JWT Bearer Token From Azure AD Example

Here's how to integrate Azure AD authentication with a Node.js REST API, for example. Specifically, here are the details on verifying an Azure AD-generated JWT Bearer Token.

TL;DR

• git clone or download the project
• In index.js paste your Bearer token string (Base64, no "Bearer " prefix) into the token variable
• Paste your public key X.509 Certificate string (without PEM prefix/suffix) into the x5cString variable
• Run npm install and then node . from the command-line

If your token is printed out on the console then verification succeeded. Otherwise, an error message will be displayed.

Consider using additional verify options for improved security once you have the basic public key verification working.

Recommended Reading - Background

Let's say that you have an API endpoint using Node.js (Express, LoopBack, Feathers, etc.) and you want to accept JWT Bearer Tokens issued by Azure AD. This is a terrific, stateless way of doing single-sign-on (SSO) between say, Microsoft Office 365 or SharePoint and your own custom single-page application (SPA).

You'll find some good examples for using ADAL within the browser to get the Microsoft Azure AD-signed Bearer Token.

However, what is perhaps not so clear is how to verify the Bearer token on the Node.js side in your API code. This little example and README is a show-and-tell on the "trick" needed to perform the JWT Bearer token verification. Further details beyond the tidbits offered below are available here and here.

Start Down the Rabbit Hole

I'm going to say right off the bat that Microsoft seems to make this whole thing a bit more complicated than it has to be. I started by reading this article, which tells me to examine this online JSON document that contains a "jwks_uri" value pointing to this URI where the actual public keys are stored.

The public keys are provided in JSON format as well, and you must cross-reference into them to find the right one for your particular Office 365 tenancy presumably.

Let me explain what I mean by "cross-reference". Here's what the public keys JSON file looks like. Notice that there are 3 elements (objects) in the array of public key data (as of this writing).

{
  "keys": [
    {
      "kty":"RSA",
      "use":"sig",
      "kid":"z44wMdHu8wKsumrbfaK98qxs5YI",
      "x5t":"z44wMdHu8wKsumrbfaK98qxs5YI",
      "n":"p3HhMQsqmgSjeiDsZ1ay...",
      "e":"AQAB",
      "x5c": ["MIIDBTCCAe2gAwIBAgIQaD0..."]
    },
    {
      "kty":"RSA",
      "use":"sig",
      "kid":"SSQdhI1cKvhQEDSJxE2gGYs40Q0",
      "x5t":"SSQdhI1cKvhQEDSJxE2gGYs40Q0",
      "n":"pJUB90EMxiNjgkVz5CLL...",
      "e":"AQAB",
      "x5c":["MIIDBTCCAe2gAwIBAgIQHJ7yHxN..."]
    },
    {
      "kty":"RSA",
      "use":"sig",
      "kid":"2S4SCVGs8Sg9LS6AqLIq6DpW-g8",
      "x5t":"2S4SCVGs8Sg9LS6AqLIq6DpW-g8",
      "n":"oZ-QQrNuB4ei9ATYrT61ebPt...",
      "e":"AQAB",
      "x5c":["MIIDKDCCAhCgAwIBAgIQBH..."]
    }
  ]
}

The main index into the objects in the array is the "x5t" value (X.509 "Thumbprint", see this spec):

• z44wMdHu8wKsumrbfaK98qxs5YI,
• SSQdhI1cKvhQEDSJxE2gGYs40Q0, or
• 2S4SCVGs8Sg9LS6AqLIq6DpW-g8

Following so far? Great.

Cross-Reference Azure AD-Issued Token to get the Correct Public Key

To determine which public key your particular Bearer token can be verified with, examine the corresponding "x5t" value in the header section of your Bearer token.

Then, from the matching object in the keys array (shown above and as mentioned above, available here), take the "x5c" value to construct your actual public key for token verification purposes.

Construct the Public Key

Now that you have the correct "x5c" string you're almost ready to verify your Bearer token! Wow, right?

The "trick" in this final step is to note that the "x5c" string is, as the label implies, an X.509 Certificate.

Therefore, to successfully use this "x5c" string as a public key argument to a JWT token verification call, you need to perform a small string concatenation to put it into the expected PEM format (also see here).

var token = '...';
var x5cString = '...';
var publicKey = '-----BEGIN CERTIFICATE-----\n' + x5cString + '\n-----END CERTIFICATE-----';
...

// Verify
verifiedToken = jwt.verify(token, publicKey);