How to create a "challenge" for my Cloud Functions server - ios

Question

I'm trying use Apple's new DeviceCheck API to verify that network calls in my app are actually coming from an uncompromised version of my app.
Documentation
After successfully verifying a key’s attestation, your server can
require the app to assert its legitimacy for any or all future server
requests. The app does this by signing the request. In the app, first
obtain a unique, one-time challenge from the server. You use a
challenge here, like for attestation, to avoid replay attacks. Then
combine the challenge with the server request to create a hash:
let challenge = <# A string from your server #>
let request = [ "action": "getGameLevel",
"levelId": "1234",
"challenge": challenge ]
guard let clientData = try? JSONEncoder().encode(request) else { return }
let clientDataHash = Data(SHA256.hash(data: clientData))
Use this hash and the key identifier that you generated earlier to
create an assertion object by calling the
generateAssertion(_:clientDataHash:completionHandler:) method:
service.generateAssertion(keyId, clientDataHash: clientDataHash) { assertion, error in
guard error == nil else { /* Handle the error. */ }
// Send the assertion and request to your server.
}
I'm trying to add this assertion functionality to my Swift function, which is a helper function that calls a Firebase Cloud Function.
I want the assertion object to be passed as data to the Cloud Function, to verify that the Cloud Function is being called from an uncompromised version of my app:
func callFunction(name: String, data: [String:Any?], completion: #escaping (HTTPSCallableResult?, Error?)->()){
var functions = Functions.functions()
functions.httpsCallable(name).call(data){ (result, error) in
completion(result, error)
}
}
(Example of callFunction() being used below):
let data: [String:Any?] = [
"gameId": self.game?.id,
"answer": answer,
"answeredAt": Date().millisecondsSince1970
]
callFunction(name: "answerQuestion", data: data){ res, err in
print("Submitted answer: \(res.debugDescription) | Error: \(err)")
if let err = err {
self.game?.question?.state = .initial
}
}
To generate the assertion object to send to my server (cloud function), it requires me to generate a challenge as stated above. However I'm not sure how to generate this challenge.
Apple says it should be "A string from your server". But I'm not sure what the string should be. Is it meant to be a dynamic string based on the user's UID? A Base64-encoded string of the user ID and a static secret string? And when I try to retrieve this string from the server, the user will just be able to read it as they can see incoming network JSON (I presume I would retrieve the string with a Cloud Function call) - so it seems pointless as it's not a secret string anymore?
Any idea how I can make the challenge work securely?

Answer 1

The point of the challenge is to avoid replay attacks, so it can be any randomised string. A UUID would be fine. It doesn't need to be a secret.
The challenge string is combined with the transaction data and a hash is generated. You send the hash to and you send that to generateAssertion and receive the assertion object. You then send this to your server along with the request data.
Now your server can combine the received request data with the challenge (which it knows, since it sent it to the client initially), generate the same hash and validate the attestation.
The server-side attestation article provides detail on the challenge data:
Provide a Challenge
Every time your app needs to communicate attestation data to your server, the app first asks the server for a unique, one-time challenge. App Attest integrates this challenge into the objects that it provides, and that your app sends back to your server for validation. This makes it harder for an attacker to implement a replay attack.
When asked for a challenge, provide your app with a randomized data value, and remember the value for use when verifying the corresponding attestation or assertion objects sent by the client. How you use the challenge data depends on the kind of object that you need to validate.