(Swift 3,4,5...) 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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func chilkatTest() {
    // 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"
    // }

    let jsonToken = CkoJsonObject()!
    var success: Bool = jsonToken.loadFile("qa_data/tokens/google_sample_id_token.json")
    if success == false {
        print("Failed to load the JSON file...")
        return
    }

    // Use Chilkat's JWT API to examine the id_token..
    let jwt = CkoJwt()!
    var idToken: String? = jsonToken.string(of: "id_token")

    // Extract the JOSE header..
    var jose: String? = jwt.getHeader(idToken)

    let jsonHeader = CkoJsonObject()!
    jsonHeader.load(jose)
    jsonHeader.emitCompact = false
    print("\(jsonHeader.emit()!)")

    // The JOSE header looks like this:

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

    var claims: String? = jwt.getPayload(idToken)

    let jsonClaims = CkoJsonObject()!
    jsonClaims.load(claims)
    jsonClaims.emitCompact = false
    print("\(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.

    var token_to_hash: String? = jsonToken.string(of: "access_token")
    var token_hash_expected: String? = jsonClaims.string(of: "at_hash")

    // Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
    let crypt = CkoCrypt2()!
    let bdHash = CkoBinData()!

    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), encoding: "hex")
    var sz: Int = bdHash.numBytes.intValue

    var token_hash_computed: String? = bdHash.getEncodedChunk(0, numBytes: sz / 2, encoding: "base64url")

    // If the hashes are identical, then the access_token as issued for the given id_token.
    print("token_hash_expected: \(token_hash_expected!)")
    print("token_hash_computed: \(token_hash_computed!)")

}