Question

Good morning,

I have to create a random number from a given 256HASH using a secret key. Everyting is fine, until I have to “convert” the hash into an integer between 0 and 15.000.000 (which is the random number). I have been playing with ord() and bytes but I don’t get anything that suits me.

My original idea was to cast the SHA256 string into an integer, and then apply a divisor to obtain the modulus. But I need a random number between a very big range. 0 to 15.000.000 (fifteen million). How would you do it?

Thanks!

Answer 1

A hash i basically a number, just with a base of 16. Having said that, you just have to convert it to an int.

The problem is, when you convert a hash into a int like so:

echo intval(hash('sha256','asdf'),16);

You will always get 2147483647 for 32-bit systems, which is the maximum value for intval.

My suggestion would be to cut the value of the hash to a few first characters like so

 echo intval(substr(hash('sha256','bsbaf'),0,6),16);

This is now the seed for the random number, you can get the random by:

 $hash = hash('sha256','bsbaf');
 $seed = intval(substr($hash,0,6),16);
 mt_srand($seed);
 echo mt_rand(0,15000000);

Note that in some cases, you can have collisions, because you are using only the first 6 chars of the hash, but for most uses this will not be a problem. It’s up to you to decide if this is acceptable since I do not know your specific use case.

UPDATE: An alternative is to make modulo from the resulting seed – like so:

 $hash = hash('sha256','bsbaf');
 $seed = intval(substr($hash,0,6),16);
 echo $seed % 15000000;