Cred: Pluggable Authentication

Goals

Cred is a pluggable authentication system for servers. It allows any number of network protocols to connect and authenticate to a system, and communicate to those aspects of the system which are meaningful to the specific protocol. For example, Twisted's POP3 support passes a username and password set of credentials to get back a mailbox for the specified email account. IMAP does the same, but retrieves a slightly different view of the same mailbox, enabling those features specific to IMAP which are not available in other mail protocols.

Cred is designed to allow both the backend implementation of the business logic - called the avatar - and the authentication database - called the credential checker - to be decided during deployment. For example, the same POP3 server should be able to authenticate against the local UNIX password database or an LDAP server without having to know anything about how or where mail is stored.

To sketch out how this works - a Realm corresponds to an application domain and is in charge of avatars, which are network-accessible business logic objects. To connect this to an authentication database, a top-level object called a Portal stores a realm, and a number of credential checkers. Something that wishes to log in, such as a Protocol, stores a reference to the portal. Login consists of passing credentials and a request interface (e.g. POP3's IMailbox) to the portal. The portal passes the credentials to the appropriate credential checker, which returns an avatar ID. The ID is passed to the realm, which returns the appropriate avatar. For a Portal that has a realm that creates mailbox objects and a credential checker that checks /etc/passwd, login consists of passing in a username/password and the IMailbox interface to the portal. The portal passes this to the /etc/passwd credential checker, gets back a avatar ID corresponding to an email account, passes that to the realm and gets back a mailbox object for that email account.

Putting all this together, here's how a login request will typically be processed:

Cred objects

The Portal

This is the the core of login, the point of integration between all the objects in the cred system. There is one concrete implementation of Portal, and no interface - it does a very simple task. A Portal associates one (1) Realm with a collection of CredentialChecker instances. (More on those later.)

If you are writing a protocol that needs to authenticate against something, you will need a reference to a Portal, and to nothing else. This has only 2 methods -

• login(credentials, mind, *interfaces)

  The docstring is quite expansive (see twisted.cred.portal), but in brief, this is what you call when you need to call in order to connect a user to the system. Typically you only pass in one interface, and the mind is None. The interfaces are the possible interfaces the returned avatar is expected to implement, in order of preference. The result is a deferred which fires a tuple of:

  • interface the avatar implements (which was one of the interfaces passed in the *interfaces tuple)
  • an object that implements that interface (an avatar)
  • logout, a 0-argument callable which disconnects the connection that was established by this call to login

  The logout method has to be called when the avatar is logged out. For POP3 this means when the protocol is disconnected or logged out, etc..

• registerChecker(checker, *credentialInterfaces)

  which adds a CredentialChecker to the portal. The optional list of interfaces are interfaces of credentials that the checker is able to check.

The CredentialChecker

This is an object implementing ICredentialsChecker which resolves some Credentials to an avatar ID. Some examples of CredentialChecker implementations would be: InMemoryUsernamePassword, ApacheStyleHTAccessFile, UNIXPasswordDatabase, SSHPublicKeyDatabase. A credential checker stipulates some requirements of the credentials it can check by specifying a credentialInterfaces attribute, which is a list of interfaces. Credentials passed to its requestAvatarId method must implement one of those interfaces.

For the most part, these things will just check usernames and passwords and produce the username as the result, but hopefully we will be seeing some public-key, challenge-response, and certificate based credential checker mechanisms soon.

A credential checker should raise an error if it cannot authenticate the user, and return twisted.cred.checkers.ANONYMOUS for anonymous access.

The Credentials

Oddly enough, this represents some credentials that the user presents. Usually this will just be a small static blob of data, but in some cases it will actually be an object connected to a network protocol. For example, a username/password pair is static, but a challenge/response server is an active state-machine that will require several method calls in order to determine a result.

Twisted comes with a number of credentials interfaces and implementations in the twisted.cred.credentials module, such as IUsernamePassword and IUsernameHashedPassword.

The Realm

A realm is an interface which connects your universe of business objects to the authentication system.

IRealm is another one-method interface:

• requestAvatar(avatarId, mind, *interfaces)

  This method will typically be called from 'Portal.login'. The avatarId is the one returned by a CredentialChecker.

  Note: Note that avatarId must always be a string. In particular, do not use unicode strings. If internationalized support is needed, it is recommended to use UTF-8, and take care of decoding in the realm.

  The important thing to realize about this method is that if it is being called, the user has already authenticated. Therefore, if possible, the Realm should create a new user if one does not already exist whenever possible. Of course, sometimes this will be impossible without more information, and that is the case that the interfaces argument is for.

Since requestAvatar should be called from a Deferred callback, it may return a Deferred or a synchronous result.

The Avatar

An avatar is a business logic object for a specific user. For POP3, it's a mailbox, for a first-person-shooter it's the object that interacts with the game, the actor as it were. Avatars are specific to an application, and each avatar represents a single user.

The Mind

As mentioned before, the mind is usually None, so you can skip this bit if you want.

Masters of Perspective Broker already know this object as the ill-named client object. There is no mind class, or even interface, but it is an object which serves an important role - any notifications which are to be relayed to an authenticated client are passed through a 'mind'. In addition, it allows passing more information to the realm during login in addition to the avatar ID.

The name may seem rather unusual, but considering that a Mind is representative of the entity on the other end of a network connection that is both receiving updates and issuing commands, I believe it is appropriate.

Although many protocols will not use this, it serves an important role. It is provided as an argument both to the Portal and to the Realm, although a CredentialChecker should interact with a client program exclusively through a Credentials instance.

Unlike the original Perspective Broker client object, a Mind's implementation is most often dictated by the protocol that is connecting rather than the Realm. A Realm which requires a particular interface to issue notifications will need to wrap the Protocol's mind implementation with an adapter in order to get one that conforms to its expected interface - however, Perspective Broker will likely continue to use the model where the client object has a pre-specified remote interface.

(If you don't quite understand this, it's fine. It's hard to explain, and it's not used in simple usages of cred, so feel free to pass None until you find yourself requiring something like this.)

Responsibilities

Server protocol implementation

The protocol implementor should define the interface the avatar should implement, and design the protocol to have a portal attached. When a user logs in using the protocol, a credential object is created, passed to the portal, and an avatar with the appropriate interface is requested. When the user logs out or the protocol is disconnected, the avatar should be logged out.

The protocol designer should not hardcode how users are authenticated or the realm implemented. For example, a POP3 protocol implementation would require a portal whose realm returns avatars implementing IMailbox and whose credential checker accepts username/password credentials, but that is all. Here's a sketch of how the code might look - note that USER and PASS are the protocol commands used to login, and the DELE command can only be used after you are logged in:

from zope.interface import Interface

from twisted.protocols import basic
from twisted.python import log
from twisted.cred import credentials, error
from twisted.internet import defer

class IMailbox(Interface):
    """Interface specification for mailbox."""
    def deleteMessage(index): pass


class POP3(basic.LineReceiver):
    # ...
    def __init__(self, portal):
        self.portal = portal

    def do_DELE(self, i):
        # uses self.mbox, which is set after login
        i = int(i)-1
        self.mbox.deleteMessage(i)
        self.successResponse()

    def do_USER(self, user):
        self._userIs = user
        self.successResponse('USER accepted, send PASS')

    def do_PASS(self, password):
        if self._userIs is None:
            self.failResponse("USER required before PASS")
            return
        user = self._userIs
        self._userIs = None
        d = defer.maybeDeferred(self.authenticateUserPASS, user, password)
        d.addCallback(self._cbMailbox, user)

    def authenticateUserPASS(self, user, password):
        if self.portal is not None:
            return self.portal.login(
                cred.credentials.UsernamePassword(user, password),
                None,
                IMailbox
            )
        raise error.UnauthorizedLogin()

    def _cbMailbox(self, ial, user):
        interface, avatar, logout = ial

        if interface is not IMailbox:
            self.failResponse('Authentication failed')
            log.err("_cbMailbox() called with an interface other than IMailbox")
            return

        self.mbox = avatar
        self._onLogout = logout
        self.successResponse('Authentication succeeded')
        log.msg("Authenticated login for " + user)

Application implementation

The application developer can implement realms and credential checkers. For example, she might implement a realm that returns IMailbox implementing avatars, using MySQL for storage, or perhaps a credential checker that uses LDAP for authentication. In the following example, the Realm for a simple remote object service (using Twisted's Perspective Broker protocol) is implemented:

from twisted.spread import pb
from twisted.cred.portal import IRealm

class SimplePerspective(pb.Avatar):

    def perspective_echo(self, text):
        print 'echoing',text
        return text

    def logout(self):
        print self, "logged out"


class SimpleRealm:
    implements(IRealm)

    def requestAvatar(self, avatarId, mind, *interfaces):
        if pb.IPerspective in interfaces:
            avatar = SimplePerspective()
            return pb.IPerspective, avatar, avatar.logout 
        else:
            raise NotImplementedError("no interface")

Deployment

Deployment involves tying together a protocol, an appropriate realm and a credential checker. For example, a POP3 server can be constructed by attaching to it a portal that wraps the MySQL-based realm and an /etc/passwd credential checker, or perhaps the LDAP credential checker if that is more useful. The following example shows how the SimpleRealm in the previous example is deployed using an in-memory credential checker:

from twisted.spread import pb
from twisted.internet import reactor
from twisted.cred.portal import Portal
from twisted.cred.checkers import InMemoryUsernamePasswordDatabaseDontUse

portal = Portal(SimpleRealm())
checker = InMemoryUsernamePasswordDatabaseDontUse()
checker.addUser("guest", "password")
portal.registerChecker(checker)
reactor.listenTCP(9986, pb.PBServerFactory(portal))
reactor.run()