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Code to talk to a U2F tag directly from Swift, without a browser
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What is this?

I was toying with some code that used a U2F key directly from an application, rather than having to work via a web browser. The code in this repository was rudely extracted from a project I was working on, but I think includes the parts you need to get going.

It isn't presently in the form of a supported library, or a Cocoa pod, or a Swift package, or anything like that; it isn't particularly extensively commented, or documented, and I haven't had time to write a nice sample application either. It supports U2F, not the newer protocol, and not the older one either. But you can probably see how to extend it if you wish.

What's the license?

It's MIT licensed. See LICENSE.

If you use it somewhere, it'd be nice to know that fact.

How do I use it?

You'll need to create a U2FManager object and you'll probably want to register for the U2FDeviceListDidChange notification, to detect when the user inserts (or removes) a U2F device.

To register a device, you call U2FManager's "register" method, passing in the challenge dictionary, which has two keys, "challenge" and "origin"; the former should be a Base64-encoded string containing the challenge data, and the latter should be your facet ID (for a Mac app, typically something like "").

The register method also accepts a block that gives you the device, challenge data (in JSON form), public key, key handle, attestation certificate and signature. So the register call looks something like this:

let challenge : [String:Any] = [
    "challenge": challengeData.base64EncodedString(),
    "origin": ""
u2fManager.register(challenge: challenge) {
  (device, challengeJSON, publicKey, keyHandle, attestationCertificate,
   signature) in

To authenticate using U2F, you use the "authenticate" method, which again takes a challenge, a list of key handles your system knows about and a block that is used to handle the response. So it looks like:

let challenge : [String:Any] = [
    "challenge": challengeData.base64EncodedString(),
    "origin": ""
u2fManager.authenticate(challenge: challenge,
	          keyHandles: keyHandles) {
  (status, challenge, responses) in
  if status == .failed {
    // Tell the user somehow

  if responses.count == 0 {
    // No responses; maybe retry? [A]

  // Otherwise, responses is a list of U2FAuthResponse structs containing
  // the relevant information

In practice you may want to write the callback as a separate function/method because that makes it easy to trigger a retry from [A], above, as you can just call u2fManager.authenticate again.

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