How do you generate cryptographically secure rando

Question

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Secure random number generation in PHP

We need to generate a cryptographically random string to use as an authentication token, which will be tied to session data in the database. We are using PHP, which doesn't appear to have a suitable random number generator built-in. How can we generate a cryptographically secure random string of N length using php?

Also note, due to the nature of our application, shell_exec is off the table.

Answer 1

what about uniqid? docs have an example of how it can be used for cookies/sessions.

Answer 2

Depending on your platform, you may use /dev/urandom or CAPICOM. This is nicely summarized in this comment from Mark Seecof:

"If you need some pseudorandom bits for security or cryptographic purposes (e.g.g., random IV for block cipher, random salt for password hash) mt_rand() is a poor source. On most Unix/Linux and/or MS-Windows platforms you can get a better grade of pseudorandom bits from the OS or system library, like this:

<?php
// get 128 pseudorandom bits in a string of 16 bytes

$pr_bits = '';

// Unix/Linux platform?
$fp = @fopen('/dev/urandom','rb');
if ($fp !== FALSE) {
    $pr_bits .= @fread($fp,16);
    @fclose($fp);
}

// MS-Windows platform?
if (@class_exists('COM')) {
    // http://msdn.microsoft.com/en-us/library/aa388176(VS.85).aspx
    try {
        $CAPI_Util = new COM('CAPICOM.Utilities.1');
        $pr_bits .= $CAPI_Util->GetRandom(16,0);

        // if we ask for binary data PHP munges it, so we
        // request base64 return value.  We squeeze out the
        // redundancy and useless ==CRLF by hashing...
        if ($pr_bits) { $pr_bits = md5($pr_bits,TRUE); }
    } catch (Exception $ex) {
        // echo 'Exception: ' . $ex->getMessage();
    }
}

if (strlen($pr_bits) < 16) {
    // do something to warn system owner that
    // pseudorandom generator is missing
}
?>

NB: it is generally safe to leave both the attempt to read /dev/urandom and the attempt to access CAPICOM in your code, though each will fail silently on the other's platform. Leave them both there so your code will be more portable."

Answer 3

Off the top of my head: take micro time, multiply it by microtime % 100 and do few randoms on sha1 of received result.