently of
any other block. This mode exposes frequency of symbols
in your plaintext. Other modes (e.g. <em>CBC</em>) should be used instead.
</td>
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<span class="summary-type"> </span>
</td><td class="summary">
<a href="Crypto.Cipher.blockalgo-module.html#MODE_CBC" class="summary-name">MODE_CBC</a> = <code title="2">2</code><br />
<em>Cipher-Block Chaining (CBC)</em>. Each of the ciphertext blocks depends
on the current and all previous plaintext blocks. An Initialization Vector
(<em>IV</em>) is required.
</td>
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<span class="summary-type"> </span>
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<a href="Crypto.Cipher.blockalgo-module.html#MODE_CFB" class="summary-name">MODE_CFB</a> = <code title="3">3</code><br />
<em>Cipher FeedBack (CFB)</em>. This mode is similar to CBC, but it transforms
the underlying block cipher into a stream cipher. Plaintext and ciphertext
are processed in <em>segments</em> of <strong>s</strong> bits. The mode is therefore sometimes
labelled <strong>s</strong>-bit CFB. An Initialization Vector (<em>IV</em>) is required.
</td>
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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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<a href="Crypto.Cipher.blockalgo-module.html#MODE_OFB" class="summary-name">MODE_OFB</a> = <code title="5">5</code><br />
<em>Output FeedBack (OFB)</em>. This mode is very similar to CBC, but it
transforms the underlying block cipher into a stream cipher.
The keystream is the iterated block encryption of an Initialization Vector (<em>IV</em>).
</td>
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<span class="summary-type"> </span>
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<a href="Crypto.Cipher.blockalgo-module.html#MODE_CTR" class="summary-name">MODE_CTR</a> = <code title="6">6</code><br />
<em>CounTeR (CTR)</em>. This mode is very similar to ECB, in that
encryption of one block is done independently of all other blocks.
Unlike ECB, the block <em>position</em> contributes to the encryption and no
information leaks about symbol frequency.
</td>
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<span class="summary-type"> </span>
</td><td class="summary">
<a href="Crypto.Cipher.blockalgo-module.html#MODE_OPENPGP" class="summary-name">MODE_OPENPGP</a> = <code title="7">7</code><br />
OpenPGP. This mode is a variant of CFB, and it is only used in PGP and <a class="rst-reference external" href="http://tools.ietf.org/html/rfc4880" target="_top">OpenPGP</a> applications.
An Initialization Vector (<em>IV</em>) is required.
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<span class="summary-type"> </span>
</td><td class="summary">
<a name="__package__"></a><span class="summary-name">__package__</span> = <code title="'Crypto.Cipher'"><code class="variable-quote">'</code><code class="variable-string">Crypto.Cipher</code><code class="variable-quote">'</code></code>
</td>
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</table>
<!-- ==================== VARIABLES DETAILS ==================== -->
<a name="section-VariablesDetails"></a>
<table class="details" border="1" cellpadding="3"
cellspacing="0" width="100%" bgcolor="white">
<tr bgcolor="#70b0f0" class="table-header">
<td align="left" colspan="2" class="table-header">
<span class="table-header">Variables Details</span></td>
</tr>
</table>
<a name="MODE_ECB"></a>
<div>
<table class="details" border="1" cellpadding="3"
cellspacing="0" width="100%" bgcolor="white">
<tr><td>
<h3 class="epydoc">MODE_ECB</h3>
<p><em>Electronic Code Book (ECB)</em>.
This is the simplest encryption mode. Each of the plaintext blocks
is directly encrypted into a ciphertext block, independently of
any other block. This mode exposes frequency of symbols
in your plaintext. Other modes (e.g. <em>CBC</em>) should be used instead.</p>
<p>See <a class="rst-reference external" href="http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf" target="_top">NIST SP800-38A</a> , Section 6.1 .</p>
<dl class="fields">
</dl>
<dl class="fields">
<dt>Value:</dt>
<dd><table><tr><td><pre class="variable">
1
</pre></td></tr></table>
</dd>
</dl>
</td></tr></table>
</div>
<a name="MODE_CBC"></a>
<div>
<table class="details" border="1" cellpadding="3"
cellspacing="0" width="100%" bgcolor="white">
<tr><td>
<h3 class="epydoc">MODE_CBC</h3>
<p><em>Cipher-Block Chaining (CBC)</em>. Each of the ciphertext blocks depends
on the current and all previous plaintext blocks. An Initialization Vector
(<em>IV</em>) is required.</p>
<p>The <em>IV</em> is a data block to be transmitted to the receiver.
The <em>IV</em> can be made public, but it must be authenticated by the receiver and
it should be picked randomly.</p>
<p>See <a class="rst-reference external" href="http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf" target="_top">NIST SP800-38A</a> , Section 6.2 .</p>
<dl class="fields">
</dl>
<dl class="fields">
<dt>Value:</dt>
<dd><table><tr><td><pre class="variable">
2
</pre></td></tr></table>
</dd>
</dl>
</td></tr></table>
</div>
<a name="MODE_CFB"></a>
<div>
<table class="details" border="1" cellpadding="3"
cellspacing="0" width="100%" bgcolor="white">
<tr><td>
<h3 class="epydoc">MODE_CFB</h3>
<p><em>Cipher FeedBack (CFB)</em>. This mode is similar to CBC, but it transforms
the underlying block cipher into a stream cipher. Plaintext and ciphertext
are processed in <em>segments</em> of <strong>s</strong> bits. The mode is therefore sometimes
labelled <strong>s</strong>-bit CFB. An Initialization Vector (<em>IV</em>) is required.</p>
<p>When encrypting, each ciphertext segment contributes to the encryption of
the next plaintext segment.</p>
<p>This <em>IV</em> is a data block to be transmitted to the receiver.
The <em>IV</em> can be made public, but it should be picked randomly.
Reusing the same <em>IV</em> for encryptions done with the same key lead to
catastrophic cryptographic failures.</p>
<p>See <a class="rst-reference external" href="http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf" target="_top">NIST SP800-38A</a> , Section 6.3 .</p>
<dl class="fields">
</dl>
<dl class="fields">
<dt>Value:</dt>
<dd><table><tr><td><pre class="variable">
3
</pre></td></tr></table>
</dd>
</dl>
</td></tr></table>
</div>
<a name="MODE_OFB"></a>
<div>
<table class="details" border="1" cellpadding="3"
cellspacing="0" width="100%" bgcolor="white">
<tr><td>
<h3 class="epydoc">MODE_OFB</h3>
<p><em>Output FeedBack (OFB)</em>. This mode is very similar to CBC, but it
transforms the underlying block cipher into a stream cipher.
The keystream is the iterated block encryption of an Initialization Vector (<em>IV</em>).</p>
<p>The <em>IV</em> is a data block to be transmitted to the receiver.
The <em>IV</em> can be made public, but it should be picked randomly.</p>
<p>Reusing the same <em>IV</em> for encryptions done with the same key lead to
catastrophic cryptograhic failures.</p>
<p>See <a class="rst-reference external" href="http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf" target="_top">NIST SP800-38A</a> , Section 6.4 .</p>
<dl class="fields">
</dl>
<dl class="fields">
<dt>Value:</dt>
<dd><table><tr><td><pre class="variable">
5
</pre></td></tr></table>
</dd>
</dl>
</td></tr></table>
</div>
<a name="MODE_CTR"></a>
<div>
<table class="details" border="1" cellpadding="3"
cellspacing="0" width="100%" bgcolor="white">
<tr><td>
<h3 class="epydoc">MODE_CTR</h3>
<p><em>CounTeR (CTR)</em>. This mode is very similar to ECB, in that
encryption of one block is done independently of all other blocks.
Unlike ECB, the block <em>position</em> contributes to the encryption and no
information leaks about symbol frequency.</p>
<p>Each message block is associated to a <em>counter</em> which must be unique
across all messages that get encrypted with the same key (not just within
the same message). The counter is as big as the block size.</p>
<p>Counters can be generated in several ways. The most straightword one is
to choose an <em>initial counter block</em> (which can be made public, similarly
to the <em>IV</em> for the other modes) and increment its lowest <strong>m</strong> bits by
one (modulo <em>2^m</em>) for each block. In most cases, <strong>m</strong> is chosen to be half
the block size.</p>
<p>Reusing the same <em>initial counter block</em> for encryptions done with the same
key lead to catastrophic cryptograhic failures.</p>
<p>See <a class="rst-reference external" href="http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf" target="_top">NIST SP800-38A</a> , Section 6.5 (for the mode) and Appendix B (for how
to manage the <em>initial counter block</em>).</p>
<dl class="fields">
</dl>
<dl class="fields">
<dt>Value:</dt>
<dd><table><tr><td><pre class="variable">
6
</pre></td></tr></table>
</dd>
</dl>
</td></tr></table>
</div>
<a name="MODE_OPENPGP"></a>
<div>
<table class="details" border="1" cellpadding="3"
cellspacing="0" width="100%" bgcolor="white">
<tr><td>
<h3 class="epydoc">MODE_OPENPGP</h3>
<p>OpenPGP. This mode is a variant of CFB, and it is only used in PGP and <a class="rst-rst-reference external rst-reference external" href="http://tools.ietf.org/html/rfc4880" target="_top">OpenPGP</a> applications.
An Initialization Vector (<em>IV</em>) is required.</p>
<p>Unlike CFB, the IV is not transmitted to the receiver. Instead, the <em>encrypted</em> IV is.
The IV is a random data block. Two of its bytes are duplicated to act as a checksum
for the correctness of the key. The encrypted IV is therefore 2 bytes longer than
the clean IV.</p>
<dl class="fields">
</dl>
<dl class="fields">
<dt>Value:</dt>
<dd><table><tr><td><pre class="variable">
7
</pre></td></tr></table>
</dd>
</dl>
</td></tr></table>
</div>
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