Daily Archives: January 7, 2018

AES-OFB

I previously added AES-CTR to my library WjCryptLib. AES-CTR is by far the best way to use AES as stream cipher. However it was not the first mode of operation devised for using a block cipher as a stream cipher. Output-Feedback-Mode (OFB) was one of the original modes of operation specified for the original block ciphers like DES. The way OFB works is to start with an IV the size of a block and repeatedly encrypt it. Each encryption produces another block worth of stream bytes.

When running as a single thread AES-OFB is exactly the same speed as AES-CTR. However it can not be parallelised, nor can the stream be synced to an arbitrary location. So in pretty much every situation AES-CTR is a better choice than AES-OFB. However if you are required to use AES-OFB due to a pre specified protocol then there are times you may need it. I have added AES-OFB to WjCryptLib.

Public domain C source code for AES-OFB:

These depend on the AES block cipher files:

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