linux-kernel - Re: [PATCH 1/3] random: replace non-blocking pool with a Chacha20-based CRNG

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Date:	Wed, 04 May 2016 08:24:28 +0200
From:	Stephan Mueller <smueller@...onox.de>
To:	Theodore Ts'o <tytso@....edu>
Cc:	linux-kernel@...r.kernel.org, herbert@...dor.apana.org.au,
	andi@...stfloor.org, sandyinchina@...il.com,
	cryptography@...edaemon.net, jsd@...n.com, hpa@...or.com,
	linux-crypto@...r.kernel.org
Subject: Re: [PATCH 1/3] random: replace non-blocking pool with a Chacha20-based CRNG

Am Dienstag, 3. Mai 2016, 11:36:12 schrieb Stephan Mueller:

Hi Ted,

> > +
> > +static ssize_t extract_crng_user(void __user *buf, size_t nbytes)
> > +{
> > +	ssize_t ret = 0, i;
> > +	__u8 tmp[CHACHA20_BLOCK_SIZE];
> > +	int large_request = (nbytes > 256);
> > +
> > +	while (nbytes) {
> > +		if (large_request && need_resched()) {
> > +			if (signal_pending(current)) {
> > +				if (ret == 0)
> > +					ret = -ERESTARTSYS;
> > +				break;
> > +			}
> > +			schedule();
> > +		}
> > +
> > +		extract_crng(tmp);
> > +		i = min_t(int, nbytes, CHACHA20_BLOCK_SIZE);
> > +		if (copy_to_user(buf, tmp, i)) {
> > +			ret = -EFAULT;
> > +			break;
> > +		}
> > +
> > +		nbytes -= i;
> > +		buf += i;
> > +		ret += i;
> > +	}
> > +
> > +	/* Wipe data just written to memory */
> > +	memzero_explicit(tmp, sizeof(tmp));
> 
> Would it make sense to add another chacha20_block() call here at the end?
> Note, the one thing about the SP800-90A DRBG I really like is the enhanced
> backward secrecy support which is implemented by "updating" the internal
> state (the key / state) used for one or more random number generation
> rounds after one request for random numbers is satisfied.
> 
> This means that even if the state becomes known or the subsequent caller
> manages to deduce the state of the RNG to some degree of confidence, he
> cannot backtrack the already generated random numbers.
> 
> I see that the ChaCha20 RNG implicitly updates its state while it operates.
> But for the last round of the RNG, there is no more shuffling of the
> internal state. As one round is 64 bytes in size and many callers just want
> 16 or 32 bytes (as seen during testing), a lot of callers trigger only one
> round of the RNG.

After doing some performance tests, I see that we reach a performance of north 
of 200 MB/s on my system (compare that to 12 MB/s for the SHA-1 version).

Thus, I would assume adding another call to chacha20_block should not hurt.

Ciao
Stephan