I of course have no idea what the real answer is, but if you play out the hypothetical this is hardly a paradox.
NK hacks Sony.
US uses technique x to attribute the attack to NK.
In the course of a shared investigation NK learns what technique x is.
NK changes their future operations to counter technique x.
The US loses the ability to use technique x to attribute attacks.
NK hacks Sony. US uses technique x to attribute the attack to NK. In the course of a shared investigation NK learns what technique x is. NK changes their future operations to counter technique x. The US loses the ability to use technique x to attribute attacks.