When does mod_php in apache seed the random number generator?

Question

I am using the php rand() function to generate coupon codes for my e commerce system. It worked fine for a while but now I am getting a lot of errors that the code is already in the system.

This is the function I use:

function generateRandomString($length) {
    $characters = '0123456789abcdefghijklmnopqrstuvwxyz';
    $randomString = '';
    for ($i = 0; $i < $length; $i++) {
        $randomString .= $characters[rand(0, strlen($characters) - 1)];
    }
    return $randomString;
}   

And my codes are 32 characters long.

I did a sample of ~150 tries and noticed that more than 50% of the generated codes where laready in the system.

I have 4212 codes in the system. The odds of a 32 character random string with 36 different symbols producing a collision are basically zero, and I get 50% collisions.

When I re-seeded the random number generator in my function by calling srand(); I did not have any collisions any more.

But on the man page of php it cleary says:

Note: As of PHP 4.2.0, there is no need to seed the random number generator with srand() or mt_srand() as this is now done automatically.

I am running php version PHP 5.5.9

So my thoughts where something like that seeding is done, but only once per webserver worker, and then when the process is forked, it is not reseeded or something like that. But that would be clearly a bug in apache...

I am running php as apache modul in apache version Apache/2.4.7 (Ubuntu) and the mpm_prefork_module module

So do I still need to call srand() at the top of every script dispite the manpages saying other wise, and why? Is it apaches fault or PHP's?

And yes, I am aware that I should not use this function for this purpose, and I will update it to use cryptographically secure numbers. But I think this should not happen anyway and I am still interested in what is going on!

Answer 1

If your codes are 32 characters long, then why don't you simply encrypt the current microtime with md5 ?

  $coupon = md5( microtime() );

One line simple. And if you want a touch of randomness, just throw a

   $coupon = md5( microtime() . mt_rand( 0, 10000) );

On there like a salt. That will almost guarantee you will never duplicate. As for the why it is not as random.

PHP’s random number generators are seeded only once per process.

See this posting ...

http://phpsecurity.readthedocs.org/en/latest/Insufficient-Entropy-For-Random-Values.html

by the way I don't think you need cryptographically secure hashes, only sufficiently random ones that cant be easily guessed. Even with a cryptographic hash, users will enter said hash into the cart for the coupon, it's a simple matter to brute force even a cryptographically secure hash then, you'd do better to invest time in only allowing "n" attempts, or "n" attempts per second etc. To reduce the rate a brute force attack can be done.

For example, I would just try all combinations of 32 character hashes. So it doesn't matter in the end, because you are not using plaintext entries like a password, and then hiding your salting and encryption method. The number of coupons active would determine my success rate and the time it takes me in either case ... If you follow.

IMPLIED IN MY ANSWER IS THIS

PHP’s random number generators are seeded only once per process. Forking does not create a new process but copies the current processes state.

See
Calling rand/mt_rand on forked children yields identical results
and
http://wiki.openssl.org/index.php/Random_fork-safety
and
http://www.reddit.com/r/shittyprogramming/comments/2jvzgq/sometimes_it_takes_real_shitty_code_to_expose_an/

Additionally this is not an issue specific to php but more so to psudorandom number generation in general.

Answer 2

See this: https://github.com/php/php-src/blob/d0cb715373c3fbe9dc095378ec5ed8c71f799f67/ext/standard/rand.c#L66-L68

Apparently RNG is being reseeded on first call to rand() (or explicitly calling srand()).

Since fork copies parent's memory, child also gets parent's seed - never getting reseeded.

Answer 3

function generateRandomString($length) {
    $characters = '0123456789abcdefghijklmnopqrstuvwxyz';
    $randomString = '';
    for ($i = 0; $i < $length; $i++) {
        $randomString .= $characters[rand(0, strlen($characters) - 1)];
    }
    return substr(time().$randomString,0,$length);
}