md5(uniqid) makes sense for random unique tokens?

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

To answer your question, the problem is you can't have a generator that is guaranteed random and unique as random by itself, i.e., md5(mt_rand()) can lead to duplicates. What you want is "random appearing" unique values. uniqid gives the unique id, rand() affixes a random number making it even harder to guess, md5 masks the result to make it yet even harder to guess. Nothing is unguessable. We just need to make it so hard that they wouldn't even want to try.

Answer 2

I ran into an interesting idea a couple of years ago.
Storing two hash values in the datebase, one generated with md5($a) and the other with sha($a). Then chek if both the values are corect. Point is, if the attacker broke your md5(), he cannot break your md5 AND sha in the near future.
Problem is: how can that concept be used with the token generating needed for your problem?

Answer 3

Define "unique". If you mean that two tokens cannot have the same value, then hashing isn't enough - it should be backed with a uniqueness test. The fact that you supply the hash algorithm with unique inputs does not guarantee unique outputs.

Answer 4

This is a copy of another question I found that was asked a few months before this one. Here is a link to the question and my answer: https://stackoverflow.com/a/13733588/1698153.

I do not agree with the accepted answer. According to PHPs own website "[uniqid] does not generate cryptographically secure tokens, in fact without being passed any additional parameters the return value is little different from microtime(). If you need to generate cryptographically secure tokens use openssl_random_pseudo_bytes()."

I do not think the answer could be clearer than this, uniqid is not secure.

Answer 5

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;
}

Answer 6

MD5 is a decent algorithm for producing data dependent IDs. But in case you have more than one item which has the same bitstream (content), you will be producing two similar MD5 "ids".

So if you are just applying it to a rand() function, which is guaranteed not to create the same number twice, you are quite safe.

But for a stronger distribution of keys, I'd personally use SHA1 or SHAx etc'... but you will still have the problem of similar data leads to similar keys.