Re: [PATCH v2 2/2] random: use BLAKE2s instead of SHA1 in extraction

["Theodore Ts'o" <tytso@mit.edu>]

On Thu, Dec 23, 2021 at 03:11:13PM +0100, Jason A. Donenfeld 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.
> 
> This commit aims to fix both of these issues while also keeping the
> general structure and semantics as close to the original as possible.
> Specifically:
> 
>    a) Rather than overwriting the hash IV with RDRAND, we put it into
>       BLAKE2's documented "salt" and "personal" fields, which were
>       specifically created for this type of usage.
>    b) Since this function feeds the full hash result back into the
>       entropy collector, we only return from it half the length of the
>       hash, just as it was done before. This increases the
>       construction's forward secrecy from 2^80 to a much more
>       comfortable 2^128.
>    c) Rather than using the raw "sha1_transform" function alone, we
>       instead use the full proper BLAKE2s function, with finalization.
> 
> This also has the advantage of supplying 16 bytes at a time rather than
> SHA1's 10 bytes, which, in addition to having a faster compression
> function to begin with, means faster extraction in general. On an Intel
> i7-11850H, this commit makes initial seeding around 131% faster.
> 
> BLAKE2s itself has the nice property of internally being based on the
> ChaCha permutation, which the RNG is already using for expansion, so
> there shouldn't be any issue with newness, funkiness, or surprising CPU
> behavior, since it's based on something already in use.
> 
> [1] https://eprint.iacr.org/2005/010.pdf
> [2] https://www.iacr.org/archive/crypto2005/36210017/36210017.pdf
> [3] https://eprint.iacr.org/2015/967.pdf
> [4] https://shattered.io/static/shattered.pdf
> [5] https://www.usenix.org/system/files/sec20-leurent.pdf
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Reviewed-by: Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Looks good, thanks for the work!

Reviewed-by: Theodore Ts'o <tytso@mit.edu>

						- Ted