Re: [PATCH v2] siphash: update the hsiphash documentation

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Apr 22, 2022 at 02:40:36AM +0200, Jason A. Donenfeld wrote:
> 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

That's fine, assuming that you're just talking about SipHash-1-3, not also
HalfSipHash-1-3.  The section title is "HalfSipHash - SipHash's insecure younger
cousin", so the word "insecure" is applied to HalfSipHash already.

- Eric