(Java) 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.

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import com.chilkatsoft.*;

public class ChilkatExample {

  static {
    try {
        System.loadLibrary("chilkat");
    } catch (UnsatisfiedLinkError e) {
      System.err.println("Native code library failed to load.\n" + e);
      System.exit(1);
    }
  }

  public static void main(String argv[])
  {
    // 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"
    // }

    CkJsonObject jsonToken = new CkJsonObject();
    boolean success = jsonToken.LoadFile("qa_data/tokens/google_sample_id_token.json");
    if (success == false) {
        System.out.println("Failed to load the JSON file...");
        return;
        }

    // Use Chilkat's JWT API to examine the id_token..
    CkJwt jwt = new CkJwt();
    String idToken = jsonToken.stringOf("id_token");

    // Extract the JOSE header..
    String jose = jwt.getHeader(idToken);

    CkJsonObject jsonHeader = new CkJsonObject();
    jsonHeader.Load(jose);
    jsonHeader.put_EmitCompact(false);
    System.out.println(jsonHeader.emit());

    // The JOSE header looks like this:

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

    String claims = jwt.getPayload(idToken);

    CkJsonObject jsonClaims = new CkJsonObject();
    jsonClaims.Load(claims);
    jsonClaims.put_EmitCompact(false);
    System.out.println(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`).
    CkCrypt2 crypt = new CkCrypt2();
    CkBinData bdHash = new CkBinData();

    crypt.put_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.put_EncodingMode("hex");

    success = bdHash.AppendEncoded(crypt.hashStringENC(token_to_hash),"hex");
    int sz = bdHash.get_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.
    System.out.println("token_hash_expected: " + token_hash_expected);
    System.out.println("token_hash_computed: " + token_hash_computed);
  }
}