Re: [PATCH] arc4random.3: New page documenting the arc4random(3) family of functions

[Alejandro Colomar <alx.manpages@gmail.com>]

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On 3/17/23 22:44, Alejandro Colomar wrote:
> Hi Jakub,
> 
> On 3/17/23 22:31, Jakub Wilk wrote:
>> * Alejandro Colomar <alx.manpages@gmail.com>, 2023-01-01 17:27:
>>> arc4random_uniform() returns a random number less than upper_bound for 
>>> valid input, or 0 when upper_bound is invalid.
>>
>> Is the "or 0 ..." thing part of the API?
> 
> Yes, it is part of the (undocumented) API.  At least, their authors
> claim to have thought about it when designing it, and purposefully took
> the decision of returning 0.  They fail to acknowledge that it's a bug,
> also fail to acknowledge that their documentation doesn't document this
> behavior, and don't have any intention of changing the API because
> "we don't know what can possibly fail; you'd have to audit all software
> in the world to confirm that none depends on that detail".
> 
> I have serious doubts that any software can depend on that, because
> mathematically it doesn't make any sense, so algorithms will likely
> have to purposefully special-case arc4random_uniform(0), but can't know
> for sure, because well, I haven't audited all software in the world.
> 
> I didn't find any case in OpenBSD's source code that depends on that,
> however.
> 
>> I could find anything like that 
>> in glibc docs or BSD man pages.
> 
> <https://inbox.sourceware.org/libc-alpha/20230101162627.28031-1-alx@kernel.org/>

Sorry, I pasted the wrong link.  I wanted to paste this one:

<https://inbox.sourceware.org/libc-alpha/20221231023653.41877-1-alx@kernel.org/T/>

> 
> Their manual page is bogus, and the funny thing is that one of Theo's
> arguments to reject a proposal to fix the bug in the API was that I
> wouldn't be able to document it reasonably.  Well, as you saw, it's the
> current behavior that isn't well documented, and I had to special-case
> it in BUGS.
> 
>>
>>> STANDARDS
>>>       These nonstandard functions are present in several Unix systems.
>>
>> That's not really helpful. Also, the VERSIONS section is missing ("every 
>> new interface should include a VERSIONS section").
> 
> I thought of that this morning, while doing some reorganization of that
> section globally.  I'll add the version.
> 
>>
>> In contrast, the libbsd man page is much more informative:
>>
>>> These functions first appeared in OpenBSD 2.1, FreeBSD 3.0, NetBSD 
>>> 1.6, and DragonFly 1.0.  The functions arc4random(), arc4random_buf() 
>>> and arc4random_uniform() appeared in glibc 2.36.
> 
> Yup.  :)
> 
> Thanks a lot for this thorough review!
> 
> Alex
> 
>>>
>>> The original version of this random number generator used the RC4 (also 
>>> known as ARC4) algorithm.  In OpenBSD 5.5 it was replaced with the 
>>> ChaCha20 cipher, and it may be replaced again in the future as 
>>> cryptographic techniques advance.  A good mnemonic is “A Replacement 
>>> Call for Random”.
>>
> 

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