'Generate validation for the challenge.
Subclasses must implement this method, but they are likely to
return completely different data structures, depending on what\'s
necessary to complete the challenge. Interpretation of that
return value must be known to the caller.
:param JWK account_key:
:returns: Challenge-specific validation.'

@abc.abstractmethod
def validation(self, account_key, **kwargs):

    raise NotImplementedError()
