linux-kernel - Re: [PATCH v2] random: mix build-time latent entropy into pool at init

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Date:   Mon, 4 Apr 2022 16:27:44 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Sandy Harris <sandyinchina@...il.com>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH v2] random: mix build-time latent entropy into pool at
 init

On Sat, Apr 02, 2022 at 12:44:42PM +0800, Sandy Harris wrote:
> Yarrow is a good design, but it has limitations; in particular
> the Yarrow paper says the cryptographic strength is limited
> to the size of the hash context, 160 bits for their SHA-1 &
> 512 for our Blake.
> 
> 512 bits is more than enough for nearly all use cases, but
> we may have some where it is not. How many random bits
> are needed to generate a 4k-bit PGP key?
> 
> Will some users try to generate one-time pads from /dev/random?
> The OTP security proof requires truly random data as long as the
> message; with anything short of that the proof fails & you get
> a stream cipher.

All the data from /dev/{u,}random is generated by ChaCha20, which is a 256-bit
stream cipher.  We don't target, or need to target, more than 256-bit security.
So the entropy pool itself doesn't need to be more than 256 bits, provided that
it is implemented properly using a cryptographic hash function, which it now is.

- Eric