Security of REST authentication schemes

Question

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 1

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.

Answer 2

REST means working with the standards of the web, and the standard for "secure" transfer on the web is SSL. Anything else is going to be kind of funky and require extra deployment effort for clients, which will have to have encryption libraries available.

Once you commit to SSL, there's really nothing fancy required for authentication in principle. You can again go with web standards and use HTTP Basic auth (username and secret token sent along with each request) as it's much simpler than an elaborate signing protocol, and still effective in the context of a secure connection. You just need to be sure the password never goes over plain text; so if the password is ever received over a plain text connection, you might even disable the password and mail the developer. You should also ensure the credentials aren't logged anywhere upon receipt, just as you wouldn't log a regular password.

HTTP Digest is a safer approach as it prevents the secret token being passed along; instead, it's a hash the server can verify on the other end. Though it may be overkill for less sensitive applications if you've taken the precautions mentioned above. After all, the user's password is already transmitted in plain-text when they log in (unless you're doing some fancy JavaScript encryption in the browser), and likewise their cookies on each request.

Note that with APIs, it's better for the client to be passing tokens - randomly generated strings - instead of the password the developer logs into the website with. So the developer should be able to log into your site and generate new tokens that can be used for API verification.

The main reason to use a token is that it can be replaced if it's compromised, whereas if the password is compromised, the owner could log into the developer's account and do anything they want with it. A further advantage of tokens is you can issue multiple tokens to the same developers. Perhaps because they have multiple apps or because they want tokens with different access levels.

(Updated to cover implications of making the connection SSL-only.)

Answer 3

Or you could use the known solution to this problem and use SSL. Self-signed certs are free and its a personal project right?

Answer 4

If you require the hash of the body as one of the parameters in the URL and that URL is signed via a private key, then a man-in-the-middle attack would only be able to replace the body with content that would generate the same hash. Easy to do with MD5 hash values now at least and when SHA-1 is broken, well, you get the picture.

To secure the body from tampering, you would need to require a signature of the body, which a man-in-the-middle attack would be less likely to be able to break since they wouldn't know the private key that generates the signature.

Answer 5

In fact, the original S3 auth does allow for the content to be signed, albeit with a weak MD5 signature. You can simply enforce their optional practice of including a Content-MD5 header in the HMAC (string to be signed).

http://s3.amazonaws.com/doc/s3-developer-guide/RESTAuthentication.html

Their new v4 authentication scheme is more secure.

http://docs.aws.amazon.com/general/latest/gr/signature-version-4.html

Answer 6

Remember that your suggestions makes it difficult for clients to communicate with the server. They need to understand your innovative solution and encrypt the data accordingly, this model is not so good for public API (unless you are amazon\yahoo\google..).

Anyways, if you must encrypt the body content I would suggest you to check out existing standards and solutions like:

XML encryption (W3C standard)

XML Security