[Pass] Custom Authenticator for WebAuthn

[pandres95]

Motivation

Based on #5, the Authenticator trait was released on fc-traits-authn. In order to provide integrations with usable authentication mechanisms for pallet-pass, it's suggested to implement an authenticator that validates

Design

Define a structure that implements the required methods by the Authenticator trait.

pub trait Authenticator {
    fn get_device_id(&self, device: Vec<u8>) -> Option<DeviceId>;
    fn authenticate(
        &self,
        device: Vec<u8>,
        challenge: &[u8],
        payload: &[u8],
    ) -> Result<(), AuthenticateError>;
}

Implementing get_device_id

For get_device_id, we'll need to get an [u8; 32] that represents the PublicKeyCredentials object, encoded in the device parameter. The method returns Some if the device is able to be authenticated by this authenticator (i.e. this device is an actual encoded representation of some PublicKeyCredentials).

Implementing authenticate

For authenticate, the method returns a Result with a unit (()) success whenever the payload sent by the device resolves the challenge given by pallet-pass, using a device provided by the user trying to authenticate, or an error when the challenge can't correctly be resolved by the given device.

For example, Alice sends an authentication request with a given device: the public key ABC, and a payload that is the signature of the challenge, using the private key that derives this public key. The result for this method should be a success.

On the other hand, Bob sends an authentication request, with a given device: the public key DEF, but the payload is a signature of the challenge using a different private key from the one that produced DEF. In this case, the signature is not correctly assesed, and the challenge fails, therefore the method returns an error.

Finally, Charlie sends an authentication request, with a given device: the sub id 123456. This is not a PublicKeyCredentials representation, but something else. The method should return an error.