Security of REST authentication schemes
Background:
I'm designing the authentication scheme for a REST web service. This doesn't "really" need to be secure (it's more of a personal project) but I want to make it as secure as possible as an exercise/learning experience. I don't want to use SSL since I don't want the hassle and, mostly, the expense of setting it up.
These SO questions were especially useful to get me started:
- RESTful Authentication
- Best Practices for securing a REST API / web service
- Examples of the best SOAP/REST/RPC web APIs? And why do you like them? And what’s wrong with them?
I'm thinking of using a simplified version of Amazon S3's authentication (I like OAuth but it seems too complicated for my needs). I'm adding a randomly generated nonce, supplied by the server, to the request, to prevent replay attacks.
To get to the question:
Both S3 and OAuth rely on signing the request URL along with a few selected headers. Neither of them sign the request body for POST or PUT requests. Isn't this vulnerable to a man-in-the-middle attack, which keeps the url and headers and replaces the request body with any data the attacker wants?
It seems like I can guard against this by including a hash of the request body in the string that gets signed. Is this secure?
Answer
A previous answer only mentioned SSL in the context of data transfer and didn't actually cover authentication.
You're really asking about securely authenticating REST API clients. Unless you're using TLS client authentication, SSL alone is NOT a viable authentication mechanism for a REST API. SSL without client authc only authenticates the server, which is irrelevant for most REST APIs because you really want to authenticate the client.
If you don't use TLS client authentication, you'll need to use something like a digest-based authentication scheme (like Amazon Web Service's custom scheme) or OAuth 1.0a or even HTTP Basic authentication (but over SSL only).
These schemes authenticate that the request was sent by someone expected. TLS (SSL) (without client authentication) ensures that the data sent over the wire remains untampered. They are separate - but complementary - concerns.
For those interested, I've expanded on an SO question about HTTP Authentication Schemes and how they work.