Currency, the specific instruments of money used as a medium of exchange, has come in many forms—grain, beads, cowry shells, peppercorns, cigarettes, cans of mackerel and potatoes. While digital money comes in many forms—bank account records, foreign exchange, commercial currencies run by platform companies, cryptocurrencies—they share the common characteristic of compressing time-space through technologies. Blockchain-based cryptocurrencies are fundamentally predicated on a logic of algorithmic proofs, rather than centralized human governance. To achieve workable decentralized systems, cryptocurrencies rely on three key technologies: distributed databases, for recording interactions; networks of computer nodes; and encryption protocols. Cryptocurrencies have the standard characteristics expected from currency—readily divisible into smaller parts, easily transportable—and offer some new protections against fraud, counterfeiting, or seizure by a centralized authority. The design of Bitcoin and subsequent cryptocurrencies emerges from, and is embedded in, a particular segment of society working to mainstream some rather fringe ideas about money and currency.