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Date: Mon, 1 Apr 2019 13:06:56 +0300
Message-ID: <CAPhkiZx1dnMqG+Ws55zqCHkcKf6_VkQW1eOiMccJ1EcGZp46SA@mail.gmail.com>
To: Benjamin Morel <benjamin.morel@gmail.com>
Cc: Pierre Joye <pierre.php@gmail.com>, Arvids Godjuks <arvids.godjuks@gmail.com>, 
	David Rodrigues <david.proweb@gmail.com>, PHP internals <internals@lists.php.net>
Content-Type: text/plain; charset="UTF-8"
Subject: Re: [PHP-DEV] random_seed()
From: narf@devilix.net (Andrey Andreev)

Hi,

On Mon, Apr 1, 2019 at 11:08 AM Benjamin Morel <benjamin.morel@gmail.com> wrote:
>
> Seeds could even be dangerous here, as these numbers are supposed to be
> cryptographically secure. If you need a seedable PRNG for testing, just use
> rand().
>

Not only it could be dangerous, it would beat the entire purpose of
random_bytes()/random_int(). Just to clarify for readers not familiar
with the topic:

Seed-based RNGs are deterministic.
deterministic === predictable
predictable === not secure

Whether you want to seed for testing purposes, or someone has beaten
it into you to use random_*() instead of (mt_)rand() and now your code
doesn't work the same way, you're likely blindly following best
practices without consideration. Not everything is 100% testable and
not every problem can have the same solution.

If you need to generate secure tokens of some kind - use random_bytes().
If you need to generate unpredictable random numbers - use random_int().
Don't worry about testing either of those.
If you need seed-based, reproducible outcomes - use mt_rand(), that's
perfectly fine for e.g. re-creating the same "random" map layout in a
video game - a valid use case; but it's not for security.

Cheers,
Andrey.