REST and authentication variants


I am currently working on a REST library for .net, and I would like to hear some opinions about an open point I have: REST and authentication.

Here is an example of an RESTful interface used with the library:

public interface IUserInterface
  void Add(User user);

  int[] List();

  User Get(int id);

  void Delete(int id);

The server code then just implements the interface and the clients can obtain the same interface through a factory. Or if the client is not using the library a standard HTTP request also works.

I know that there are the major ways of either using HTTP Basic Auth or sending a token to requests requiring authenticated users.

The first method (HTTP Basic Auth), has the following issues (partly web browser specific):

  • The password is transmitted with every request – even with SSL this has some kind of "bad feeling".
  • Since the password is transmitted with a request header, it would be easy for an local attacker to look at the transmitted headers to gain the password.
  • The password is available in the browsers memory.
  • No standard way to expire user "sessions".
  • Login with a browser interrupts the look and feel of a page.

The issues for the second method are more focused on implementation and library use:

  • Each request URI which needs authentication must have a parameter for the token, which is just very repetitive.
  • There is a lot more code to write if each method implementation needs to check if a token is valid.
  • The interface will become less specific e.g. [RestGet("/get/{id}")] vs. [RestGet("/get/{id}/{token}")].
  • Where to put the token: at the end of the URI? after the root? somewhere else?

My idea was to pass the token as parameter to the URL like http:/server/user/get/1234?token=token_id.

Another possibility would be to send the parameter as an HTTP header, but this would complicate usage with plain HTTP clients I guess.

The token would get passed back to the client as a custom HTTP header ("X-Session-Id") on each request.

This then could be completely abstracted from the interface, and any implementation needing authentication could just ask which user the token (if given) belongs to.

Do you think this would violate REST too much or do you have any better ideas?

Best Solution

I tend to believe that authentication details belong in the header, not the URI. If you rely on a token being placed on the URI, then every URI in your application will need to be encoded to include the token. It would also negatively impact caching. Resources with a token that is constantly changing will no longer be able to be cached. Resource related information belongs in the URI, not application related data such as credentials.

It seems you must be targeting web browsers as a client? If so you could investigate using HTTP Digest access authentication or issuing clients their own SSL certificates to uniquely identify and authenticate them. Also, I don't think that session cookies are necessarily a bad thing. Especially when having to deal with a browser. As long as you isolate the cookie handling code and make the rest of the application not rely on it you would be fine. The key is only store the user's identity in the session, nothing else. Do not abuse server side session state.

If you are targeting clients other than the browser then there are a number of approaches you can take. I've had luck with using Amazon's S3 Authentication mechanism.

This is all very subjective of course. Purity and following REST to the letter can sometimes be impractical. As long as you minimize and isolate such behavior, the core of your application can still be RESTful. I highly recommend RESTful Web Services as a great source of REST information and approaches.