Creating random keys with md5 but sometimes creates a repeated key

Question

I'm trying to generate a process in order to create aproximately 5000 unique keys into a table with rand_md5 function. Sometimes it is giving a repeated "unique" key constraint violation. What can I do to solve this?

function rand_md5($length) {
  $max = ceil($length / 32);
  $random = '';
  for ($i = 0; $i < $max; $i ++) {
    $random .= md5(microtime(true).mt_rand(10000,90000));
  }
return substr($random, 0, $length);

This function is called inside a for loop from 1 to 5000 iterations (for example).

Any ideas how to solve this?

http://stackoverflow.com/questions/1155008/how-unique-is-uuid — anonymous, May 13 '15 at 16:03
So don't generate unique stuff using PHP, just use MySQL for that. `INSERT INTO table (my_unique_column) values (MD5(UUID());` and there you go, unique. No language should have the authority for generating unique stuff for a database. — N.B., May 13 '15 at 16:31

score 1 · Answer 1 · answered May 13 '15 at 16:08

You have to keep track of the allocated values and check for each newly generated one if it was already used.

function rand_md5($length) {
  $max = ceil($length / 32);
  $random = '';
  for ($i = 0; $i < $max; $i ++) {
    $random .= md5(microtime(true).mt_rand(10000,90000));
  }
  return substr($random, 0, $length);
}

$randoms = [];

do {
  $rnd = rand_md5($length);
} while( in_array( $rnd, $randoms ) );
$randoms[] = $rnd;

score 0 · Answer 2 · answered May 13 '15 at 16:07

First suggestion, use a library better at generating unique tokens, i..e, uuid().

Secondly, ensure that your database column is long enough to store the string. Whilst probable you will generate collisions with your algorithm, frequently seems to imply you (or the database) are truncating the string.

score 0 · Answer 3 · 2015-05-15T02:33:48.043

0

Using openssl_random_pseudo_bytes:

function randomKey($length) {

    //Need a length divisible by two.
    $length = intval($length);
    if (!$length || $length % 2)
        return false;

    //Get random bytes & hex encode.
    $key = openssl_random_pseudo_bytes($length / 2);
    return (is_string($key) ? bin2hex($key) : false);

}

edited May 15 '15 at 02:33

answered May 15 '15 at 01:39

Anything involve to have an ssl connection? – Rodolfo Velasco May 15 '15 at 16:42
If your PHP installation doesn't have OpenSSL, you can use `mcrypt_create_iv($length / 2, MCRYPT_DEV_URANDOM)` instead. – May 15 '15 at 22:48

score -1 · Answer 4 · answered May 13 '15 at 16:21

-1

I think it can help you

function rand_md5($length) {
  $random = md5(microtime(true).mt_rand(10000,90000));
  $random = str_shuffle($random);
  return substr($random, 0, $length);
}

answered May 13 '15 at 16:21

user3419778

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Creating random keys with md5 but sometimes creates a repeated key

4 Answers4