linux-kernel - Re: [PATCH v2] siphash: update the hsiphash documentation

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Message-ID: <YmH5hDrUZKBymqYE@zx2c4.com>
Date:   Fri, 22 Apr 2022 02:40:36 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH v2] siphash: update the hsiphash documentation

Hi Eric,

On Thu, Apr 21, 2022 at 05:27:31PM -0700, Eric Biggers wrote:
> +On 64-bit kernels, the hsiphash functions actually implement SipHash-1-3, a
> +reduced-round variant of SipHash, instead of HalfSipHash-1-3. This is because in
> +64-bit code, SipHash-1-3 is no slower than HalfSipHash-1-3, and can be faster.
> +Note, this does *not* mean that in 64-bit kernels the hsiphash functions are the
> +same as the siphash ones, or that they are secure; the hsiphash functions still
> +use an insecure reduced-round algorithm and truncate their outputs to 32 bits.
 
Small nit: "less secure" rather than "insecure", as I don't think
there's a super realistic attack against 1-3. I can make that change
when committing if that's fine by you.

Jason