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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Tue, 11 Jan 2022 12:38:40 +0100
Message-ID: <CAMuHMdU0spv9X_wErkBBWQ9kV9f1zE_YNcu5nPbTG_64Lh_h0w@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] random: use BLAKE2s instead of SHA1 in extraction
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Theodore Tso <tytso@mit.edu>,
        Greg KH <gregkh@linuxfoundation.org>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>
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Hi Jsaon,

On Sat, Dec 25, 2021 at 1:52 AM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> This commit addresses one of the lower hanging fruits of the RNG: its
> usage of SHA1.
>
> BLAKE2s is generally faster, and certainly more secure, than SHA1, which
> has [1] been [2] really [3] very [4] broken [5]. Additionally, the
> current construction in the RNG doesn't use the full SHA1 function, as
> specified, and allows overwriting the IV with RDRAND output in an
> undocumented way, even in the case when RDRAND isn't set to "trusted",
> which means potential malicious IV choices. And its short length means
> that keeping only half of it secret when feeding back into the mixer
> gives us only 2^80 bits of forward secrecy. In other words, not only is
> the choice of hash function dated, but the use of it isn't really great
> either.

m68k bloat-o-meter:

blake2s_compress_generic                       -    4448   +4448
blake2s256_hmac                                -     302    +302
blake2s_update                                 -     156    +156
blake2s_final                                  -     124    +124
blake2s_init.constprop                         -      94     +94
__ksymtab_blake2s_update                       -      12     +12
__ksymtab_blake2s_final                        -      12     +12
__ksymtab_blake2s_compress_generic             -      12     +12
__ksymtab_blake2s256_hmac                      -      12     +12
blake2s_mod_init                               -       4      +4
__initcall__kmod_libblake2s__101_82_blake2s_mod_init6       -       4      +4

Unfortunately we cannot get rid of the sha1 code yet (lib/sha1.o is
built-in unconditionally), as there are other users...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds