Cryptographically secure unique id

Question

I want to generate cryptographically secure unique uuids using php.

uniqid() provides unique but not secure ids and openssl_random_pseudo_bytes() provides secure but not unique ids. Is the combination of the two(following code) a proper approach or is there a better solution?

uniqid(bin2hex(openssl_random_pseudo_bytes(10)), true);

Answer 1

I want to generate cryptographically secure unique uuids using php.

Okay, that's easily done.

uniqid() provides unique but not secure ids and openssl_random_pseudo_bytes() provides secure but not unique ids.

What makes you think a cryptographically secure pseudorandom number isn't unique?

/**
 * Return a UUID (version 4) using random bytes
 * Note that version 4 follows the format:
 *     xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx
 * where y is one of: [8, 9, A, B]
 * 
 * We use (random_bytes(1) & 0x0F) | 0x40 to force
 * the first character of hex value to always be 4
 * in the appropriate position.
 * 
 * For 4: http://3v4l.org/q2JN9
 * For Y: http://3v4l.org/EsGSU
 * For the whole shebang: https://3v4l.org/LNgJb
 * 
 * @ref https://stackoverflow.com/a/31460273/2224584
 * @ref https://paragonie.com/b/JvICXzh_jhLyt4y3
 * 
 * @return string
 */
function uuidv4()
{
    return implode('-', [
        bin2hex(random_bytes(4)),
        bin2hex(random_bytes(2)),
        bin2hex(chr((ord(random_bytes(1)) & 0x0F) | 0x40)) . bin2hex(random_bytes(1)),
        bin2hex(chr((ord(random_bytes(1)) & 0x3F) | 0x80)) . bin2hex(random_bytes(1)),
        bin2hex(random_bytes(6))
    ]);
}

The above example conforms to the UUIDv4 specification and uses PHP7's random_bytes() function.

For PHP 5 projects, you can use random_compat to polyfill random_bytes() from PHP 7.