private function _generateEncryptedContentKey($aesKey, $protectionKey)
    {
        $cert = openssl_x509_read($protectionKey);

        $cryptedContentKey = '';
        openssl_public_encrypt(
            $aesKey,
            $cryptedContentKey,
            $cert,
            OPENSSL_PKCS1_OAEP_PADDING
        );

        $this->_encryptedContentKey = base64_encode($cryptedContentKey);
    }