I'm currently trying to understand the difference between "regular" OFB with full-block feedback and OFB with 8-bit feedback and, in this context, the difference between this two modes regarding security.
Please correct me if I am wrong: Given an example using AES256 and a blocksize of 128 bytes, the general procedure would be the following:
IV _____________ | | | v | v Key -> AES-256 | Key -> AES-256 |______| |____.... | | v v P -> xor P -> xor | | v v C C
With full-block feedback, I would get a 128 byte long output from the first AES block, which would then be XORed with 128 bytes of plaintext, which would result in 128 bytes of cipher text.
When using OFB in 8-bit mode, only the first 8-bit of the plaintext would be XORed with the first 8-bit from the output of the first AES block.
The main difference I can see is that it would take longer to encrypt / decrypt the plaintext since it would need more iterations, but what exactly is the problem regarding security?
The main reason for this question is this Post.