def _toString_OPENSSH(self, extra):
        """
        Return a public or private OpenSSH string.  See
        _fromString_PUBLIC_OPENSSH and _fromString_PRIVATE_OPENSSH for the
        string formats.  If extra is present, it represents a comment for a
        public key, or a passphrase for a private key.

        @param extra: Comment for a public key or passphrase for a
            private key
        @type extra: L{bytes}

        @rtype: L{bytes}
        """
        data = self.data()
        if self.isPublic():
            if self.type() == 'EC':
                if not extra:
                    extra = b''
                return (self._keyObject.public_bytes(
                    serialization.Encoding.OpenSSH,
                    serialization.PublicFormat.OpenSSH
                    ) + b' ' + extra).strip()

            b64Data = encodebytes(self.blob()).replace(b'\n', b'')
            if not extra:
                extra = b''
            return (self.sshType() + b' ' + b64Data + b' ' + extra).strip()
        else:

            if self.type() == 'EC':
                # EC keys has complex ASN.1 structure hence we do this this way.
                if not extra:
                    # unencrypted private key
                    encryptor = serialization.NoEncryption()
                else:
                    encryptor = serialization.BestAvailableEncryption(extra)

                return self._keyObject.private_bytes(
                    serialization.Encoding.PEM,
                    serialization.PrivateFormat.TraditionalOpenSSL,
                    encryptor)

            lines = [b''.join((b'-----BEGIN ', self.type().encode('ascii'),
                               b' PRIVATE KEY-----'))]
            if self.type() == 'RSA':
                p, q = data['p'], data['q']
                objData = (0, data['n'], data['e'], data['d'], p, q,
                           data['d'] % (p - 1), data['d'] % (q - 1),
                           data['u'])
            else:
                objData = (0, data['p'], data['q'], data['g'], data['y'],
                           data['x'])
            asn1Sequence = univ.Sequence()
            for index, value in izip(itertools.count(), objData):
                asn1Sequence.setComponentByPosition(index, univ.Integer(value))
            asn1Data = berEncoder.encode(asn1Sequence)
            if extra:
                iv = randbytes.secureRandom(8)
                hexiv = ''.join(['%02X' % (ord(x),) for x in iterbytes(iv)])
                hexiv = hexiv.encode('ascii')
                lines.append(b'Proc-Type: 4,ENCRYPTED')
                lines.append(b'DEK-Info: DES-EDE3-CBC,' + hexiv + b'\n')
                ba = md5(extra + iv).digest()
                bb = md5(ba + extra + iv).digest()
                encKey = (ba + bb)[:24]
                padLen = 8 - (len(asn1Data) % 8)
                asn1Data += (chr(padLen) * padLen).encode('ascii')

                encryptor = Cipher(
                    algorithms.TripleDES(encKey),
                    modes.CBC(iv),
                    backend=default_backend()
                ).encryptor()

                asn1Data = encryptor.update(asn1Data) + encryptor.finalize()

            b64Data = encodebytes(asn1Data).replace(b'\n', b'')
            lines += [b64Data[i:i + 64] for i in range(0, len(b64Data), 64)]
            lines.append(b''.join((b'-----END ', self.type().encode('ascii'),
                                   b' PRIVATE KEY-----')))
            return b'\n'.join(lines)

        Return a public or private OpenSSH string.  See
        _fromString_PUBLIC_OPENSSH and _fromString_PRIVATE_OPENSSH for the
        string formats.  If extra is present, it represents a comment for a
        public key, or a passphrase for a private key.

        @param extra: Comment for a public key or passphrase for a
            private key
        @type extra: L{bytes}

        @rtype: L{bytes}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/ssh/keys.py