How do I implement authentication the restful way?

Question

I'm building a picture diary on web application google app engine using python. Users can sign up and post pictures to their diary.

Also, I'm trying to conform as much as I can to the REST architecture of doing things.

The authentication scheme is based like this for the web application:
1. Post username/password from the frontend
2. Backend sets up a cookie if authentication is successful
3. The rest of the AJAX calls made are authenticated using this cookie.

Is there any way to conform to REST without using cookies ?

Now, I'm also building an android application where users can sign in and post/view their picture diary. I need to expose the data from web application's datastore so I'll be building a webservice to fetch data from the datastore.

The authentication scheme for the android client:
OPTION a
1. Post username/password over https to the web service
2. Web service returns a unique authorization token (store the token in the username/pwd table on the datastore)
3. Request subsequent services by adding this token to the Request Header of the request
4. Server maps the token to the username/pwd table and returns data if token is found
5. Authorization token expires after a certain period of time

OPTION b
1. Set up a secret key on the client and server side
2. Use "username:hash of password and secret key" in the authorization header of every request
3. server generates the password by extracting the password from the hash value using the same hash algorithm ; if successful returns data
btw, I didn't wanna use basic authorization because of its security vulnerabilities.

Which is better ?

Are there other significantly better ways to accomplish what I'm trying to do ? Security is quite a concern for me btw.
I'd appreciate if anyone has any insight into this issue. thanks.

---

I've been doing some research myself as to what would be the best solution. I think the 2-legged oauth might work in my case as Leonm suggested.
In this case the server has to provide the client with a consumer key/secret which in my case is hardcoded in the app.

The steps now would be:
1. Generate a signature using the oauth_parameters(consumer_key, signature_method, timestamp), request url, request parameters, and the SECRET.
2. Include the signature, oauth parameters when making a request.
3. Server verifies the request by generating the signature again except in this case it uses the SECRET that corresponds to the key

I think this way I am pretty much confirming to the REST principles. The server is statless as I far I understand now.

What are the pros/cons on doing things this way?

Answer 1

If "security is a concern" then I would say that you'd be a lot better off using open standards and a library to achieve what you want. The main reason for this is that if you do it yourself, you're very likely to forget something; these standards have had a lot of eyes looking at them, looking for holes.

Your options include (in increasing level of complexity)

Basic authentication and HTTPS

Everything is encrypted, which makes it impossible to compress or look into, it increases the overhead somewhat, using more horsepower on the server, and more perhaps battery power on the client. Simple to implement, since it's well supported by libraries.

Digest authentication

Unencrypted messages pass the wire, but the authentication is securely managed in the Authorization headers. See the wikipedia entry for more information.

OAuth

See how Google is providing OAuth for installed applications. I believe it isn't what you're looking for, since you're not asking to share data between applications, just authenticating users.

Roll your own

If you want to roll your own, I suggest looking at e.g. how Google's (now deprecated ?) ClientLogin used to work.

1. Clients would GET a protected resource, and get a 401 with instructions to perform a GoogleLogin authentication, including a URI for where to perform the login itself
2. Clients (knowing how to do this) POST a request in a specific manner to that URI
3. The server responds with a specific response including a (long) token
4. The client can now perform GET requests to the protected resource with that token.

Statelessness

You cite REST, which dictates that requests should not specifically depend on prior interaction: "... each request from client to server must contain all of the information necessary to understand the request, and cannot take advantage of any stored context on the server." (fielding) This means that a server shouldn't store conversational context (like an authentication token) in a table.

One way of fixing this is by using any of the token based approaches (where the server tells the client about a token it should use for future requests) where the token is not a random number, but a message to the server itself. To protect yourself from client tampering, it can be signed, and if you're afraid of clients looking at it, you can encrypt it.

Edit: Although I'm not certain, it seems unlikely that Google has a table of all authentication tokens ever issued; The length of their tokens suggests that the token is some encrypted message proving that whoever holds this token actually provided real credentials in some realm at some time.

Answer 2

OAuth does exactly what you want to do in a standard way.