261 lines
8.3 KiB
HTML
Executable File
261 lines
8.3 KiB
HTML
Executable File
<?xml version="1.0" ?>
|
|
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
|
|
<html xmlns="http://www.w3.org/1999/xhtml">
|
|
<head>
|
|
<title>des_modes</title>
|
|
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
|
|
<link rev="made" href="mailto:root@localhost" />
|
|
</head>
|
|
|
|
<body style="background-color: white">
|
|
|
|
|
|
<!-- INDEX BEGIN -->
|
|
<div name="index">
|
|
<p><a name="__index__"></a></p>
|
|
|
|
<ul>
|
|
|
|
<li><a href="#name">NAME</a></li>
|
|
<li><a href="#description">DESCRIPTION</a></li>
|
|
<li><a href="#overview">OVERVIEW</a></li>
|
|
<ul>
|
|
|
|
<li><a href="#electronic_codebook_mode__ecb_">Electronic Codebook Mode (ECB)</a></li>
|
|
<li><a href="#cipher_block_chaining_mode__cbc_">Cipher Block Chaining Mode (CBC)</a></li>
|
|
<li><a href="#cipher_feedback_mode__cfb_">Cipher Feedback Mode (CFB)</a></li>
|
|
<li><a href="#output_feedback_mode__ofb_">Output Feedback Mode (OFB)</a></li>
|
|
<li><a href="#triple_ecb_mode">Triple ECB Mode</a></li>
|
|
<li><a href="#triple_cbc_mode">Triple CBC Mode</a></li>
|
|
</ul>
|
|
|
|
<li><a href="#notes">NOTES</a></li>
|
|
<li><a href="#see_also">SEE ALSO</a></li>
|
|
<li><a href="#copyright">COPYRIGHT</a></li>
|
|
</ul>
|
|
|
|
<hr name="index" />
|
|
</div>
|
|
<!-- INDEX END -->
|
|
|
|
<p>
|
|
</p>
|
|
<hr />
|
|
<h1><a name="name">NAME</a></h1>
|
|
<p>des_modes - the variants of DES and other crypto algorithms of OpenSSL</p>
|
|
<p>
|
|
</p>
|
|
<hr />
|
|
<h1><a name="description">DESCRIPTION</a></h1>
|
|
<p>Several crypto algorithms for OpenSSL can be used in a number of modes. Those
|
|
are used for using block ciphers in a way similar to stream ciphers, among
|
|
other things.</p>
|
|
<p>
|
|
</p>
|
|
<hr />
|
|
<h1><a name="overview">OVERVIEW</a></h1>
|
|
<p>
|
|
</p>
|
|
<h2><a name="electronic_codebook_mode__ecb_">Electronic Codebook Mode (ECB)</a></h2>
|
|
<p>Normally, this is found as the function <em>algorithm</em><code>_ecb_encrypt()</code>.</p>
|
|
<ul>
|
|
<li>
|
|
<p>64 bits are enciphered at a time.</p>
|
|
</li>
|
|
<li>
|
|
<p>The order of the blocks can be rearranged without detection.</p>
|
|
</li>
|
|
<li>
|
|
<p>The same plaintext block always produces the same ciphertext block
|
|
(for the same key) making it vulnerable to a 'dictionary attack'.</p>
|
|
</li>
|
|
<li>
|
|
<p>An error will only affect one ciphertext block.</p>
|
|
</li>
|
|
</ul>
|
|
<p>
|
|
</p>
|
|
<h2><a name="cipher_block_chaining_mode__cbc_">Cipher Block Chaining Mode (CBC)</a></h2>
|
|
<p>Normally, this is found as the function <em>algorithm</em><code>_cbc_encrypt()</code>.
|
|
Be aware that <code>des_cbc_encrypt()</code> is not really DES CBC (it does
|
|
not update the IV); use <code>des_ncbc_encrypt()</code> instead.</p>
|
|
<ul>
|
|
<li>
|
|
<p>a multiple of 64 bits are enciphered at a time.</p>
|
|
</li>
|
|
<li>
|
|
<p>The CBC mode produces the same ciphertext whenever the same
|
|
plaintext is encrypted using the same key and starting variable.</p>
|
|
</li>
|
|
<li>
|
|
<p>The chaining operation makes the ciphertext blocks dependent on the
|
|
current and all preceding plaintext blocks and therefore blocks can not
|
|
be rearranged.</p>
|
|
</li>
|
|
<li>
|
|
<p>The use of different starting variables prevents the same plaintext
|
|
enciphering to the same ciphertext.</p>
|
|
</li>
|
|
<li>
|
|
<p>An error will affect the current and the following ciphertext blocks.</p>
|
|
</li>
|
|
</ul>
|
|
<p>
|
|
</p>
|
|
<h2><a name="cipher_feedback_mode__cfb_">Cipher Feedback Mode (CFB)</a></h2>
|
|
<p>Normally, this is found as the function <em>algorithm</em><code>_cfb_encrypt()</code>.</p>
|
|
<ul>
|
|
<li>
|
|
<p>a number of bits (j) <= 64 are enciphered at a time.</p>
|
|
</li>
|
|
<li>
|
|
<p>The CFB mode produces the same ciphertext whenever the same
|
|
plaintext is encrypted using the same key and starting variable.</p>
|
|
</li>
|
|
<li>
|
|
<p>The chaining operation makes the ciphertext variables dependent on the
|
|
current and all preceding variables and therefore j-bit variables are
|
|
chained together and can not be rearranged.</p>
|
|
</li>
|
|
<li>
|
|
<p>The use of different starting variables prevents the same plaintext
|
|
enciphering to the same ciphertext.</p>
|
|
</li>
|
|
<li>
|
|
<p>The strength of the CFB mode depends on the size of k (maximal if
|
|
j == k). In my implementation this is always the case.</p>
|
|
</li>
|
|
<li>
|
|
<p>Selection of a small value for j will require more cycles through
|
|
the encipherment algorithm per unit of plaintext and thus cause
|
|
greater processing overheads.</p>
|
|
</li>
|
|
<li>
|
|
<p>Only multiples of j bits can be enciphered.</p>
|
|
</li>
|
|
<li>
|
|
<p>An error will affect the current and the following ciphertext variables.</p>
|
|
</li>
|
|
</ul>
|
|
<p>
|
|
</p>
|
|
<h2><a name="output_feedback_mode__ofb_">Output Feedback Mode (OFB)</a></h2>
|
|
<p>Normally, this is found as the function <em>algorithm</em><code>_ofb_encrypt()</code>.</p>
|
|
<ul>
|
|
<li>
|
|
<p>a number of bits (j) <= 64 are enciphered at a time.</p>
|
|
</li>
|
|
<li>
|
|
<p>The OFB mode produces the same ciphertext whenever the same
|
|
plaintext enciphered using the same key and starting variable. More
|
|
over, in the OFB mode the same key stream is produced when the same
|
|
key and start variable are used. Consequently, for security reasons
|
|
a specific start variable should be used only once for a given key.</p>
|
|
</li>
|
|
<li>
|
|
<p>The absence of chaining makes the OFB more vulnerable to specific attacks.</p>
|
|
</li>
|
|
<li>
|
|
<p>The use of different start variables values prevents the same
|
|
plaintext enciphering to the same ciphertext, by producing different
|
|
key streams.</p>
|
|
</li>
|
|
<li>
|
|
<p>Selection of a small value for j will require more cycles through
|
|
the encipherment algorithm per unit of plaintext and thus cause
|
|
greater processing overheads.</p>
|
|
</li>
|
|
<li>
|
|
<p>Only multiples of j bits can be enciphered.</p>
|
|
</li>
|
|
<li>
|
|
<p>OFB mode of operation does not extend ciphertext errors in the
|
|
resultant plaintext output. Every bit error in the ciphertext causes
|
|
only one bit to be in error in the deciphered plaintext.</p>
|
|
</li>
|
|
<li>
|
|
<p>OFB mode is not self-synchronizing. If the two operation of
|
|
encipherment and decipherment get out of synchronism, the system needs
|
|
to be re-initialized.</p>
|
|
</li>
|
|
<li>
|
|
<p>Each re-initialization should use a value of the start variable
|
|
different from the start variable values used before with the same
|
|
key. The reason for this is that an identical bit stream would be
|
|
produced each time from the same parameters. This would be
|
|
susceptible to a 'known plaintext' attack.</p>
|
|
</li>
|
|
</ul>
|
|
<p>
|
|
</p>
|
|
<h2><a name="triple_ecb_mode">Triple ECB Mode</a></h2>
|
|
<p>Normally, this is found as the function <em>algorithm</em>_ecb3_encrypt().</p>
|
|
<ul>
|
|
<li>
|
|
<p>Encrypt with key1, decrypt with key2 and encrypt with key3 again.</p>
|
|
</li>
|
|
<li>
|
|
<p>As for ECB encryption but increases the key length to 168 bits.
|
|
There are theoretic attacks that can be used that make the effective
|
|
key length 112 bits, but this attack also requires 2^56 blocks of
|
|
memory, not very likely, even for the NSA.</p>
|
|
</li>
|
|
<li>
|
|
<p>If both keys are the same it is equivalent to encrypting once with
|
|
just one key.</p>
|
|
</li>
|
|
<li>
|
|
<p>If the first and last key are the same, the key length is 112 bits.
|
|
There are attacks that could reduce the effective key strength
|
|
to only slightly more than 56 bits, but these require a lot of memory.</p>
|
|
</li>
|
|
<li>
|
|
<p>If all 3 keys are the same, this is effectively the same as normal
|
|
ecb mode.</p>
|
|
</li>
|
|
</ul>
|
|
<p>
|
|
</p>
|
|
<h2><a name="triple_cbc_mode">Triple CBC Mode</a></h2>
|
|
<p>Normally, this is found as the function <em>algorithm</em>_ede3_cbc_encrypt().</p>
|
|
<ul>
|
|
<li>
|
|
<p>Encrypt with key1, decrypt with key2 and then encrypt with key3.</p>
|
|
</li>
|
|
<li>
|
|
<p>As for CBC encryption but increases the key length to 168 bits with
|
|
the same restrictions as for triple ecb mode.</p>
|
|
</li>
|
|
</ul>
|
|
<p>
|
|
</p>
|
|
<hr />
|
|
<h1><a name="notes">NOTES</a></h1>
|
|
<p>This text was been written in large parts by Eric Young in his original
|
|
documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed
|
|
it to:</p>
|
|
<pre>
|
|
AS 2805.5.2
|
|
Australian Standard
|
|
Electronic funds transfer - Requirements for interfaces,
|
|
Part 5.2: Modes of operation for an n-bit block cipher algorithm
|
|
Appendix A</pre>
|
|
<p>
|
|
</p>
|
|
<hr />
|
|
<h1><a name="see_also">SEE ALSO</a></h1>
|
|
<p><em>BF_encrypt(3)</em>, <em>DES_crypt(3)</em></p>
|
|
<p>
|
|
</p>
|
|
<hr />
|
|
<h1><a name="copyright">COPYRIGHT</a></h1>
|
|
<p>Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.</p>
|
|
<p>Licensed under the OpenSSL license (the "License"). You may not use
|
|
this file except in compliance with the License. You can obtain a copy
|
|
in the file LICENSE in the source distribution or at
|
|
<a href="https://www.openssl.org/source/license.html">https://www.openssl.org/source/license.html</a>.</p>
|
|
|
|
</body>
|
|
|
|
</html>
|