(Node.js) Validate the at_hash Claim of an ID Token

Demonstrates how to hash an access token to compare it with the at_hash claim of an ID token.

Install Chilkat for Node.js and Electron using npm at Chilkat npm packages for Node.js Chilkat npm packages for Electron on Windows, Linux, MacOSX, and ARM

var os = require('os');
if (os.platform() == 'win32') {  
    if (os.arch() == 'ia32') {
        var chilkat = require('@chilkat/ck-node21-win-ia32');
    } else {
        var chilkat = require('@chilkat/ck-node21-win64'); 
    }
} else if (os.platform() == 'linux') {
    if (os.arch() == 'arm') {
        var chilkat = require('@chilkat/ck-node21-arm');
    } else if (os.arch() == 'x86') {
        var chilkat = require('@chilkat/ck-node21-linux32');
    } else {
        var chilkat = require('@chilkat/ck-node21-linux64');
    }
} else if (os.platform() == 'darwin') {
    if (os.arch() == 'arm64') {
        var chilkat = require('@chilkat/ck-node21-mac-m1');
    } else {
        var chilkat = require('@chilkat/ck-node21-macosx');
    }
}

function chilkatExample() {

    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    // This example uses a Google access_token + id_token that looks like this:

    //  {
    //   "access_token": "ya29.a0...0f",
    //   "expires_in": 3599,
    //   "scope": "openid https://www.googleapis.com/auth/userinfo.email",
    //   "token_type": "Bearer",
    //   "id_token": "eyJhb...o5nQ"
    // }

    var jsonToken = new chilkat.JsonObject();
    var success = jsonToken.LoadFile("qa_data/tokens/google_sample_id_token.json");
    if (success == false) {
        console.log("Failed to load the JSON file...");
        return;
    }

    // Use Chilkat's JWT API to examine the id_token..
    var jwt = new chilkat.Jwt();
    var idToken = jsonToken.StringOf("id_token");

    // Extract the JOSE header..
    var jose = jwt.GetHeader(idToken);

    var jsonHeader = new chilkat.JsonObject();
    jsonHeader.Load(jose);
    jsonHeader.EmitCompact = false;
    console.log(jsonHeader.Emit());

    // The JOSE header looks like this:

    // {
    //   "alg": "RS256",
    //   "kid": "e8799db06287515556213c80acbcfd022fb302a9",
    //   "typ": "JWT"
    // }

    var claims = jwt.GetPayload(idToken);

    var jsonClaims = new chilkat.JsonObject();
    jsonClaims.Load(claims);
    jsonClaims.EmitCompact = false;
    console.log(jsonClaims.Emit());

    // The claims look like this:

    // {
    //   "iss": "https://accounts.google.com",
    //   "azp": "258999997753-5ni8lu5f15r7mno97d82f5lir9i9f6i1.apps.googleusercontent.com",
    //   "aud": "258999997753-5ni8lu5f15r7mno97d82f5lir9i9f6i1.apps.googleusercontent.com",
    //   "sub": "111787341816486547572",
    //   "email": "somebody@gmail.com",
    //   "email_verified": true,
    //   "at_hash": "HYJZImlW3mUK-UfjRfXjKw",
    //   "iat": 1615315968,
    //   "exp": 1615319568
    // }

    // The at_hash is the Access Token hash value. Its value is the base64url encoding of the
    // left-most half of the hash of the octets of the ASCII representation of the access_token value,
    // where the hash algorithm used is the hash algorithm used in the alg Header Parameter of the
    // ID Token's JOSE Header. For instance, if the alg is RS256, hash the access_token value with SHA-256,
    // then take the left-most 128 bits and base64url encode them. The at_hash value is a case sensitive string.

    var token_to_hash = jsonToken.StringOf("access_token");
    var token_hash_expected = jsonClaims.StringOf("at_hash");

    // Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
    var crypt = new chilkat.Crypt2();
    var bdHash = new chilkat.BinData();

    crypt.HashAlgorithm = "sha256";
    // This encoding mode must match the encoding mode passed in the 2nd arg to AppendEncoded.
    // The encoding mode can be anything, as long as they are the same in both places.
    crypt.EncodingMode = "hex";

    success = bdHash.AppendEncoded(crypt.HashStringENC(token_to_hash),"hex");
    var sz = bdHash.NumBytes;

    var token_hash_computed = bdHash.GetEncodedChunk(0,sz / 2,"base64url");

    // If the hashes are identical, then the access_token as issued for the given id_token.
    console.log("token_hash_expected: " + token_hash_expected);
    console.log("token_hash_computed: " + token_hash_computed);

}

chilkatExample();