Privateering, piracy, and state authority

Is privateering illegal under international law, and what separates a privateer from a pirate?

A privateer operates under state authorization and flag, unlike a pirate; agreements restricting privateering matter only insofar as states enforce them.

A privateer differs from a pirate through state authorization, service in a declared war, and the flag under which the vessel operates. That status is compared with other state-employed mercenary service instead of independent predation. International agreements constrain behavior only when states choose to honor and enforce them. The Armada example, where privateering-built galleons entered Royal Navy service, shows how porous the boundary could be between private enterprise and state naval power.