PHP: How to generate a random, unique, alphanumeric string?


Question

How would it be possible to generate a random, unique string using numbers and letters for use in a verify link? Like when you create an account on a website, and it sends you an email with a link, and you have to click that link in order to verify your account...yeah...one of those.

How can I generate one of those using PHP?

Update: Just remembered about uniqid(). It's a PHP function that generates a unique identifier based on the current time in microseconds. I think I'll use that.

1
347
4/15/2012 12:15:06 PM

Accepted Answer

Security Notice: This solution should not be used in situations where the quality of your randomness can affect the security of an application. In particular, rand() and uniqid() are not cryptographically secure random number generators. See Scott's answer for a secure alternative.

If you do not need it to be absolutely unique over time:

md5(uniqid(rand(), true))

Otherwise (given you have already determined a unique login for your user):

md5(uniqid($your_user_login, true))
281
8/13/2015 3:50:00 PM

I was just looking into how to solve this same problem, but I also want my function to create a token that can be used for password retrieval as well. This means that I need to limit the ability of the token to be guessed. Because uniqid is based on the time, and according to php.net "the return value is little different from microtime()", uniqid does not meet the criteria. PHP recommends using openssl_random_pseudo_bytes() instead to generate cryptographically secure tokens.

A quick, short and to the point answer is:

bin2hex(openssl_random_pseudo_bytes($bytes))

which will generate a random string of alphanumeric characters of length = $bytes * 2. Unfortunately this only has an alphabet of [a-f][0-9], but it works.


Below is the strongest function I could make that satisfies the criteria (This is an implemented version of Erik's answer).

function crypto_rand_secure($min, $max)
{
    $range = $max - $min;
    if ($range < 1) return $min; // not so random...
    $log = ceil(log($range, 2));
    $bytes = (int) ($log / 8) + 1; // length in bytes
    $bits = (int) $log + 1; // length in bits
    $filter = (int) (1 << $bits) - 1; // set all lower bits to 1
    do {
        $rnd = hexdec(bin2hex(openssl_random_pseudo_bytes($bytes)));
        $rnd = $rnd & $filter; // discard irrelevant bits
    } while ($rnd > $range);
    return $min + $rnd;
}

function getToken($length)
{
    $token = "";
    $codeAlphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    $codeAlphabet.= "abcdefghijklmnopqrstuvwxyz";
    $codeAlphabet.= "0123456789";
    $max = strlen($codeAlphabet); // edited

    for ($i=0; $i < $length; $i++) {
        $token .= $codeAlphabet[crypto_rand_secure(0, $max-1)];
    }

    return $token;
}

crypto_rand_secure($min, $max) works as a drop in replacement for rand() or mt_rand. It uses openssl_random_pseudo_bytes to help create a random number between $min and $max.

getToken($length) creates an alphabet to use within the token and then creates a string of length $length.

EDIT: I neglected to cite source - http://us1.php.net/manual/en/function.openssl-random-pseudo-bytes.php#104322

EDIT (PHP7): With the release of PHP7, the standard library now has two new functions that can replace/improve/simplify the crypto_rand_secure function above. random_bytes($length) and random_int($min, $max)

http://php.net/manual/en/function.random-bytes.php

http://php.net/manual/en/function.random-int.php

Example:

function getToken($length){
     $token = "";
     $codeAlphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
     $codeAlphabet.= "abcdefghijklmnopqrstuvwxyz";
     $codeAlphabet.= "0123456789";
     $max = strlen($codeAlphabet); // edited

    for ($i=0; $i < $length; $i++) {
        $token .= $codeAlphabet[random_int(0, $max-1)];
    }

    return $token;
}

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