Secure REST API without user authentification (no credentials)

You might want to look at Passport. It is a platform that allows you to easily add authentication to your application. There are many authentication strategies available. I am using Passport in a Node.js application implementing my own hmac strategy.

To authenticate, the client request includes an API ID to identify who the caller is and also includes an signature of a specified part of the message that includes things like the HTTP method, the API ID, a date value and some other header values, like maybe content-type. What data to include in the string to sign is up to you in your implementation, but the client and server must create and sign the same strings for the authentication to work. The signature is created by doing an hmac hash of the string using a shared secret.

On the server side, you use the API ID to retrieve the shared secret (possibly from a database or the filesystem) and perform the same hash on the request. If the hmac values match then you've authenticated the request. To prevent playback attacks, the date is included in the signed part of the request and must be within a certain window of the server's current time. For example, you might reject the request if the timestamp is more than 30 seconds old.

To enable a new user of your API, you generate a new API ID and shared secret. You give both of those to your API user and you store them for look up in your database or filesystem. The user must sign the requests with the shared secret and include the ID in the request.

The Hawk strategy provides much of this functionality, but we decided to roll our own hmac strategy.

Because you say that the user info endpoint returns active directory name, I assume you're on the Windows platform.

If so, why not use Windows integrated authentication (Kerberos) to authenticate your users without asking them for credentials? This will only work within your active directory domain, but is completely transparent to your service.

You can still call the user info endpoint and verify that the info it returns is for the same user that is calling your REST service.

If you need to call services that do not support Windows integrated auth, you could generate a security token (sign it to guarantee integrity) and make the other services trust this token.