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(Delphi ActiveX) 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 ActiveX DLL for Delphi

Chilkat non-ActiveX DLL for Delphi

* The examples here use the ActiveX DLL.

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Chilkat_v9_5_0_TLB;

...

procedure TForm1.Button1Click(Sender: TObject);
var
jsonToken: TChilkatJsonObject;
success: Integer;
jwt: TChilkatJwt;
idToken: WideString;
jose: WideString;
jsonHeader: TChilkatJsonObject;
claims: WideString;
jsonClaims: TChilkatJsonObject;
token_to_hash: WideString;
token_hash_expected: WideString;
crypt: TChilkatCrypt2;
bdHash: TChilkatBinData;
sz: Integer;
token_hash_computed: WideString;

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 := TChilkatJsonObject.Create(Self);
success := jsonToken.LoadFile('qa_data/tokens/google_sample_id_token.json');
if (success = 0) then
  begin
    Memo1.Lines.Add('Failed to load the JSON file...');
    Exit;
  end;

// Use Chilkat's JWT API to examine the id_token..
jwt := TChilkatJwt.Create(Self);
idToken := jsonToken.StringOf('id_token');

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

jsonHeader := TChilkatJsonObject.Create(Self);
jsonHeader.Load(jose);
jsonHeader.EmitCompact := 0;
Memo1.Lines.Add(jsonHeader.Emit());

// The JOSE header looks like this:

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

claims := jwt.GetPayload(idToken);

jsonClaims := TChilkatJsonObject.Create(Self);
jsonClaims.Load(claims);
jsonClaims.EmitCompact := 0;
Memo1.Lines.Add(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.

token_to_hash := jsonToken.StringOf('access_token');
token_hash_expected := jsonClaims.StringOf('at_hash');

// Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
crypt := TChilkatCrypt2.Create(Self);
bdHash := TChilkatBinData.Create(Self);

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');
sz := bdHash.NumBytes;

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.
Memo1.Lines.Add('token_hash_expected: ' + token_hash_expected);
Memo1.Lines.Add('token_hash_computed: ' + token_hash_computed);
end;

 

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