1  Introduction to codes and ciphers

1.1 Codes and Ciphers

We may often talk of “code-breaking”, or the “Enigma code”, but in fact there is a subtle distinction between the meanings of code and cipher.

A code is a mapping from some meaningful concept (a word, or a sentence), to an arbitrary symbol (perhaps a letter or a number). For example, we might have a code that assigns the sentence “It’s very cold today” to the number “67”. There’s no particular logic behind that, we just decided it, and wrote down this mapping in our code book so that it can be decoded later on.

Today though, we will be looking at ciphers. While a code operates on meanings, a cipher operates on symbols (such as individual letters). It transforms the “plaintext” symbols to their “ciphertext” counterparts using an algorithm. This algorithm will usually be a mathematical operation involving the original message and some sort of key. If someone knows (or is able to deduce) the algorithm and the key, they will be able to decipher an encrypted message.

1.2 Some basics

Suppose Alice wanted to send a message to her friend Bob, using a simple “mono-alphabetic cipher” where we replace each letter in our message with a different letter (we’ll look in more detail at this type of cipher a bit later). The message might look like:

Tiuug Cgc,

Tgz oji lgd?
Uiq’h riiq gs Rgseol!

Oupyi

Exercise: Could we use some simple logic and guesswork to have a go at decrypting this? (When might Alice and Bob be planning to meet?)

Hints:

  • What are common ways of greeting people?
  • Since we know the names of both the sender and the recipient, could we look for those somewhere in the message?
  • Look for things like double-letters, or places where the same letter appears in different words.

As a very basic step, even before we worry about what encryption algorithm to use, we can make life more difficult for someone who wants to snoop on our messages by taking a few simple steps:

  • Only use capital letters.
  • Ignore spaces, new lines, and punctuation.
  • Put letters into e.g. groups of five.

It would be much harder for someone to try to decrypt:

TIUUG CGCTZ IGILG DUIQH RIIQG SRGSE OLOUP YI

than the message above!