Audit my PHP function to generate cryptography secure passwords

Question

I wrote a function in PHP to generate a random password (only 0-9a-zA-Z) for my app. The resulting password must be cryptography secure, and as random as possible. I.E. the passwords are sensitive.

The big trick I do is shuffle $possible characters everytime, so even if mt_rand() is not truely random, it should not be predictable.

Any recommended changes or security issues in my function? Is using openssl_random_pseudo_bytes() instead of mt_rand() really going to make the algorithm stronger and more secure?

    public function generate_random($length = 15) {
        $random = "";
        $possible = "1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
        $possible = str_shuffle($possible);

        $maxlength = strlen($possible);

        if ($length > $maxlength) {
            $length = $maxlength;
        }

        $i = 0;

        while ($i < $length) {
            $random .= substr($possible, mt_rand(0, $maxlength-1), 1);
            $i++;
        }

        return $random;
    }

Thanks.

Just part of the spec. Adding special characters could be done, but let's ignore that aspect. — Justin, Jun 20 '13 at 04:35
you will still have certain letters next to each other with this method right or not at all? — DevZer0, Jun 20 '13 at 04:35
codereview.stackexchange.com is probably a better site for questions like this. — Barmar, Jun 20 '13 at 04:38
Sure, but they have to guess the shuffle and each `mt_rand()` character? Twice (not really but...) the random, right? — Justin, Jun 20 '13 at 05:04
Looks ok, but don't use mt_rand, use openssl_random_pseudo_bytes() instead. — Vlad Krasnov, Jun 20 '13 at 06:19
Related: http://stackoverflow.com/questions/14089516/should-i-use-urandom-or-openssl-random-pseudo-bytes — Jürgen Thelen, Jun 20 '13 at 12:44

score 2 · Answer 1 · answered Jun 20 '13 at 08:14

To generate something really random, you have to use the random source of the operating system. After reading from this source, you need to encode the bytes to an alphabet of your choice.

An easy conversion is base64 encoding, but this will include '+' and '/' characters. To only get characters from the alphabet and digits, you need a base62 encoding, or you can simply replace those characters with other characters.

/**
 * Generates a random string of a given length, using the random source of
 * the operating system. The string contains only characters of this
 * alphabet: +/0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
 * @param int $length Number of characters the string should have.
 * @return string A random base64 encoded string.
 */
function generateRandomBase64String($length)
{
  if (!defined('MCRYPT_DEV_URANDOM')) die('The MCRYPT_DEV_URANDOM source is required (PHP 5.3).');

  // Generate random bytes, using the operating system's random source.
  // Since PHP 5.3 this also uses the random source on a Windows server.
  // Unlike /dev/random, the /dev/urandom does not block the server, if
  // there is not enough entropy available.
  $binaryLength = (int)($length * 3 / 4 + 1);
  $randomBinaryString = mcrypt_create_iv($binaryLength, MCRYPT_DEV_URANDOM);
  $randomBase64String = base64_encode($randomBinaryString);
  return substr($randomBase64String, 0, $length);
}

The code is part of this class, have a look at the function generateRandomBase62String() for a complete example.

score 1 · Answer 2 · edited May 23 '17 at 12:22

1

Adding pseudo-randomness to a pseudo-random string won't increase the entropy at all. The only way is to use a better random number generator.

Possible duplicate: Secure random number generation in PHP

edited May 23 '17 at 12:22

Community

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answered Jun 20 '13 at 06:35

ntoskrnl

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score 1 · Answer 3 · edited Oct 07 '21 at 06:16

If by cryptographically secure, you mean you intend to use the password as a key somewhere, it is important to realize that your space isn't nearly large enough. 15 characters with 62 possibilities each is less than 90 bits, which is about as strong as RSA-1024, and is considered unsafe today.

You should, however, not be doing such a thing in the first place. If you do require a human-readable string that maps to something that can be used as a cryptographic key, use something like PBKDF2.

Lastly, shuffling the string does not increase effective randomness. As long as you do not use it directly as a key, your function is fine - remember to first check the output against a dictionary of common passwords
(like a password list from a password cracker) and reject those.

Rajeev Ranjan · Answer 4 · 2013-06-20T05:38:12.263

0

This is not an answer of your question but it seems feasible to use this function to generate random random password containing only (0-9a-z,A-Z)

$password = base64_encode(openssl_random_pseudo_bytes(20, $strong));
$newstr = preg_replace('/[^a-zA-Z0-9\']/', '', $password);
echo $newstr;

edited Jun 20 '13 at 05:38

answered Jun 20 '13 at 04:45

Rajeev Ranjan

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This will only contain `0`–`9`, `a`–`f`. – Gumbo Jun 20 '13 at 04:52
@Gumbo yes it is.actually i am using it in my app to generate random password or one time password when user register and clicks forget password. – Rajeev Ranjan Jun 20 '13 at 04:56
check my edit this should work.. – Rajeev Ranjan Jun 20 '13 at 05:38

Audit my PHP function to generate cryptography secure passwords

4 Answers4