Draft : January 11 , 2018 CHAPTER 1 . ONE - TIME PAD

Secure communication is the act of conveying information from a sender to a receiver, while simultaneously hiding it from everyone else. Secure communication is the oldest application of cryptography, and remains the centerpiece of cryptography to this day. After all, the word cryptography means “hidden writing” in Greek. So, secure communication is a natural place to start our study of cryptography. History provides us with roughly 2000 years of attempts to secure sensitive communications in the presence of eavesdroppers. Despite the many brilliant minds that lived during this period of time, almost no useful concepts remain relevant to modern cryptography. In fact, it was not clear how to even formally de ne the goal of secure communication until the 1940s. Today we use security de nitions for encryption that were developed only in 1982 and 1990. The only cryptographic method developed before 1900 that has stood the test of time is the one-time pad, which appears in some form in essentially every modern encryption scheme. In this chapter, we introduce the one-time pad and discuss its important characteristics. Along the way, we will start to get acclimated to cryptographic security de nitions.