Re: Modulo instead of XOR in CFB

From: Gregory G Rose (
Date: 08/10/05

Date: 10 Aug 2005 08:24:55 -0700

In article <42f9f6c0$0$8411$>,
Thomas <> wrote:
>In CFB mode usually the plain text is XORed with the encrypted queue
>bit. However, I want encrypt only bytes below 0xFF and the cipher should
>not contain any 0xFF.
>Therefore I thought of replacing the XOR with (mod 255).
>Are there any security concerns I might hav overseen?

Assuming that the generator is good, you'll
introduce a bias by doing this. That's because
the generator will generate 0 and 255 with equal
probability, both of which will encrypt the
plaintext the same way (in fact, by not changing
it). That means that a ciphertext byte is twice
as likely to be the same as the plaintext as it
is to be any other byte. This is a staggeringly
large bias, by crypto standards.


Greg Rose
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