protected function addToken(EntityInterface $entity, AttributeInterface $attribute, array $options = array())
    {
        $min_length = $attribute->getOption('min_length', 1);
        $max_length = $attribute->getOption('max_length', 40);
        $size = $this->faker->numberBetween($min_length, $max_length);
        $token = bin2hex(mcrypt_create_iv(ceil($size/2), MCRYPT_DEV_URANDOM));
        $truncated_token = substr($token, 0, $size);
        $this->setValue($entity, $attribute, $truncated_token, $options);
    }