Implementing a 2 Legged OAuth Provider

Question

I'm trying to find my way around the OAuth spec, its requirements and any implementations I can find and, so far, it really seems like more trouble than its worth because I'm having trouble finding a single resource that pulls it all together. Or maybe it's just that I'm looking for something more specialized than most tutorials.

I have a set of existing APIs--some in Java, some in PHP--that I now need to secure and, for a number of reasons, OAuth seems like the right way to go. Unfortunately, my inability to track down the right resources to help me get a provider up and running is challenging that theory. Since most of this will be system-to-system API usage, I'll need to implement a 2-legged provider. With that in mind...

1. Does anyone know of any good tutorials for implementing a 2-legged OAuth provider with PHP?
2. Given that I have securable APIs in 2 languages, do I need to implement a provider in both or is there a way to create the provider as a "front controller" that I can funnel all requests through?
3. When securing PHP services, for example, do I have to secure each API individually by including the requisite provider resources on each?

Thanks for your help.

Answer 1

Rob, not sure where you landed on this but wanted to add my 2 cents in case anyone else ran across this question.

I more or less had the same question a few months ago and hearing about "OAuth" for the better part of a year. I was developing a REST API I needed to secure so I started reading about OAuth... and then my eyes started to roll backwards in my head.

I probably gave it a good solid day or 2 of skimming and reading until I decided, much like you, that OAuth was confusing garbage and just gave up on it.

So then I started researching ways to secure APIs in general and started to get a better grasp on ways to do that. The most popular way seemed to be sending requests to the API along with a checksum of the entire message (encoded with a secret that only you and the server know) that the server can use to decide if the message had been tampered with on it's way from the client, like so:

1. Client sends /user.json/123?showFriends=true&showStats=true&checksum=kjDSiuas98SD987ad
2. Server gets all that, looks up user "123" in database, loads his secret key and then (using the same method the client used) re-calculates it's OWN checksum given the request arguments.
3. If the server's generated checksum and the client's sent checksum match up, the request is OK and executed, if not, it is considered tampered with and rejected.

The checksum is called an HMAC and if you want a good example of this, it is what Amazon Web Services uses (they call the argument 'signature' not 'checksum' though).

So given that one of the key components of this to work is that the client and server have to generate the HMAC in the same fashion (otherwise they won't match), there have to be rules on HOW to combine all the arguments... then I suddenly understood all that "natural byte-ordering of parameters" crap from OAuth... it was just defining the rules for how to generate the signature because it needed to.

Another point is that every param you include in the HMAC generation is a value that then can't be tampered with when you send the request.

So if you just encode the URI stem as the signature, for example:

• /user.json == askJdla9/kjdas+Askj2l8add

then the only thing in your message that cannot be tampered with is the URI, all of the arguments can be tampered with because they aren't part of the "checksum" value that the server will re-calculate.

Alternatively, even if you include EVERY param in the calculation, you still run the risk of "replay attacks" where a malicious middle man or evesdropped can intercept an API call and just keep resending it to the server over and over again.

You can fix that by adding a timestamp (always use UTC) in the HMAC calculation as well.

REMINDER: Since the server needs to calculate the same HMAC, you have to send along any value you use in the calculation EXCEPT YOUR SECRET KEY (OAuth calls it a consumer_secret I think). So if you add timestamp, make sure you send a timestamp param along with your request.

If you want to make the API secure from replay attacks, you can use a nonce value (it's a 1-time use value the server generates, gives to the client, the client uses it in the HMAC, sends back the request, the server confirms and then marks that nonce value as "used" in the DB and never lets another request use it again).

NOTE: 'nonce' are a really exact way to solve the "replay attack" problem -- timestamps are great, but because computers don't always have in-sync timestamp values, you have to allow an acceptable window on the server side of how "old" a request might be (say 10 mins, 30 mins, 1hr.... Amazon uses 15mins) before we accept or reject it. In this scenario your API is technically vulnerable during the entire window of time.

I think nonce values are great, but should only need to be used in APIs that are critical they keep their integrity. In my API, I didn't need it, but it would be trivial to add later if users demanded it... I would literally just need to add a "nonce" table in my DB, expose a new API to clients like:

• /nonce.json

and then when they send that back to me in the HMAC calculation, I would need to check the DB to make sure it had never been used before and once used, mark it as such in the DB so if a request EVER came in again with that same nonce I would reject it.

Summary

Anyway, to make a long story short, everything I just described is basically what is known as "2-legged OAuth". There isn't that added step of flowing to the authority (Twitter, Facebook, Google, whatever) to authorize the client, that step is removed and instead the server implicitly trusts the client IF the HMAC's they are sending match up. That means the client has the right secret_key and is signing it's messages with it, so the server trusts it.

If you start looking around online, this seems to be the preferred method for securing API methods now-adays, or something like it. Amazon almost exactly uses this method except they use a slightly different combination method for their parameters before signing the whole thing to generate the HMAC.

If you are interested I wrote up this entire journey and thought-process as I was learning it. That might help provide a guided thinking tour of this process.