Cryptography is a word derived from the Greek that is approximately translated as hidden writing. There are various forms and styles of cryptography; however, since ancient times the aim of cryptography has been the same: secure the transmission of information from an emitter to a receiver. A classical form of cryptography involves the sharing of a code between the emitter and the receiver. The emitter writes a message, using the shared code, and sends it to the intended receiver who uses the code to decipher the message. The integrity of the message is secured if and only if the code remains in the knowledge of the emitter and the intended receiver only. If the code is acquired, or broken, by a third party, then the message is no longer secured. An example of a simple classical numerical code is illustrated in Figure 19.1 and used to write the number π to 10 decimal places. Classical code representation for the truncated value of π to 10 decimal places. The key is included as a third item. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781315216720/7ef0b284-a9fb-4a7a-a07e-e2440def2e56/content/fig19_1.tif"/>