(Delphi DLL) 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 for Delphi Downloads Chilkat non-ActiveX DLL for Delphi Chilkat ActiveX DLL for Delphi * The examples here use the non-ActiveX DLL.

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, BinData, Jwt, JsonObject, Crypt2;

...

procedure TForm1.Button1Click(Sender: TObject);
var
jsonToken: HCkJsonObject;
success: Boolean;
jwt: HCkJwt;
idToken: PWideChar;
jose: PWideChar;
jsonHeader: HCkJsonObject;
claims: PWideChar;
jsonClaims: HCkJsonObject;
token_to_hash: PWideChar;
token_hash_expected: PWideChar;
crypt: HCkCrypt2;
bdHash: HCkBinData;
sz: Integer;
token_hash_computed: PWideChar;

begin
// 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"
// }

jsonToken := CkJsonObject_Create();
success := CkJsonObject_LoadFile(jsonToken,'qa_data/tokens/google_sample_id_token.json');
if (success = False) then
  begin
    Memo1.Lines.Add('Failed to load the JSON file...');
    Exit;
  end;

// Use Chilkat's JWT API to examine the id_token..
jwt := CkJwt_Create();
idToken := CkJsonObject__stringOf(jsonToken,'id_token');

// Extract the JOSE header..
jose := CkJwt__getHeader(jwt,idToken);

jsonHeader := CkJsonObject_Create();
CkJsonObject_Load(jsonHeader,jose);
CkJsonObject_putEmitCompact(jsonHeader,False);
Memo1.Lines.Add(CkJsonObject__emit(jsonHeader));

// The JOSE header looks like this:

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

claims := CkJwt__getPayload(jwt,idToken);

jsonClaims := CkJsonObject_Create();
CkJsonObject_Load(jsonClaims,claims);
CkJsonObject_putEmitCompact(jsonClaims,False);
Memo1.Lines.Add(CkJsonObject__emit(jsonClaims));

// 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.

token_to_hash := CkJsonObject__stringOf(jsonToken,'access_token');
token_hash_expected := CkJsonObject__stringOf(jsonClaims,'at_hash');

// Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
crypt := CkCrypt2_Create();
bdHash := CkBinData_Create();

CkCrypt2_putHashAlgorithm(crypt,'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.
CkCrypt2_putEncodingMode(crypt,'hex');

success := CkBinData_AppendEncoded(bdHash,CkCrypt2__hashStringENC(crypt,token_to_hash),'hex');
sz := CkBinData_getNumBytes(bdHash);

token_hash_computed := CkBinData__getEncodedChunk(bdHash,0,sz / 2,'base64url');

// If the hashes are identical, then the access_token as issued for the given id_token.
Memo1.Lines.Add('token_hash_expected: ' + token_hash_expected);
Memo1.Lines.Add('token_hash_computed: ' + token_hash_computed);

CkJsonObject_Dispose(jsonToken);
CkJwt_Dispose(jwt);
CkJsonObject_Dispose(jsonHeader);
CkJsonObject_Dispose(jsonClaims);
CkCrypt2_Dispose(crypt);
CkBinData_Dispose(bdHash);

end;