“The encryption genie is out of the bottle.”—Jan Koum, WhatsApp founder


Before we start writing cipher programs, let’s look at the process of encrypting and decrypting with just pencil and paper. This will help you understand how ciphers work and the math that goes into producing their secret messages. In this chapter, you’ll learn what we mean by cryptog­raphy and how codes are different from ciphers. Then you’ll use a simple cipher called the Caesar cipher to encrypt and decrypt messages using paper and pencil.

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