B4X
Verify a Google JWT Using Google's Public Key

Demonstrates how to verify a JWT that was signed using Google's RSA private key.
This example verifies the RSA signature. It also does the following:
Checks to see if the time constraints ("nbf" and "exp") are valid.
Recovers the original JOSE header.
Recovers the original claims JSON.
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B4X
Dim success As Boolean = False

'  This example assumes the Chilkat API to have been previously unlocked.
'  See Global Unlock Sample for sample code.

Dim http As ChilkatHttp
http.Initialize("http")
Dim sbPubKeys As ChilkatStringBuilder
sbPubKeys.Initialize

success = http.QuickGetSb("https://www.googleapis.com/oauth2/v3/certs", sbPubKeys)
If success = False Then
    Log(http.LastErrorText)
    Return
End If


Log(sbPubKeys.GetAsString)

'  Here are the keys:

'  {
'    "keys": [
'      {
'        "e": "AQAB",
'        "n": "4bAT6C6EeX8Dspje3FrAXw-nnhNk04e1RmNa4kjc0CHf6Pk7ryARlwA-6YilyPABqQfYHx60s8oSnxvUVprFfQ2-Q8aAZO7bPKSxnoGlcKERL2oLNA4Msvc89N9Y5ycThZUplf_QC19e6jyYXN6Nz-UnJSCLrtQY8tVhhVRs61j4A2N_p-enAi-r704Qi1-v-DKV4eVRkClKViploo8NyjUaT9L4vbBssPCjyimJzsWnEe1fED5c4LnHeArYzA_FEn3JJotqDIz9t2VnvZNTMhizHEX4VnORlEWMEfR8n4CEHQx7PcQUOmfqyw08gWeXQl1-uTjtIGaE-sRIv9u_vQ",
'        "kty": "RSA",
'        "use": "sig",
'        "alg": "RS256",
'        "kid": "2af90e87be140c20038898a6efa11283dab6031d"
'      },
'      {
'        "n": "nzGsrziOYrMVYMpvUZOwkKNiPWcOPTYRYlDSdRW4UpAHdWPbPlyqaaphYhoMB5DXrVxI3bdvm7DOlo-sHNnulmAFQa-7TsQMxrZCvVdAbyXGID9DZYEqf8mkCV1Ohv7WY5lDUqlybIk1OSHdK7-1et0QS8nn-5LojGg8FK4ssLf3mV1APpujl27D1bDhyRb1MGumXYElwlUms7F9p9OcSp5pTevXCLmXs9MJJk4o9E1zzPpQ9Ko0lH9l_UqFpA7vwQhnw0nbh73rXOX2TUDCUqL4ThKU5Z9Pd-eZCEOatKe0mJTpQ00XGACBME_6ojCdfNIJr84Y_IpGKvkAEksn9w",
'        "use": "sig",
'        "kid": "87bbe0815b064e6d449cac999f0e50e72a3e4374",
'        "e": "AQAB",
'        "alg": "RS256",
'        "kty": "RSA"
'      }
'    ]
'  }

'  -------------------------------------------------------------------------------------------
'  Replace this with your actual token.
'  This sample token contains a kid that does not match any of the above Google public keys.
'  -------------------------------------------------------------------------------------------
Dim token As String = "eyJhbGciOiJSUzI1NiIsImtpZCI6IjQyZmY5MGQ3ZDM0OGM5NzM4MWE3YzExOWVmMWY1MzI0ZWEzZjViZWIifQ.eyJpc3MiOiJodHRwczovL2FjY291bnRzLmdvb2dsZS5jb20iLCJzdWIiOiIxMTExMTExMTExMTExMTExMTExMTExMTExMTExMTExMTExMTExMTExMTExIiwiaWF0IjoxNjExMTE1MTQzLCJleHAiOjE2MTEyMDE1NDMsImF1ZCI6IjQyOTc1NzExNTE0ODg0OSJ9.pLem5i0bx3M7lJYj7jKv2Nq7c07X5YpZz-x1uM5RniW-v4LsX-lKIVvOq2x3-WoPqkzLXJfP0kG0dx1uD2q1NfFQK60YwKH4FnFtB6INnUP1dRVpP9_pTTKyAE28I3s5Tay4PbPdrCl7ZLCIJzCfpCW1TiWeVoPjp5HgZKTBHdP_sEkN_yO5dQerQXAkFJkV3kNgF9jI3ayT-KPqOIH6GVoWXjHFDyA2EYgJPEFRo5WSe6XycJ85p5duwT-OoBcb_kJZG9PxYd91eHlPCzp8vGxzIb2dVROCBxyM8e8W0cd9v15hfmpg9R-eG9vCM5y63ZLChZLFeHFx0Pd7hvAqfKg"

Dim jwt As ChilkatJwt
jwt.Initialize
Dim header As String = jwt.GetHeader(token)

Log(header)

'  Sample header:
'  {"alg":"RS256","kid":"87bbe0815b064e6d449cac999f0e50e72a3e4374"}

'  Load the public key matching the "kid" into a Chilkat public key object, then verify..

Dim json As ChilkatJsonObject
json.Initialize
json.Load(header)
Dim kid As String = json.StringOf("kid")

Log("kid = " & kid)

Dim jsonPubKeys As ChilkatJsonObject
jsonPubKeys.Initialize
jsonPubKeys.LoadSb(sbPubKeys)

Dim jsonKey As ChilkatJsonObject = jsonPubKeys.FindRecord("keys", "kid", kid, True)
If jsonPubKeys.LastMethodSuccess = False Then
    Log("Did not find a matching public key based on the kid.")
    Return
End If


Log(jsonKey.Emit)

'  Load the matching public key into a Chilkat public key object.
Dim pubKey As ChilkatPublicKey
pubKey.Initialize
success = pubKey.LoadFromString(jsonKey.Emit)

If success = False Then
    Log(pubKey.LastErrorText)
    Return
End If


'  ----------------------------------------------------------------------------------------
'  Now we can validate the JWT using Google's public key as shown in this example:
'  (Except we use the public key obtained as shown above instead of a public key loaded from a PEM file.
'  
'  See Verify JWT Using an RSA Public Key