Re: What do you think CSPRNG in PHP

Hi On 7/25/22 13:14, Go Kudo wrote:

However, I still think the overhead of the getrandom syscall in a Linux environment is significant and should be considered.

I disagree. On my Intel(R) Core(TM) i5-2430M with Ubuntu 20.04 with Linux 5.4.0-123-generic I can call random_bytes(16) (128 Bits of randomness which is sufficient for ~everything) 100000 times in ~140ms: <?php for ($i = 0; $i < 100000; $i++) {

    $foo = random_bytes(16);

} The same script modified to just set $foo = is_int(1) runs in 20ms: <?php for ($i = 0; $i < 100000; $i++) { $foo = is_int(1); } Thus the time of syscalling getrandom() on my machine (which definitely is not modern hardware) 100k times is 120ms or 1.2us per call.

I would suggest deprecating mt_srand()/srand() and using php_random_bytes() in sessions etc. for PHP 8.3 for better security.

Syscalling getrandom() a few times to seed a PRNG or to generate a session ID is not going to have a measurable effect. As I said in my previous email: "The cost of a few getrandom() syscalls is not really measurable compared to the time spent waiting for the database, file IO or template rendering."

I think this is a safe implementation due to the nature of CSPRNG, what do you think?

I'm pretty sure the implementation is unsafe when the process calls fork() which might happen with https://www.php.net/manual/en/function.pcntl-fork.php. The *only thing* I trust with actually generating proper cryptographically secure randomness is the kernel. Non-kernel implementations have proven to be insecure over and over again. Best regards Tim Düsterhus