Historical perspective – classical cryptography

Cryptography is the term for any method or technique used to secure information or communication, and specifically for the study of methods and protocols for secure communication. In the past, cryptography was used in reference to encryption, a term that refers to techniques used to encode information.

At its most basic, encryption might take the form of a substitution cipher, in which the letters or words in a message are substituted for others, based on a code shared in advance between the parties. The classic example is that of the Caesar Cipher, in which individual letters are indexed to their place in the alphabet and shifted forward a given number of characters. For example, the letter A might become the letter N, with a key of 13.

This specific form of the Caesar Cipher is known as ROT13, and it’s likely the only substitution cipher that continues to see any regular use—it provides a user with a trivially reversible way of hiding expletives or the solutions to puzzles on static websites (the same, of course, could be implemented very simply in JavaScript).

This very simple example introduces two important concepts. The first is an algorithm, which is a formal description of a specific computation with predictable, deterministic results. Take each character in the message and shift it forward by n positions in the alphabet. The second is a key: the n in that algorithm is 13. The key in this instance is a pre-shared secret, a code that the two (or more) parties have agreed to, but, as we’ll see, that is not the only kind of key.