Suggestions for Laravel Encrypter

mockup.php

<?php
// Use a (maybe?) new "Security" namespace for RandomBytes
namespace Illuminate\Security;

class RandomBytes
{
    /**
     * Generate a random string of bytes suitable for cryptographic use.
     *
     * @param int $count
     * @return string
     * @throws \RuntimeException
     */
    public function getRandomBytes($count)
    {
        $count = (int) $count;

        if ($count < 1)
        {
            throw new \RuntimeException('$count must be greater than 0');
        }

        $bytes = '';
        $hasBytes = false;

        // Try mcrypt_create_iv at first
        if (function_exists('mcrypt_create_iv'))
        {
            $tmp = mcrypt_create_iv($count, MCRYPT_DEV_URANDOM);
            if ($tmp !== false)
            {
                $bytes = $tmp;
                $hasBytes = true;
            }
        }

        // Fall back trying to read straight from urandom
        if ($hasBytes === false && file_exists('/dev/urandom') && is_readable('/dev/urandom') && (false !== ($fh = fopen('/dev/urandom', 'rb'))))
        {
            if (function_exists('stream_set_read_buffer'))
            {
                stream_set_read_buffer($fh, 0);
            }

            $tmp = fread($fh, $count);
            fclose($fh);
            if ($tmp !== false)
            {
                $bytes = $tmp;
            }
        }

        if (strlen($bytes) === $count)
        {
            return $bytes;
        }
        else
        {
            throw new \RuntimeException('Could not generate random bytes');
        }
    }
}

// Inject a new RandomBytes instance to Encrypter and call
// getRandomBytes() when random bytes are needed
$iv = $this->RandomBytes->getRandomBytes($this->getIvSize());

// Randomize the MAC to make the timing side-channel unusable
protected function validMac(array $payload)
{
    // Fetch 16 random bytes to generate a random $key to
    // randomize the to-be-compared byte string
    $key = $this->RandomBytes->getRandomBytes(16);

    // It would be good to check the length of $payload['mac'],
    // but as long as we make sure $this->hash() is called with
    // valid parameters it will be OK
    
    // EDIT: maybe it is actually good to check the user supplied
    // MAC length (to mitigate maliciously drafted overlong inputs
    // being passed to the underlying SHA-256 function). But I'll leave
    // this under your consideration

    $userSuppliedMac = $payload['mac'];
    $userSuppliedMac = hash_hmac('sha256', $userSuppliedMac, $key, true);

    $calculatedMac = $this->hash($payload['iv'], $payload['value']);
    $calculatedMac = hash_hmac('sha256', $calculatedMac, $key, true);

    return ($userSuppliedMac === $calculatedMac);
}