(PowerBuilder) 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 ActiveX Downloads ActiveX for 32-bit and 64-bit Windows

integer li_rc
oleobject loo_JsonToken
integer li_Success
oleobject loo_Jwt
string ls_IdToken
string ls_Jose
oleobject loo_JsonHeader
string ls_Claims
oleobject loo_JsonClaims
string ls_Token_to_hash
string ls_Token_hash_expected
oleobject loo_Crypt
oleobject loo_BdHash
integer li_Sz
string ls_Token_hash_computed

// 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"
// }

loo_JsonToken = create oleobject
li_rc = loo_JsonToken.ConnectToNewObject("Chilkat_9_5_0.JsonObject")
if li_rc < 0 then
    destroy loo_JsonToken
    MessageBox("Error","Connecting to COM object failed")
    return
end if
li_Success = loo_JsonToken.LoadFile("qa_data/tokens/google_sample_id_token.json")
if li_Success = 0 then
    Write-Debug "Failed to load the JSON file..."
    destroy loo_JsonToken
    return
end if

// Use Chilkat's JWT API to examine the id_token..
loo_Jwt = create oleobject
li_rc = loo_Jwt.ConnectToNewObject("Chilkat_9_5_0.Jwt")

ls_IdToken = loo_JsonToken.StringOf("id_token")

// Extract the JOSE header..
ls_Jose = loo_Jwt.GetHeader(ls_IdToken)

loo_JsonHeader = create oleobject
li_rc = loo_JsonHeader.ConnectToNewObject("Chilkat_9_5_0.JsonObject")

loo_JsonHeader.Load(ls_Jose)
loo_JsonHeader.EmitCompact = 0
Write-Debug loo_JsonHeader.Emit()

// The JOSE header looks like this:

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

ls_Claims = loo_Jwt.GetPayload(ls_IdToken)

loo_JsonClaims = create oleobject
li_rc = loo_JsonClaims.ConnectToNewObject("Chilkat_9_5_0.JsonObject")

loo_JsonClaims.Load(ls_Claims)
loo_JsonClaims.EmitCompact = 0
Write-Debug loo_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.

ls_Token_to_hash = loo_JsonToken.StringOf("access_token")
ls_Token_hash_expected = loo_JsonClaims.StringOf("at_hash")

// Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
loo_Crypt = create oleobject
li_rc = loo_Crypt.ConnectToNewObject("Chilkat_9_5_0.Crypt2")

loo_BdHash = create oleobject
li_rc = loo_BdHash.ConnectToNewObject("Chilkat_9_5_0.BinData")

loo_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.
loo_Crypt.EncodingMode = "hex"

li_Success = loo_BdHash.AppendEncoded(loo_Crypt.HashStringENC(ls_Token_to_hash),"hex")
li_Sz = loo_BdHash.NumBytes

ls_Token_hash_computed = loo_BdHash.GetEncodedChunk(0,li_Sz / 2,"base64url")

// If the hashes are identical, then the access_token as issued for the given id_token.
Write-Debug "token_hash_expected: " + ls_Token_hash_expected
Write-Debug "token_hash_computed: " + ls_Token_hash_computed


destroy loo_JsonToken
destroy loo_Jwt
destroy loo_JsonHeader
destroy loo_JsonClaims
destroy loo_Crypt
destroy loo_BdHash