[Cryptography] Shortening block cipher length...

Jon Callas jon at callas.org
Mon Mar 29 21:18:36 EDT 2021



> On Mar 29, 2021, at 16:53, Phillip Hallam-Baker <phill at hallambaker.com> wrote:
> 
> 
> 
> Same problem as with a stream cipher.
> 
> The problem here is to use a single key to permute 2^n identifiers so that nothing is given away from the sequences. 
> 
> If we use a stream cipher (or CTR mode, CFB, GCM, etc) we are effectively XORing every value with the same mask.
> 
> Permute (0) = 0 XOR k
> Permute (1) = 1 XOR k
> Permute (2) = 2 XOR k
> 
> 
> What I want is for Permute (x) to be an unpredictable bijection and to be able to do it in 8, 16, 24, 32, 40, etc chunks.

Similarly to what I said to Peter, it sounds like CFB works for you.

	Jon


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