Strong encryption was government property for most of the twentieth century. Steven Levy tells the story of the mathematicians, graduate students and activists who pulled it into public hands, and of the National Security Agency that spent two decades trying to keep the lid on.

The book moves through the turning points in order. Public key cryptography arrives first, the idea that two strangers can agree on a secret over a line anyone can listen to. RSA turns that idea into a working algorithm and then into a business. Digital signatures follow, and with them the question of what a signature even means when nothing is written by hand. Alongside the inventions sits the politics: the Data Encryption Standard, whose key length the agency helped set, and the Clipper chip proposal of the 1990s, which brought the argument between federal wiretap policy and the cypherpunks into open view.

Levy writes people, not proofs. Anyone after the mathematics will need a textbook instead. Anyone who wants to know who argued what, in which room, and what it cost them, is the reader he has in mind.

One caution on the title: this is cryptography, not cryptocurrency. Published in 2001, the book stops well before the surveillance fights of the following decade, which makes it the origin story rather than a report on the present.