  We explore the origins of blockchain technologies to better understand the
enduring needs they address. We identify the five key elements of a blockchain,
show embodiments of these elements, and examine how these elements come
together to yield important properties in selected systems. To facilitate
comparing the many variations of blockchains, we also describe the four crucial
roles of blockchain participants common to all blockchains. Our historical
exploration highlights the 1979 work of David Chaum whose vault system embodies
many of the elements of blockchains.
