#51146 [Opn->Fbk]: mcrypt doesn't do OFB mode correctly

ID: 51146 Updated by: pajoye@php.net Reported By: zelnaga at gmail dot com -Status: Open +Status: Feedback Bug Type: mcrypt related Operating System: Windows XP PHP Version: 5.3.1 New Comment: It looks like a libmcrypt problem, if it is a bug. Can you try using the mcrypt cmd line tools? If it fails and you see it as a bug, please report a bug to the mcrypt project. Let us know how it went. Previous Comments: ------------------------------------------------------------------------ [2010-02-25 18:18:35] zelnaga at gmail dot com 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))); ?> ------------------------------------------------------------------------ [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