AutoIt

Validate the at_hash Claim of an ID Token

See more JSON Web Token (JWT) Examples

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

Local $bSuccess = False

; 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"
; }

$oJsonToken = ObjCreate("Chilkat.JsonObject")
$bSuccess = $oJsonToken.LoadFile("qa_data/tokens/google_sample_id_token.json")
If ($bSuccess = False) Then
    ConsoleWrite("Failed to load the JSON file..." & @CRLF)
    Exit
EndIf

; Use Chilkat's JWT API to examine the id_token..
$oJwt = ObjCreate("Chilkat.Jwt")
Local $sIdToken = $oJsonToken.StringOf("id_token")

; Extract the JOSE header..
Local $sJose = $oJwt.GetHeader($sIdToken)

$oJsonHeader = ObjCreate("Chilkat.JsonObject")
$oJsonHeader.Load($sJose)
$oJsonHeader.EmitCompact = False
ConsoleWrite($oJsonHeader.Emit() & @CRLF)

; The JOSE header looks like this:

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

Local $sClaims = $oJwt.GetPayload($sIdToken)

$oJsonClaims = ObjCreate("Chilkat.JsonObject")
$oJsonClaims.Load($sClaims)
$oJsonClaims.EmitCompact = False
ConsoleWrite($oJsonClaims.Emit() & @CRLF)

; 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.

Local $sToken_to_hash = $oJsonToken.StringOf("access_token")
Local $sToken_hash_expected = $oJsonClaims.StringOf("at_hash")

; Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
$oCrypt = ObjCreate("Chilkat.Crypt2")
$oBdHash = ObjCreate("Chilkat.BinData")

$oCrypt.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.
$oCrypt.EncodingMode = "hex"

$bSuccess = $oBdHash.AppendEncoded($oCrypt.HashStringENC($sToken_to_hash),"hex")
Local $iSz = $oBdHash.NumBytes

Local $sToken_hash_computed = $oBdHash.GetEncodedChunk(0,$iSz / 2,"base64url")

; If the hashes are identical, then the access_token as issued for the given id_token.
ConsoleWrite("token_hash_expected: " & $sToken_hash_expected & @CRLF)
ConsoleWrite("token_hash_computed: " & $sToken_hash_computed & @CRLF)