Question

I want to create a token generator that generates tokens that cannot be guessed by the user and that are still unique (to be used for password resets and confirmation codes).

I often see this code; does it make sense?

md5(uniqid(rand(), true));

According to a comment uniqid($prefix, $moreEntopy = true) yields

first 8 hex chars = Unixtime, last 5 hex chars = microseconds.

I don’t know how the $prefix-parameter is handled..

So if you don’t set the $moreEntopy flag to true, it gives a predictable outcome.

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QUESTION: But if we use uniqid with $moreEntopy, what does hashing it with md5 buy us? Is it better than:

md5(mt_rand())

edit1: I will store this token in an database column with a unique index, so I will detect columns. Might be of interest/

Answer 1

rand() is a security hazard and should never be used to generate a security token: rand() vs mt_rand() (Look at the “static” like images). But neither of these methods of generating random numbers is cryptographically secure. To generate secure secerts an application will needs to access a CSPRNG provided by the platform, operating system or hardware module.

In a web application a good source for secure secrets is non-blocking access to an entropy pool such as /dev/urandom. As of PHP 5.3, PHP applications can use openssl_random_pseudo_bytes(), and the Openssl library will choose the best entropy source based on your operating system, under Linux this means the application will use /dev/urandom. This code snip from Scott is pretty good:

function crypto_rand_secure($min, $max) {
        $range = $max - $min;
        if ($range < 0) return $min; // not so random...
        $log = log($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=32){
    $token = "";
    $codeAlphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    $codeAlphabet.= "abcdefghijklmnopqrstuvwxyz";
    $codeAlphabet.= "0123456789";
    for($i=0;$i<$length;$i++){
        $token .= $codeAlphabet[crypto_rand_secure(0,strlen($codeAlphabet))];
    }
    return $token;
}