public function getFingerprint($amount)
    {
        if (!filter_var($amount, FILTER_VALIDATE_FLOAT, FILTER_FLAG_ALLOW_THOUSAND)) {
            throw new AuthnetInvalidAmountException('You must enter a valid amount greater than zero.');
        }

        return strtoupper(hash_hmac('sha512', sprintf('%s^%s^%s^%s^',
            $this->login,
            $this->sequence,
            $this->timestamp,
            $amount
        ), hex2bin($this->signature)));
    }