ID: 51146
User updated by: zelnaga at gmail dot com
Reported By: zelnaga at gmail dot com
Status: Open
Bug Type: mcrypt related
Operating System: Windows XP
PHP Version: 5.3.1
New Comment:
mcrypt also seems to be implementing OFB and CFB modes identically.
Although the first block produced by either mode should be the same,
subsequent blocks should be different. ie. in CFB, the second block is
XOR'd with the previous ciphertext, reencrypted with the key, whereas in
OFB, the second block is XOR'd with that which the previous text was
previously XOR'd with.
Example code:
<?php
$td = mcrypt_module_open(MCRYPT_DES, '', MCRYPT_MODE_OFB, '');
mcrypt_generic_init($td, 'aaaaaaaa', 'bbbbbbbb');
echo urlencode(mcrypt_generic($td, str_repeat("\0", 16))) . "\r\n";
$td = mcrypt_module_open(MCRYPT_DES, '', MCRYPT_MODE_CFB, '');
mcrypt_generic_init($td, 'aaaaaaaa', 'bbbbbbbb');
echo urlencode(mcrypt_generic($td, str_repeat("\0", 16)));
?>
Previous Comments:
------------------------------------------------------------------------
[2010-02-25 18:01:52] zelnaga at gmail dot com
Description:
------------
Correct me if I'm wrong, but shouldn't an ECB decryption of an OFB
encrypted string of null bytes produce a string whose first eight bytes
(assuming that that's the block size) are equal to the IV? Certainly
that's the impression I get from wikipedia.org's discussion of OFB.
http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation#Output_feedback_.28OFB.29
Reproduce code:
---------------
<?php
$td = mcrypt_module_open(MCRYPT_DES, '', MCRYPT_MODE_OFB, '');
mcrypt_generic_init($td, 'aaaaaaaa', 'bbbbbbbb');
$ciphertext = mcrypt_generic($td, "\0\0\0\0\0\0\0\0");
$td = mcrypt_module_open(MCRYPT_DES, '', MCRYPT_MODE_ECB, '');
mcrypt_generic_init($td, 'aaaaaaaa', "\0\0\0\0\0\0\0\0");
echo urlencode(mdecrypt_generic($td, $ciphertext));
?>
Expected result:
----------------
bbbbbbbb
Actual result:
--------------
5%FBdq%C7Y7%13
------------------------------------------------------------------------
--
Edit this bug report at http://bugs.php.net/?id=51146&edit=1
