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Date:   Fri, 14 Jan 2022 16:20:33 +0100
From:   "Jason A. Donenfeld" <>
To:     Ard Biesheuvel <>
Cc:     Alexei Starovoitov <>,
        Toke Høiland-Jørgensen <>,
        Network Development <>,
        LKML <>,
        Geert Uytterhoeven <>,
        Herbert Xu <>,
        Jean-Philippe Aumasson <>,
        Linux Crypto Mailing List <>,
        bpf <>
Subject: Re: [PATCH RFC v1 1/3] bpf: move from sha1 to blake2s in tag calculation

On Fri, Jan 14, 2022 at 4:08 PM Ard Biesheuvel <> wrote:
> Yeah, so the issue is that, at *some* point, SHA-1 is going to have to
> go. So it would be helpful if Alexei could clarify *why* he doesn't
> see this as a problem. The fact that it is broken means that it is no
> longer intractable to forge collisions, which likley means that SHA-1
> no longer fulfills the task that you wanted it to do in the first
> place.

I think the reason that Alexei doesn't think that the SHA-1 choice
really matters is because the result is being truncated to 64-bits, so
collisions are easy anyway, regardless of which hash function is
chosen (birthday bound and all). But from Geert's perspective, that
SHA-1 is still taking up precious bytes in m68k builds. And from my
perspective, it's poor form and clutters vmlinux, and plus, now I'm
curious about why this isn't using a more appropriately sized tag in
the first place.

On Fri, Jan 14, 2022 at 3:12 PM Jason A. Donenfeld <> wrote:
> "checksum" -- the thing is only 64-bits, and as you told Andy Polyakov

Whoops, meant Lutomirski here. x86 Andy, not crypto Andy :)

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