Using the given client and interactor (or interaction.Auto if nil), register the client account if it does not already exist.

Attempts to authorize a hostname using the given client.

Completes a given challenge, polling it until it is complete.

Sort authorization combinations by preference.

Any challenge having a preference at or below this value will never be used.

Returned if all combinations fail.

PreferFast prefers fast types.

Determines the degree to which a challenge is preferred.

TypePreferencer returns a preference according to the type of the challenge.