(C# UWP/WinRT) 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.

Chilkat Universal Windows Platform (UWP) / WinRT Downloads Chilkat for the Universal Windows Platform (UWP)

// 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"
// }

Chilkat.JsonObject jsonToken = new Chilkat.JsonObject();
bool success = jsonToken.LoadFile("qa_data/tokens/google_sample_id_token.json");
if (success == false) {
    Debug.WriteLine("Failed to load the JSON file...");
    return;
}

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

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

Chilkat.JsonObject jsonHeader = new Chilkat.JsonObject();
jsonHeader.Load(jose);
jsonHeader.EmitCompact = false;
Debug.WriteLine(jsonHeader.Emit());

// The JOSE header looks like this:

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

string claims = jwt.GetPayload(idToken);

Chilkat.JsonObject jsonClaims = new Chilkat.JsonObject();
jsonClaims.Load(claims);
jsonClaims.EmitCompact = false;
Debug.WriteLine(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.

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

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

string 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.
Debug.WriteLine("token_hash_expected: " + token_hash_expected);
Debug.WriteLine("token_hash_computed: " + token_hash_computed);