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

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Date:   Fri, 22 Apr 2022 01:08:01 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        "Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH] siphash: update the HalfSipHash documentation

Hi Eric,

On Thu, Apr 21, 2022 at 10:44 PM Eric Biggers <ebiggers@...nel.org> wrote:
> -Danger!
> +**Danger!** HalfSipHash should only be used in a very limited set of use cases
> +where nothing better is possible, namely:
>
> -Do not ever use HalfSipHash except for as a hashtable key function, and only
> -then when you can be absolutely certain that the outputs will never be
> -transmitted out of the kernel. This is only remotely useful over `jhash` as a
> -means of mitigating hashtable flooding denial of service attacks.
> +- Hashtable key functions, where the outputs will never be transmitted out of
> +  the kernel. This is only remotely useful over `jhash` as a means of mitigating
> +  hashtable flooding denial of service attacks.

I think we should actually drop this chunk of the patch. You wrote in
your commit message, "HalfSipHash-1-3 is not entirely limited to
hashtable functions, with it now being used in the interrupt entropy
accumulator." But in fact, random.c uses HalfSipHash-1, with no three
round finalization (since we use BLAKE2s for that). So it's not
_quite_ the same thing. If it were, we could have gotten away by just
calling the actual hsiphash function, but instead it's just applying
the round function as a permutation.

If you feel strongly that somebody might accidentally copy and paste
that after grepping for halfsiphash and trying to figure out how to
use it, I suppose we could keep this. But it strikes me as very much
not the same thing as the hsiphash_* family of functions.

Jason