public function sign($plaintext, $fingerprint)
    {
        if ($fingerprint === null || !isset($this->keys[$fingerprint])) {
            throw new InvalidKeyPairIdException('Invalid keypair fingerprint given', 1299095799);
        }

        $signature = '';
        openssl_sign($plaintext, $signature, $this->keys[$fingerprint]['privateKey']->getKeyString());

        return $signature;
    }