B4X

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. B4X

Dim success As Boolean = 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"
'  }

Dim jsonToken As ChilkatJsonObject
jsonToken.Initialize
success = jsonToken.LoadFile("qa_data/tokens/google_sample_id_token.json")
If success = False Then
    Log("Failed to load the JSON file...")
    Return
End If


'  Use Chilkat's JWT API to examine the id_token..
Dim jwt As ChilkatJwt
jwt.Initialize
Dim idToken As String = jsonToken.StringOf("id_token")

'  Extract the JOSE header..
Dim jose As String = jwt.GetHeader(idToken)

Dim jsonHeader As ChilkatJsonObject
jsonHeader.Initialize
jsonHeader.Load(jose)
jsonHeader.EmitCompact = False
Log(jsonHeader.Emit)

'  The JOSE header looks like this:

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

Dim claims As String = jwt.GetPayload(idToken)

Dim jsonClaims As ChilkatJsonObject
jsonClaims.Initialize
jsonClaims.Load(claims)
jsonClaims.EmitCompact = False
Log(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.

Dim token_to_hash As String = jsonToken.StringOf("access_token")
Dim token_hash_expected As String = jsonClaims.StringOf("at_hash")

'  Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
Dim crypt As ChilkatCrypt2
crypt.Initialize("crypt")
Dim bdHash As ChilkatBinData
bdHash.Initialize

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")
Dim sz As Int = bdHash.NumBytes

Dim token_hash_computed As String = bdHash.GetEncodedChunk(0, sz / 2, "base64url")

'  If the hashes are identical, then the access_token as issued for the given id_token.
Log("token_hash_expected: " & token_hash_expected)
Log("token_hash_computed: " & token_hash_computed)