Re: [PATCH v12 9/9] manual: Add documentation for arc4random functions

["Zack Weinberg" <zack@owlfolio.org>]

On Fri, Jul 22, 2022, at 1:15 PM, Adhemerval Zanella Netto wrote:
> On 22/07/22 12:33, Zack Weinberg via Libc-alpha wrote:
>> On 2022-07-22 8:21 AM, Adhemerval Zanella via Libc-alpha wrote:
>>> +Although these functions provide higher random quality than ISO, BSD, and
>>> +SVID functions, these still use a Pseudo-Random generator and should not
>>> +be used in cryptographic contexts.
>>
>> Huh? Isn't the whole point of arc4random that it _is_
>> cryptographically strong?
>
> Even OpenBSD manual is not clear about it [1].
...
> The main problem is if we state this is a CSRNG we need to certify
> somehow, which I don't have the background to do; and it might incur
> in extra certification depending of the intended glibc usage [2].
> So I think it would be a more conservative approach to state our
> implementation is not a CSRNG ...

My main concern with saying "still uses a Pseudo-Random Generator" and
"should not be used in cryptographic contexts" is that, since we
resisted adding this API for so long, people might think we did the
bare minimum and it shouldn't actually be used at all.

How about this?

# These functions use a cryptographic-strength random number
# generation _algorithm_ (currently ChaCha20) but the implementation
# has not been validated for use in security-critical contexts.

zw