Is a random string a good verification code

I'm generating a verification code to be used for account activation. You've probably seen this sort of thing before.

My question: if I were to genera开发者_StackOverflow社区te this code with a complex formula like this:

md5(md5(time().'helloguys'.rand(0,9999)));

Is it really any better than generating just a random string of 32 characters and numbers like gj3dI3OGwo5Enf...?

No, using the hash is not better. It would be more secure (less predictable) to pick 32 random characters. (Digits are characters.) Use a good ("cryptographic") random number generator, with a good seed (some bytes from /dev/random). Don't use time as a seed.

Agree with erickson, just may advise you to use

pwgen -1 -s

command on *nix which will the job muich better of any procedure you may invent.

If you want to generate some string programmatically you may take a look at

<?php    
$better_token = md5(uniqid(rand(),1));
?>

this gives very good level of randomness and prior to collisions.

If you need even higher level of security you may consider to generate random sequences on http://www.random.org/