Is there a difference between CBC and CFB modes in AES?

Question

Quoting myforwik's answer, CBC, OFB and CFB are identical, however OFB/CFB is better because you only need encryption and not decryption, which can save code space.

I want a pure implementation of AES CFB in PHP. I specifically do NOT want mcrypt.

I therefore turned to PHPSecLib. However, as per the documentation, the library only supports ECB and CBC.

I do not think myforwik meant that CBC and CFB can be used interchangeably... So, a) is there a CFB implementation out there~~, or b) can I (easily) implement CFB mode from PHPSecLib's CBC implementation~~?

score 2 · Accepted Answer · answered Oct 17 '10 at 02:59

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Check out the latest CVS of phpseclib. It supports CTR, CFB and OFB, in addition to CBC and ECB. I assume the documentation will be updated when a new version is released.

answered Oct 17 '10 at 02:59

fruzer

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Thanks for pointing that out. I should always remember to check the repo, but sometimes time makes me forget... – Félix Saparelli Oct 18 '10 at 04:58

score 1 · Answer 2 · answered Oct 17 '10 at 02:43

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CBC, OFB and CFB are not identical - though there are similarities. If they were identical, they would not have different names.

If you want AES + CFB, you cannot use AES + CBC to achieve the result. You will have to find a way to get AES + CFB implemented if it is not already available.

answered Oct 17 '10 at 02:43

Jonathan Leffler

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That's what I thought. That's b) taken care of, then. – Félix Saparelli Oct 17 '10 at 02:46
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@passcod: I've gone and annotated the x-ref. – Jonathan Leffler Oct 17 '10 at 02:54

Is there a difference between CBC and CFB modes in AES?

2 Answers2