PHP's silly PRNG

http://cod.ifies.com/2008/05/php-rand01-on-windows-openssl-rand-on.html

by zokier • 17 years ago

33 39 17 years ago

cod.ifies.com

halo• 17 years ago

For some added context, the normal rand function in PHP uses an implementation based on libc.

PHP provides a second, safer, function, mt_rand, which is based around an implementation of the Mersenne Twister.

windsurfer• 17 years ago

It would make more sense to call mt_rand simply rand, and the current rand to fast_rand.

zokier3• 17 years ago

well, that could make some sense, but mt_rand is significantly faster too than basic rand.

edit: what I meant was that imho there is not really any reason to keep that old, broken rand function lying around

ecaron• 17 years ago

The difference when using mt_rand is astonishing.

PHP 5.3 graph using rand: http://twitpic.com/gq81b

PHP 5.3 graph using mt_rand: http://twitpic.com/gq827

jacquesm11• 17 years ago

The regular rand can do that just as well. see elsewhere in this thread.

tptacek• 17 years ago

mt_rand isn't crypto-safe either. You need to be using the system's secure random number generator, or OpenSSL's.

robryan• 17 years ago

If mt_rand is really that much better, what is the point of keeping them both? Shouldn't they just make rand map to mt_rand?

idlewords• 17 years ago

There seems to be some kind of law that, for anything to do with crypto, numerics, or randomness, a language must ship with both a broken default and a correct implementation hidden deep in some library known only to the pure of heart.

Mark_B• 17 years ago

Probably a matter of mitigating risk. Despite its drawbacks, rand() is used a LOT and works...well, "good enough".

In the interest of making sure the code you wrote for PHP3 will work on current/future versions and not have anything unexpected happen.

zokier3• 17 years ago

I would like to see the code that breaks if PRNG is swapped. I mean, like is there code out there that relies on the unrandomness of current rand?

michaelcampbell• 17 years ago

> Tuesday, May 20, 2008

I assume PHP still has this issue?

ComputerGuru• 17 years ago

Yes.

blasdel• 17 years ago

Of course!

PHP only gains new issues, no warts are ever really removed.

jacquesm11• 17 years ago

Making real random data using a computer is hard, very hard.

These are pseudo random number generators, I take it that everybody using them understands that the 'P' means that you can not rely on the output of these pieces of code to be truely random.

One of the first online casinos used a PIC to sample a noisy zener diode to get to something less predictable.

And even with that as the source concept it took quite a bit of tweaking of the design to get really random bits, it's surprising how much hum from your powersupply can make it through to the bits of your a/d converter.

zokier3• 17 years ago

Yeah. Random is hard.

BUT when you actually already have far better implementation of PRNG, namely md_rand in PHP, there really is no excuse for defaulting to broken behavior.

jacquesm11• 17 years ago

That may actually have to do with not upsetting peoples unit tests that depend on this 'broken behavior'.

Let's say someone wrote an extensive body of statistical software that gets tested with the known input from the PRNG in php, if you then change the random number generator all the output will be different.

Or if someone used the PRNG in PHP to encrypt a bunch of data replacing it would break the decrypt function leaving the user with inaccessible data.

mmphosis• 17 years ago

I heard that an online casino used optical sensors and lava lamps to generate random numbers.

Scaevolus• 17 years ago

The problem that you see here is mostly that rand() gets the range in a way biased towards the lower-order bits, which in more LCGs are less random than the higher-order bits.

jacquesm11• 17 years ago

Not only that, they invoke a bunch of float code that then has to balance on getting the rounding of the lowest bit right after a whole bunch of float scaling operations.

That's simply not the way to do it when you are only interested in one bit of data to check the random number generator.

That's only handy if you want to limit the range of your random numbers between two numbers with a bit more space between them than just the lower order bit.

Say 500 and 10000 or 0 and 500 or something like that.

yan• 17 years ago

Zalewski's "Silence On The Wire" (which is totally fantastic in its own right) graphs a few PRNGs and discusses their issues.

http://www.amazon.com/exec/obidos/tg/detail/-/1593270461

jacquesm11• 17 years ago

The only thing silly here is this article.

First off, the random generator in PHP is simply a 2^32 period random number generator, nothing special or wrong about that. Yes, there are better ones but it's good enough for its intended purpose.

About the image in the article:

The image is generated by using the 'scale' function of the PHP PRNG to sample the 'lowest' bit.

You do not do a scale when you should be doing a one bit and.

It pays off to study an issue for a while, once you raise a stink it's hard to undo the damage you've done.

By replacing the line in the code that plots the pixel with:

   imagesetpixel($im,$x,$y, rand() & 1);

this image appears:

http://ww.com/random.php

Problem gone, no fuss.

That's on a linux box running php 4.4.2, the results look pretty random to me, it isn't perfect but the aliasing that was displayed in the original test image is clearly gone using the 'regular' (so not mt_rand) random number generator.

To make the result reproducible I've seeded the PRNG with '0' using srand(0); at the beginning of the program.

If you want to use the 'scale' function in PHP for something that you should be doing by a one bit and then you will reap the results of that, you are effectively sampling the lowest bit after a bunch of float operations, which is clearly sub-optimal.

PHP can't correct for incorrect use.

As for the rest of the code, the $x=0; line can be dropped, there is no need to 'reset' your loop variable at the end of a loop, last I checked.

I'm no PHP fan but before you criticize a project make sure you understand the material.

PHP's silly PRNG

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