# Password-generation function using custom seed

After reading an article on Skull Security mentioning the potential weakness of php's mt_rand function because of weak auto-seeding (http://ow.ly/4nrne), I decided to see what -- if any -- entropy I could find available from within php. The idea is to have enough (weak) sources that even if one or two are manipulated, lost or recovered, there's enough left to thwart brute force against the resulting passwords later.

Hopefully the result is both readable and usable, although I don't expect it to be production-quality.

<?php
/**
*
* v1.01
* Jumps through many hoops to attempt to overcome autoseed weakness of php's mt_rand function
*
*/
// Change this for each installation
$localsecret = 'qTgppE9T2c'; // Determine length of generated password$pwlength = 10;
$pwchars = 'ABCDEFGHJKLMNPQRSTUVWXYZabcdefghjkmnpqrstuvwxyz0123456789';$l = strlen( $pwchars ) - 1; // Get a little bit of entropy from sources that should be inaccessible to outsiders and non-static$dat = getrusage(); // gather some information from the running system
$datline = md5(implode($dat)); // wash using md5 -- it's fast and there's not enough entropy to warrant longer hash
$hardToGuess =$datline;
$self = __FILE__; // a file the script should have read access to (itself)$stat = stat($self); // information about file such as inode, accessed time, uid, guid$statline = md5(implode($stat)); // wash$hardToGuess .= $statline;$preseed = md5(microtime()) . getmypid() . $hardToGuess . memory_get_usage() . disk_free_space('.') .$localsecret;
$seed = sha1($preseed ); // final wash, longer hash
// Seed the mt_rand() function with a better seed than the standard one
mt_srand ($seed); // Pick characters from the lineup, using the seeded mt_rand function$pw = '';
for ( $i = 0;$i < $pwlength;$i++ ) {
$pw .=$pwchars{ mt_rand( 0, $l ) }; } // Return the result return$pw;
}

?>


Revision 1.01 adds a local secret.

• Just realized, while reviewing my own code the day after, that the whole thing is currently useless because the mt_srand function expects an integer as seed. – Niels2000 Mar 28 '11 at 10:34
• It's easy to cast the seed as a number (using hexdec, for example) but the problem is that the number will be too large to fit as an integer and become a float. – Niels2000 Mar 28 '11 at 11:04
• So, basically, I could use a suggestion on how to reduce the float value to an integer that fits into the current system's integer size, but does not lose too much of the gathered entropy in the process. – Niels2000 Mar 28 '11 at 11:12
• Having done some searching, I believe the best implementation of a crc64-algorithm is currently this one, written in C: bioinfadmin.cs.ucl.ac.uk/downloads/crc64 All I have to do now is convert it to php. Too bad I don't know C, this is going to be a challenge. – Niels2000 Apr 2 '11 at 12:14
• OK, so I learned elsewhere that the rand() and mt_rand() functions are actually separately seeded. This should mean that using both means you have to attack both, at the same time, to recover the result (unless attacking the 32-bit-problem). Essentially, we can do no worse by adding rand() to the mix, and probably make attacks quite a bit harder. I've implemented this by using str_shuffle on the character set, because that invokes the rand() function, not mt_rand(), according to the documentation. As a side effect, this code now "works" on 64-bit systems, but very weakly. – Niels2000 Apr 3 '11 at 10:34

I actually don't get the point of injecting so much system information into the mt_srand function. Looks like a total (and maybe even pointless) paranoia :)

But here you go with a cleaner code:

<?php
/**
* v2.0
*/

define('APP_SECRET_KEY', 'qTgppE9T2c');

function randomPassword($length=10) {$charset = 'ABCDEFGHJKLMNPQRSTUVWXYZabcdefghjkmnpqrstuvwxyz0123456789';
$charsetSize = strlen($charset) - 1;

// Seeding the generator with a bunch of different system data and the secret key
mt_srand(crc32(md5(microtime())
. getmypid()
. md5(implode(getrusage()))
. md5(implode(stat(__FILE__)))
. memory_get_usage()
. disk_free_space('.')
. APP_SECRET_KEY)
);

$password = ''; foreach (range(1,$length) as $_)$password .= $charset{mt_rand(0,$charsetSize)};

return $password; } echo randomPassword(), "\n";  Maybe you'll like the more perverted superslow version which returns CRC32 of randomly ordered entropy each time you generate a new symbol. <?php /** * Random password generator * v2.1 */ function randomPassword($length=10) {
$charset = 'ABCDEFGHJKLMNPQRSTUVWXYZabcdefghjkmnpqrstuvwxyz0123456789';$charsetSize = strlen($charset) - 1;$seeders = array(
function () { return md5(microtime()); },
function () { return md5(getmypid()); },
function () { return md5(implode(getrusage())); },
function () { return memory_get_usage(); },
function () { return disk_free_space('.'); }
);

$randomSeed = function () use ($seeders) {
shuffle($seeders);$entropy = '';
foreach ($seeders as$seeder)
$entropy .=$seeder();

return crc32($entropy); };$password = '';
foreach (range(1, $length) as$_) {
mt_srand($randomSeed());$password .= $charset{mt_rand(0,$charsetSize)};
}