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Date: Thu, 4 Aug 2022 20:33:34 +0200
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To: Anton Smirnov <sandfox@sandfox.me>,
 PHP Internals List <internals@lists.php.net>, Go Kudo <zeriyoshi@gmail.com>
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Subject: Re: [PHP-DEV] xoshiro** edge case (all zeros)
From: tim@bastelstu.be (=?UTF-8?Q?Tim_D=c3=bcsterhus?=)

Hi

On 8/4/22 10:09, Anton Smirnov wrote:
> xoshiro** has a known edge case: all-zero seed

Indeed, good catch. I had that in mind, but forgot about it.

> <?php
> 
> $engine = new \Random\Engine\Xoshiro256StarStar(str_repeat("\0", 32));
> 
> while (true) {
>      echo hex2bin($engine->generate()), PHP_EOL; // 0000000000000000
> }
> 
> It should be documented and/or handled
> 
> It's only for a string seed, int seed is not affected
> 

I've created a PR here:

https://github.com/php/php-src/pull/9250

I've opted to throw a ValueError in that case, as that's the only safe 
option that does not introduce a bias.

The 32xNUL seed basically should only happen for manually written 
testing input and not happen otherwise. An actual random seed will 
result in 32 NUL bytes with just a 2**-256 chance and when relying on 
the implicit CSPRNG seeding (`null` as seed parameter) my PR will just 
retry to catch even that edge case.

Best regards
Tim Düsterhus