Twisted Documentation: Using SSL in Twisted

Overview

This document describes how to use SSL in Twisted servers and clients. It assumes that you know what SSL is, what some of the major reasons to use it are, and how to generate your own SSL certificates, in particular self-signed certificates. It also assumes that you are comfortable with creating TCP servers and clients as described in the server howto and client howto. After reading this document you should be able to create servers and clients that can use SSL to encrypt their connections, switch from using an unencrypted channel to an encrypted one mid-connection, and require client authentication.

Using SSL in Twisted requires that you have pyOpenSSL installed. A quick test to verify that you do is to run from OpenSSL import SSL at a python prompt and not get an error.

Twisted provides SSL support as a transport - that is, as an alternative to TCP. When using SSL, use of the TCP APIs you're already familiar with, TCP4ClientEndpoint and TCP4ServerEndpoint - or reactor.listenTCP and reactor.connectTCP - is replaced by use of parallel SSL APIs. To create an SSL server, use SSL4ServerEndpoint or listenSSL. To create an SSL client, use SSL4ClientEndpoint or connectSSL.

SSL connections require SSL contexts. As with protocols, these context objects are created by factories - so that each connection can be given a unique context, if necessary. The context factories typically also keep state which is necessary to properly configure an SSL context object for its desired use - for example, private key or certificate data. The context factory is passed as a mandatory argument to any and all of the SSL APIs mentioned in the previous paragraph. twisted.internet.ssl.CertificateOptions is one commonly useful context factory for both clients and servers. twisted.internet.ssl.PrivateCertificate.options is a convenient way to create a CertificateOptions instance configured to use a particular key and certificate.

Those are the big immediate differences between TCP and SSL connections, so let's look at an example. In it and all subsequent examples it is assumed that keys and certificates for the server, certificate authority, and client should they exist live in a keys/ subdirectory of the directory containing the example code, and that the certificates are self-signed.

SSL echo server and client without client authentication

Authentication and encryption are two separate parts of the SSL protocol. The server almost always needs a key and certificate to authenticate itself to the client but is usually configured to allow encrypted connections with unauthenticated clients who don't have certificates. This common case is demonstrated first by adding SSL support to the echo client and server in the core examples.

SSL echo server

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if __name__ == '__main__':
    import echoserv_ssl
    raise SystemExit(echoserv_ssl.main())

import sys

from twisted.internet import reactor
from twisted.internet.protocol import Factory
from twisted.internet import ssl, reactor
from twisted.python import log

import echoserv

def main():
    with open('server.pem') as keyAndCert:
        cert = ssl.PrivateCertificate.loadPEM(keyAndCert.read())

    log.startLogging(sys.stdout)
    factory = Factory()
    factory.protocol = echoserv.Echo
    reactor.listenSSL(8000, factory, cert.options())
    reactor.run()

SSL echo client

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if __name__ == '__main__':
    import echoclient_ssl
    raise SystemExit(echoclient_ssl.main())

import sys

from twisted.internet.protocol import ClientFactory
from twisted.protocols.basic import LineReceiver
from twisted.internet import ssl, reactor


class EchoClient(LineReceiver):
    end="Bye-bye!"
    def connectionMade(self):
        self.sendLine("Hello, world!")
        self.sendLine("What a fine day it is.")
        self.sendLine(self.end)

    def connectionLost(self, reason):
        print 'connection lost (protocol)'

    def lineReceived(self, line):
        print "receive:", line
        if line==self.end:
            self.transport.loseConnection()

class EchoClientFactory(ClientFactory):
    protocol = EchoClient

    def clientConnectionFailed(self, connector, reason):
        print 'connection failed:', reason.getErrorMessage()
        reactor.stop()

    def clientConnectionLost(self, connector, reason):
        print 'connection lost:', reason.getErrorMessage()
        reactor.stop()

def main():
    factory = EchoClientFactory()
    reactor.connectSSL('localhost', 8000, factory, ssl.CertificateOptions())
    reactor.run()

Notice how all of the protocol code from the TCP version of the echo client and server examples is the same (imported or repeated) in these SSL versions - only the reactor method used to initiate a network action is different.

One part of the SSL connection contexts control is which version of the SSL protocol will be used. This is often called the context's "method". By default, CertificateOptions creates contexts which will select the TLSv1 protocol. CertificateOptions also supports the older SSLv3 protocol (which may be required interoperate with an existing service or piece of software), just pass OpenSSL.SSL.SSLv3_METHOD to its initializer: CertificateOptions(..., method=SSLv3_METHOD). SSLv23_METHOD is also supported (to enable either SSLv3 or TLSv1 based on negotiation). SSLv2 is explicitly not supported.

Using startTLS

If you want to switch from unencrypted to encrypted traffic mid-connection, you'll need to turn on SSL with startTLS on both ends of the connection at the same time via some agreed-upon signal like the reception of a particular message. You can readily verify the switch to an encrypted channel by examining the packet payloads with a tool like Wireshark.

startTLS server

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from twisted.internet import reactor, ssl
from twisted.internet.protocol import ServerFactory
from twisted.protocols.basic import LineReceiver

class TLSServer(LineReceiver):
    def lineReceived(self, line):
        print "received: " + line

        if line == "STARTTLS":
            print "-- Switching to TLS"
            self.sendLine('READY')

            self.transport.startTLS(self.factory.contextFactory)


if __name__ == '__main__':
    with open("keys/server.key") as keyFile:
        with open("keys/server.crt") as certFile:
            cert = PrivateCertificate.loadPEM(
                keyFile.read() + certFile.read())

    factory = ServerFactory()
    factory.protocol = TLSServer
    factory.contextFactory = cert.options()
    reactor.listenTCP(8000, factory)
    reactor.run()

startTLS client

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from twisted.internet import reactor, ssl
from twisted.internet.protocol import ClientFactory
from twisted.protocols.basic import LineReceiver

class TLSClient(LineReceiver):
    pretext = [
        "first line",
        "last thing before TLS starts",
        "STARTTLS"]

    posttext = [
        "first thing after TLS started",
        "last thing ever"]

    def connectionMade(self):
        for l in self.pretext:
            self.sendLine(l)

    def lineReceived(self, line):
        print "received: " + line
        if line == "READY":
            self.transport.startTLS(ssl.CertificateOptions())
            for l in self.posttext:
                self.sendLine(l)
            self.transport.loseConnection()

class TLSClientFactory(ClientFactory):
    protocol = TLSClient

    def clientConnectionFailed(self, connector, reason):
        print "connection failed: ", reason.getErrorMessage()
        reactor.stop()

    def clientConnectionLost(self, connector, reason):
        print "connection lost: ", reason.getErrorMessage()
        reactor.stop()

if __name__ == "__main__":
    factory = TLSClientFactory()
    reactor.connectTCP('localhost', 8000, factory)
    reactor.run()

startTLS is a transport method that gets passed a context factory. It is invoked at an agreed-upon time in the data reception method of the client and server protocols. The server uses PrivateCertificate.options to create a context factory which will use a particular certificate and private key (a common requirement for SSL servers). The client creates an uncustomized CertificateOptions which is all that's necessary for an SSL client to interact with an SSL server, although it is missing some verification settings necessary to ensure correct authentication of the server and confidentiality of data exchanged.

Client authentication

Server and client-side changes to require client authentication fall largely under the dominion of pyOpenSSL, but few examples seem to exist on the web so for completeness a sample server and client are provided here.

Client-authenticating server

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from twisted.internet import ssl, reactor
from twisted.internet.protocol import Factory, Protocol

class Echo(Protocol):
    def dataReceived(self, data):
        self.transport.write(data)

if __name__ == '__main__':
    factory = Factory()
    factory.protocol = Echo

    with open("keys/ca.pem") as certAuthCertFile:
        certAuthCert = ssl.Certificate.loadPEM(certAuthCertFile.read())

    with open("keys/server.key") as keyFile:
        with open("keys/server.crt") as certFile:
            serverCert = ssl.PrivateCertificate.loadPEM(
                keyFile.read() + certFile.read())

    contextFactory = serverCert.options(certAuthCert)

    reactor.listenSSL(8000, factory, contextFactory)
    reactor.run()

When one or more certificates are passed to PrivateCertificate.options, the resulting context factory will use those certificates as trusted authorities and require that the peer present a certificate with a valid chain terminating in one of those authorities.

Client with certificates

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from twisted.internet import ssl, reactor
from twisted.internet.protocol import ClientFactory, Protocol

class EchoClient(Protocol):
    def connectionMade(self):
        print "hello, world"
        self.transport.write("hello, world!")

    def dataReceived(self, data):
        print "Server said:", data
        self.transport.loseConnection()


class EchoClientFactory(ClientFactory):
    protocol = EchoClient

    def clientConnectionFailed(self, connector, reason):
        print "Connection failed - goodbye!"
        reactor.stop()

    def clientConnectionLost(self, connector, reason):
        print "Connection lost - goodbye!"
        reactor.stop()


if __name__ == '__main__':
    with open("keys/server.key") as keyFile:
        with open("keys/server.crt") as certFile:
            clientCert = ssl.PrivateCertificate.loadPEM(
                keyFile.read() + certFile.read())

    ctx = clientCert.options()
    factory = EchoClientFactory()
    reactor.connectSSL('localhost', 8000, factory, ctx)
    reactor.run()

Notice this client code does not pass any certificate authority certificates to PrivateCertificate.options. This means that it will not validate the server's certificate, it will only present its certificate to the server for validation.

Other facilities

twisted.protocols.amp supports encrypted connections and exposes a startTLS method one can use or subclass. twisted.web has built-in SSL support in its client, http, and xmlrpc modules.

Conclusion

After reading through this tutorial, you should be able to:

Use listenSSL and connectSSL to create servers and clients that use SSL
Use startTLS to switch a channel from being unencrypted to using SSL mid-connection
Add server and client support for client authentication