RESTful web service - how to authenticate requests from other services?
Any solution to this problem boils down to a shared secret. I also don't like the hard-coded user-name and password option but it does have the benefit of being quite simple. The client certificate is also good but is it really much different? There's a cert on the server and one on the client. It's main advantage is that it's harder to brute force. Hopefully you've got other protections in place to protect against that though.
I don't think your point A for the client certificate solution is difficult to resolve. You just use a branch. if (client side certificat) { check it } else { http basic auth } I'm no java expert and I've never worked with it to do client side certificates. However a quick Google leads us to this tutorial which looks right up your alley.
Despite all of this "what's best" discussion, let me just point out that there is another philosophy that says, "less code, less cleverness is better." (I personally hold this philosophy). The client certificate solution sounds like a lot of code.
I know you expressed questions about OAuth, but the OAuth2 proposal does include a solution to your problem called "bearer tokens" which must be used in conjunction with SSL. I think, for the sake of simplicity, I'd choose either the hard-coded user/pass (one per app so that they can be revoked individually) or the very similar bearer tokens.
There are several different approaches you can take.
The RESTful purists will want you to use BASIC authentication, and send credentials on every request. Their rationale is that no one is storing any state.
The client service could store a cookie, which maintains a session ID. I don't personally find this as offensive as some of the purists I hear from - it can be expensive to authenticate over and over again. It sounds like you're not too fond of this idea, though.
From your description, it really sounds like you might be interested in OAuth2 My experience so far, from what I've seen, is that it's kind of confusing, and kind of bleeding edge. There are implementations out there, but they're few and far between. In Java, I understand that it has been integrated into Spring3's security modules. (Their tutorial is nicely written.) I've been waiting to see if there will be an extension in Restlet, but so far, although it's been proposed, and may be in the incubator, it's still not been fully incorporated.
After reading your question, I would say, generate special token to do request required. This token will live in specific time (lets say in one day).
Here is an example from to generate authentication token:
(day * 10) + (month * 100) + (year (last 2 digits) * 1000)
for example: 3 June 2011
(3 * 10) + (6 * 100) + (11 * 1000) =
30 + 600 + 11000 = 11630
then concatenate with user password, example "my4wesomeP4ssword!"
11630my4wesomeP4ssword!
Then do MD5 of that string:
05a9d022d621b64096160683f3afe804
When do you call a request, always use this token,
https://mywebservice.com/?token=05a9d022d621b64096160683f3afe804&op=getdata
This token is always unique everyday, so I guess this kind of protection is more than sufficient to always protect ur service.
Hope helps
:)