phc-discussions - Re: [PHC] Combining sliding window and bit-reversal

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Date: Mon, 27 Jan 2014 23:49:37 +0000
From: Samuel Neves <sneves@....uc.pt>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Combining sliding window and bit-reversal

On 27-01-2014 23:22, Solar Designer wrote:
> Speaking of bit reversal instructions, it appears that we still don't
> have them on x86, not even with planned extensions.  Yet there are such
> instructions e.g. on ARM, Epiphany, AMD GCN (both scalar and vector).
>
> To be more x86-friendly, something crazy like a variable block size
> Feistel cipher using CRC-32 (one instruction on modern x86) as the F
> function might work better.  e.g. 2 rounds might be enough.  Drawback:
> complexity.
You can get decent bit reversal speed using PSHUFB, by reversing up to
32 4-bit chunks per cycle. Considering other architectures have native
bit reversal instructions, it may be an acceptable solution compared to
an ad hoc permutation.