How to verify an Azure OAuth Access JWT Token in native Javascript without using a library?

Question

I have an OAuth Access Token (its from Azure). I would like to verify the signature.

I've decoded the token

Header

{
  "typ": "JWT",
  "alg": "RS256",
  "x5t": "abc123",
  "kid": "abc123"
}

And the payload

{
  "aud": "api://xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
  "iss": "https://sts.windows.net/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/",
  "iat": 1580132587,
  "nbf": 1580132587,
  "exp": 1580136487,
  "acct": 0,
  "acr": "1",
  "aio": "xxxxxxxxxxxxxxxxxxxxxxxx/xxxxxxxxxxxxxxxxxxxxxxxxx==",
  "amr": [
    "pwd"
  ],
  "appid": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxxf",
  "appidacr": "0",
  "ipaddr": "yyy.yyy.yyy.yyy",
  "name": "test",
  "oid": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
  "scp": "user_impersonation",
  "sub": "xxxxxxxxxxxxxxxxxxxxxxxxxxx",
  "tid": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
  "unique_name": "test@mycompany.onmicrosoft.com",
  "upn": "test@mycompany.onmicrosoft.com",
  "uti": "xxxxxxxxxxxxx-xxxxxxxxx",
  "ver": "1.0"
}

(After verifying the intended audience (aud)) I need to verify the signature. To do that I first need to calculate the signature. To do that

1. I need to call the Azure metadata document to confirm the public key in the header.

https://login.microsoftonline.com/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/v2.0/.well-known/openid-configuration

2. From that I get the public keys (JWKS URI), eg

https://login.microsoftonline.com/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/discovery/v2.0/keys

3. This gives me an array

   [{ "kty": "RSA", "use": "sig", "kid": "abc123", "x5t": "abc123", "n": "...", "e": "...", "x5c": [..."], "issuer": " https: //login.microsoftonline.com/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/v2.0" }, { "kty": "RSA", "use": "sig", "kid": "...", "x5t": "...", "n": "....", "e": "...", "x5c": ["."], "issuer": "https://login.microsoftonline.com/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/v2.0" }, ]

4. I check that kid and x5t match the fields in the header.

Questions

1. How to calculate the actual signature? In my case the signing algorithm is RS256, so I got to do something like this

RSASHA256( base64UrlEncode(header) + "." + base64UrlEncode(payload), public_key)

2. What is the public_key?
3. And in native Javascript is there a function RSASHA256 ?

Answer 1

Get the x5t that matches the kid of the received token. That is the public key, though it typically needs translating to PEM format.

Most people then use a certified library to validate the signature with the public key. Here is some code of mine that does that to validate an Azure token.

By native JavaScript I assume you mean NodeJS, since access tokens are validated by APIs and not UIs.

Ultimately I believe libraries like jsonwebtoken call underlying operating system crypto code - not for the faint hearted ..