Hi Dominik, On Fri, May 13, 2022 at 08:22:36AM +0200, Dominik Brodowski wrote: > Am Wed, May 04, 2022 at 01:16:44PM +0200 schrieb Jason A. Donenfeld: > > Before, the first 64 bytes of input, regardless of how entropic it was, > > would be used to mutate the crng base key directly, and none of those > > bytes would be credited as having entropy. Then 256 bits of credited > > input would be accumulated, and only then would the rng transition from > > the earlier "fast init" phase into being actually initialized. > > > > The thinking was that by mixing and matching fast init and real init, an > > attacker who compromised the fast init state, considered easy to do > > given how little entropy might be in those first 64 bytes, would then be > > able to bruteforce bits from the actual initialization. By keeping these > > separate, bruteforcing became impossible. > > > > However, by not crediting potentially creditable bits from those first 64 > > bytes of input, we delay initialization, and actually make the problem > > worse, because it means the user is drawing worse random numbers for a > > longer period of time. > > > > Instead, we can take the first 128 bits as fast init, and allow them to > > be credited, and then hold off on the next 128 bits until they've > > accumulated. This is still a wide enough margin to prevent bruteforcing > > the rng state, while still initializing much faster. > > > > Then, rather than trying to piecemeal inject into the base crng key at > > various points, instead just extract from the pool when we need it, for > > the crng_init==0 phase. Performance may even be better for the various > > inputs here, since there are likely more calls to mix_pool_bytes() then > > there are to get_random_bytes() during this phase of system execution. > > Have you evaluated this closer, also for systems where it takes ages to > reach crng_init = 1? And might it make sense to only call extract_entropy() > if there has been new input between two calls to get_random_bytes()? Yes. On those systems, the extra calls to extract_entropy() are actually helping the otherwise abysmal state, because they're adding in some new cycle counter values on every call. Performance-wise, it's not really that bad. Actually, by *far* the most expensive thing that extract_entropy() does is RDSEED/RDRAND, but systems that have those instructions don't stay in crng_init==CRNG_EARLY for very long anyway. So all and all, it works out quite nicely. Jason
