Doc #51146 [Asn->Ana]: mcrypt doesn't do OFB mode correctly

Edit report at https://bugs.php.net/bug.php?id=51146&edit=1 ID: 51146 Updated by: kalle@php.net Reported by: zelnaga at gmail dot com Summary: mcrypt doesn't do OFB mode correctly -Status: Assigned +Status: Analyzed Type: Documentation Problem Package: mcrypt related Operating System: Windows XP PHP Version: 5.3.1 -Assigned To: leigh +Assigned To: Block user comment: N Private report: N Previous Comments: ------------------------------------------------------------------------ [2017-01-10 16:30:16] nikic@php.net Reclassifying as documentation problem for part 1). As the mcrypt extension has been killed, we won't be adding new constants -- the documentation should state that the string should be passed directly for this. ------------------------------------------------------------------------ [2017-01-10 14:28:26] leigh@php.net Ok, so what this boils down to is: 1) A documentation issue that mcrypt's implementation of OFB and CFB (as represented by MODE_OFB and MODE_CFB) operate in 8-bit mode, and that NCFB operates on full blocks. 2) A feature request to wrap mcrypt's MODE_NCFB constant. ------------------------------------------------------------------------ [2017-01-10 12:06:25] php at haravikk dot me I'm sorry but I believe this has been closed prematurely; as I have pointed out, mcrypt is in fact operating correctly, the issue here is that the behaviour of its CFB and OFB modes is misleading as it is per-byte, rather than per-state size block. The solution is to use the MCRYPT_MODE_NOFB constant, or to request ncfb mode via string, as the n signifies that these are scaled to the size of the state/key, so 128, 192 or 256 bits. As I stated in my earlier comment, the problem is in fact on the PHP side in so far as there is no MCRYPT_MODE_NCFB constant, and that the documentation for the CFB and OFB constants do not clarify that these are per-byte modes of operation. I believe that these are therefore issues with PHP's libmcrypt wrapper, not libmcrypt itself, and should be resolvable (especially after nearly seven years!) ------------------------------------------------------------------------ [2016-12-14 23:47:59] leigh@php.net Closing because it's not PHP implementing the encryption modes, it's libmcrypt which is unmaintained. The assumption that encrypting in OFB and decrypting in ECB should yield the IV at the start of the plaintext (where the original plaintext is entirely null bytes) is correct. If mcrypt does not do this, it is a problem with the third party library, the PHP module is just a wrapper. ------------------------------------------------------------------------ [2013-11-25 19:38:06] ywarnier at beeznest dot org Sorry, I forgot to mention (if that has any incidence on this report) I use PHP 5.5.3 (mod_php5) with the mcrypt module for 5.4.6. How is that even possible, I don't know, but that's what my dear Ubuntu is telling me... moving back to Debian soon, I swear :-p ------------------------------------------------------------------------ The remainder of the comments for this report are too long. To view the rest of the comments, please view the bug report online at https://bugs.php.net/bug.php?id=51146 -- Edit this bug report at https://bugs.php.net/bug.php?id=51146&edit=1