best practice to generate random token for forgot password

Question

I want to generate identifier for forgot password . I read i can do it by using timestamp with mt_rand(), but some people are saying that time stamp might not be unique every time. So i am bit of confused here. Can i do it with using time stamp with this ?

Question
What's best practice to generate random/unique tokens of custom length?

I know there are lot of questions asked around here but i am getting more confused after reading different opinion from the different people.

Answer 1

In PHP, use random_bytes(). Reason: your are seeking the way to get a password reminder token, and, if it is a one-time login credentials, then you actually have a data to protect (which is - whole user account)

So, the code will be as follows:

//$length = 78 etc $token = bin2hex(random_bytes($length)); 

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Update: previous versions of this answer was referring to uniqid() and that is incorrect if there is a matter of security and not only uniqueness. uniqid() is essentially just microtime() with some encoding. There are simple ways to get accurate predictions of the microtime() on your server. An attacker can issue a password reset request and then try through a couple of likely tokens. This is also possible if more_entropy is used, as the additional entropy is similarly weak. Thanks to @NikiC and @ScottArciszewski for pointing this out.

For more details see

• http://phpsecurity.readthedocs.org/en/latest/Insufficient-Entropy-For-Random-Values.html

Answer 2

This answers the 'best random' request:

Adi's answer¹ from Security.StackExchange has a solution for this:

Make sure you have OpenSSL support, and you'll never go wrong with this one-liner

$token = bin2hex(openssl_random_pseudo_bytes(16)); 

_{1. Adi, Mon Nov 12 2018, Celeritas, "Generating an unguessable token for confirmation e-mails", Sep 20 '13 at 7:06, https://security.stackexchange.com/a/40314/}