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
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