Subject: Re: [PATCH 1/2] wrapper: add a helper to generate numbers from a CSPRNG

From: Junio C Hamano <[email-masked]>
Date: Mon, 22 Nov 2021 01:10:26 -0800

"brian m. carlson" <[email-masked]> writes:

> On 2021-11-18 at 07:19:08, Junio C Hamano wrote:
>> Presumably csprn_bytes() grabs bytes from underlying mechanism in
>> smaller chunk, but would not return until it fills the buffer---ah,
>> your "make sure our buffer handling is correct" is primarily about
>> the check that we get full 1k bytes in the loop?  We ask 1k chunk 64
>> times and we must get full 1k chunk every time?
>
> Yes, that's what we'd expect to happen.
>
>> What I was wondering about was the other half of the check, ensuring
>> all buckets[] are painted that gave us the cute 10^-100 math.
>
> Say the buffer handling is incorrect and we read only a few bytes
> instead of the full 1 KiB.  Then we'll end up filling only some of the
> buckets, and the check will fail much of the time, because we won't get
> sufficient number of random bytes to fill all the buckets.

... meaning (64 * a few bytes) is small enough such that some slots
in buckets[] will be left untouched (and the remainder of 1kB is
untouched --- but the buffer[] is not initialized in any way, so
it's not like such an "oops, we only fed a few bytes" bug would
leave the rest to NUL or anything)?

> The check is that we got enough data that looks like random bytes over
> the course of our requests.

If the check were doing so, yes, I would have understood (whether I
agreed with it or not), but the check is "if we taint each and every
bucket[] even once, we are OK", not "bucket[] should be more or less
evenly touched", and that is why I do/did not understand the test.