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# HTTP Authentication

Many of the previous examples have looked at how to serve content by using existing resource classes or implementing new ones. In this example we’ll use Twisted Web’s basic or digest HTTP authentication to control access to these resources.

guard , the Twisted Web module which provides most of the APIs that will be used in this example, helps you to add authentication and authorization to a resource hierarchy. It does this by providing a resource which implements getChild to return a dynamically selected resource . The selection is based on the authentication headers in the request. If those headers indicate that the request is made on behalf of Alice, then Alice’s resource will be returned. If they indicate that it was made on behalf of Bob, his will be returned. If the headers contain invalid credentials, an error resource is returned. Whatever happens, once this resource is returned, URL traversal continues as normal from that resource.

The resource that implements this is HTTPAuthSessionWrapper , though it is directly responsible for very little of the process. It will extract headers from the request and hand them off to a credentials factory to parse them according to the appropriate standards (eg HTTPAuthentication: Basic and Digest Access Authentication ) and then hand the resulting credentials object off to a Portal , the core of Twisted Cred , a system for uniform handling of authentication and authorization. We won’t discuss Twisted Cred in much depth here. To make use of it with Twisted Web, the only thing you really need to know is how to implement an IRealm .

You need to implement a realm because the realm is the object that actually decides which resources are used for which users. This can be as complex or as simple as it suitable for your application. For this example we’ll keep it very simple: each user will have a resource which is a static file listing of the `public_html` directory in their UNIX home directory. First, we need to import `implements` from `zope.interface` and `IRealm` from `twisted.cred.portal` . Together these will let me mark this class as a realm (this is mostly - but not entirely - a documentation thing). We’ll also need File for the actual implementation later.

    from zope.interface import implements

    from twisted.cred.portal import IRealm
    from twisted.web.static import File

    class PublicHTMLRealm(object):
        implements(IRealm)

A realm only needs to implement one method: requestAvatar . This method is called after any successful authentication attempt (ie, Alice supplied the right password). Its job is to return the avatar for the user who succeeded in authenticating. An avatar is just an object that represents a user. In this case, it will be a `File` . In general, with `Guard` , the avatar must be a resource of some sort.

    ...
        def requestAvatar(self, avatarId, mind, *interfaces):
            if IResource in interfaces:
                return (IResource, File("/home/%s/public_html" % (avatarId,)), lambda: None)
            raise NotImplementedError()

A few notes on this method:

- The `avatarId` parameter is essentially the username. It’s the job of some other code to extract the username from the request headers and make sure it gets passed here.
- The `mind` is always `None` when writing a realm to be used with `Guard` . You can ignore it until you want to write a realm for something else.
- `Guard` is always passed `IResource` as the `interfaces` parameter. If `interfaces` only contains interfaces your code doesn’t understand, raising `NotImplementedError` is the thing to do, as above. You’ll only need to worry about getting a different interface when you write a realm for something other than `Guard` .
- If you want to track when a user logs out, that’s what the last element of the returned tuple is for. It will be called when this avatar logs out. `lambda: None` is the idiomatic no-op logout function.
- Notice that the path handling code in this example is written very poorly. This example may be vulnerable to certain unintentional information disclosure attacks. This sort of problem is exactly the reason FilePath exists. However, that’s an example for another day...

We’re almost ready to set up the resource for this example. To create an `HTTPAuthSessionWrapper` , though, we need two things. First, a portal, which requires the realm above, plus at least one credentials checker:

    from twisted.cred.portal import Portal
    from twisted.cred.checkers import FilePasswordDB

    portal = Portal(PublicHTMLRealm(), [FilePasswordDB('httpd.password')])

FilePasswordDB is the credentials checker. It knows how to read `passwd(5)` -style (loosely) files to check credentials against. It is responsible for the authentication work after `HTTPAuthSessionWrapper` extracts the credentials from the request.

Next we need either BasicCredentialFactory or DigestCredentialFactory . The former knows how to challenge HTTP clients to do basic authentication; the latter, digest authentication. We’ll use digest here:

    from twisted.web.guard import DigestCredentialFactory

    credentialFactory = DigestCredentialFactory("md5", "example.org")

The two parameters to this constructor are the hash algorithm and the HTTP authentication realm which will be used. The only other valid hash algorithm is “sha” (but be careful, MD5 is more widely supported than SHA). The HTTP authentication realm is mostly just a string that is presented to the user to let them know why they’re authenticating (you can read more about this in the RFC ).

With those things created, we can finally instantiate `HTTPAuthSessionWrapper` :

    from twisted.web.guard import HTTPAuthSessionWrapper

    resource = HTTPAuthSessionWrapper(portal, [credentialFactory])

There’s just one last thing that needs to be done here. When rpy scripts were introduced, it was mentioned that they are evaluated in an unusual context. This is the first example that actually needs to take this into account. It so happens that `DigestCredentialFactory` instances are stateful. Authentication will only succeed if the same instance is used to both generate challenges and examine the responses to those challenges. However, the normal mode of operation for an rpy script is for it to be re-executed for every request. This leads to a new `DigestCredentialFactory` being created for every request, preventing any authentication attempt from ever succeeding.

There are two ways to deal with this. First, and the better of the two ways, we could move almost all of the code into a real Python module, including the code that instantiates the `DigestCredentialFactory` . This would ensure that the same instance was used for every request. Second, and the easier of the two ways, we could add a call to `cache()` to the beginning of the rpy script:

    cache()

`cache` is part of the globals of any rpy script, so you don’t need to import it (it’s okay to be cringing at this point). Calling `cache` makes Twisted re-use the result of the first evaluation of the rpy script for subsequent requests too - just what we want in this case.

Here’s the complete example (with imports re-arranged to the more conventional style):

    cache()

    from zope.interface import implements

    from twisted.cred.portal import IRealm, Portal
    from twisted.cred.checkers import FilePasswordDB
    from twisted.web.static import File
    from twisted.web.resource import IResource
    from twisted.web.guard import HTTPAuthSessionWrapper, DigestCredentialFactory

    class PublicHTMLRealm(object):
        implements(IRealm)

        def requestAvatar(self, avatarId, mind, *interfaces):
            if IResource in interfaces:
                return (IResource, File("/home/%s/public_html" % (avatarId,)), lambda: None)
            raise NotImplementedError()

    portal = Portal(PublicHTMLRealm(), [FilePasswordDB('httpd.password')])

    credentialFactory = DigestCredentialFactory("md5", "localhost:8080")
    resource = HTTPAuthSessionWrapper(portal, [credentialFactory])

And voila, a password-protected per-user Twisted Web server.