DRKey: Check that current time is within Epoch (+ Grace period)

[JordiSubira]

This is not urgent because this is part of the FakeProvider implementation. However, it is likely that the FakeProvider will evolve to an actual provider.

The explanation below makes sense on the context of SCMP where the terminology is used, however, if we want this provider to be of general purpose (which is debatable) the reasoning (even if some parameters change, e.g., timestamp_size < 48 bits) still applies for the general case.

Currently, the GetKeyWithinAcceptanceWindow(·) method does not check whether that we are within the Epoch ( + grace period) of the selected. Since the AcceptanceWindow may be bigger than the grace period, we should check that the current time is also within the Epoch + grace period. Just refuting the current implementation by counter-example. Let's say EpochDuration := 1 day and AcceptanceWindow:= 1 minute and current time t="E_{i}+1minute". Timestamp can encode up to 3 days duration and the Grace_period is set to 5 seconds. If the sender sends a packet s.t. timestamp encodes "1 day and 30 second", the method will assume it was sent at t_s ="E_{i}+30 seconds" (i.e., 30 seconds ago) using K_{i-1} (the key for the current epoch). However, this key is not valid anymore at t_s and we should return an error.