php: number only hash?

Question

In php is there a way to give a unique hash from a string, but that the hash was made up from numbers only?

example:

return md5(234); // returns 098f         


        

Answer 1

There are some good answers but for me the approaches seem silly.
They first force php to create a Hex number, then convert this back (hexdec) in a BigInteger and then cut it down to a number of letters... this is much work!

Instead why not

Read the hash as binary:

$binhash = md5('[input value]', true);

then using

$numhash = unpack('N2', $binhash); //- or 'V2' for little endian

to cast this as two INTs ($numhash is an array of two elements). Now you can reduce the number of bits in the number simply using an AND operation. e.g:

$result = $numhash[1] & 0x000FFFFF; //- to get numbers between 0 and 1048575

But be warned of collisions! Reducing the number means increasing the probability of two different [input value] with the same output.

I think that the much better way would be the use of "ID-Crypting" with a Bijectiv function. So no collisions could happen! For the simplest kind just use an Affine_cipher

Example with max input value range from 0 to 25:

function numcrypt($a)
{
   return ($a * 15) % 26;
}

function unnumcrypt($a)
{
   return ($a * 7) % 26;
}

Output:

numcrypt(1) : 15
numcrypt(2) : 4
numcrypt(3) : 19

unnumcrypt(15) : 1
unnumcrypt(4)  : 2
unnumcrypt(19) : 3

e.g.

$id = unnumcrypt($_GET('userid'));

... do something with the ID ...

echo '<a href="do.php?userid='. numcrypt($id) . '"> go </a>';

of course this is not secure, but if no one knows the method used for your encryption then there are no security reasons then this way is faster and collision safe.