Cryptocurrencies like bitcoin have few fans in Washington. At a July congressional hearing, Senator Elizabeth Warren warned that cryptocurrency "puts the [financial] system at the whims of some shadowy, faceless group of super-coders." Treasury secretary Janet Yellen likewise asserted that the "reality" of cryptocurrencies is that they "have been used to launder the profits of online drug traffickers; they've been a tool to finance terrorism." Thus far, Bitcoin's supporters remain undeterred. (The term "Bitcoin" with a capital "B" is used here and throughout to refer to the system of...