Software is eating the world. Technology mediates the conversations and commerce that define modern living. The importance and liability of software has led to the rise of a new kind of software application. Rather than making any one machine or person a central point of failure, emerging systems operate through the cooperation of all computers interacting with the software. Cryptography is necessary to make sure that everyone plays by the rules. From BitTorrent to Bitcoin, this book is a whirlwind tour of this new domain of software. By the end of reading this, you'll be able to dream up your own!