This document sets out use cases and requirements for a new type of identifier that has 4 essential characteristics: decentralized: there should be no central issuing agency; persistent: the identifier should be inherently persistent, not requiring the continued operation of an underling organization; cryptographically verifiable: it should be possible to prove control of the identifier cryptographically; resolvable: it should be possible to discover metadata about the identifier. Although existing identifiers may display some of these characteristics, none currently displays all four.