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(VB.NET) 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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' 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 New Chilkat.JsonObject
Dim success As Boolean = jsonToken.LoadFile("qa_data/tokens/google_sample_id_token.json")
If (success = False) Then
    Debug.WriteLine("Failed to load the JSON file...")
    Exit Sub
End If


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

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

Dim jsonHeader As New Chilkat.JsonObject
jsonHeader.Load(jose)
jsonHeader.EmitCompact = False
Debug.WriteLine(jsonHeader.Emit())

' The JOSE header looks like this:

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


Dim claims As String = jwt.GetPayload(idToken)

Dim jsonClaims As New Chilkat.JsonObject
jsonClaims.Load(claims)
jsonClaims.EmitCompact = False
Debug.WriteLine(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 New Chilkat.Crypt2
Dim bdHash As New Chilkat.BinData

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 Integer = 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.
Debug.WriteLine("token_hash_expected: " & token_hash_expected)
Debug.WriteLine("token_hash_computed: " & token_hash_computed)

 

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