Improving Substitution

A cipher based on the use of a secret alphabet is not very secure; such ciphers are presented as puzzles in crossword puzzle magazines. To achieve security it is required to do something better.

Today, even to people not acquainted with cryptography, a number of possibilities suggest themselves. Originally, though, the new ideas came one at a time, separated by hundreds or thousands of years.

The basic ways to improve on simple substitution are the following:

• Instead of using just 26 substitutes, make the problem harder by using a bigger substitution. This is divided into several cases:
  • Use several substitutes for each letter (homophonic substitution)
  • Replace every two letters, or every three letters, by something else that stands for that combination of two letters or three letters (polygraphic substitution)
  • Replace common combinations of letters, or words, or phrases, by their own substitutes (nomenclators and codes)
• Instead of using the same set of substitutes all the time, change from one secret alphabet to another as you encipher a message (polyalphabetic substitution).

Another way of improving on simple substitution is less obvious. Today, text is often converted from the letters, punctuation marks, digits, and other symbols you find on a typewriter to the binary bits of ASCII. Before that, other representations of text were used to substitute for the printed word, such as Morse code. The ancient Greeks used the Polybius square for signalling, by means of which each letter was represented by two groups of from one to five signal fires.

If a letter can be broken up into smaller pieces for purposes of signalling, those smaller pieces can also be used in a cipher. For example, one can take the letters of a message apart into smaller pieces, transpose the smaller pieces, and then put the pieces back together again into letters.

This is called fractionation, and is closely related to polygraphic substitution for two reasons; one is that both deal with different sized units - parts of letters and letters, or letters and pairs of letters - and the other is that fractionation is sometimes used as a method of polygraphic substitution.

• Homophones and Nomenclators
• Polygraphic Ciphers and Fractionation
  • Playfair and its Relatives
  • The Bifid, the Trifid, and the Straddling Checkerboard
  • Fractionated Morse, and Other Oddities
  • The VIC Cipher
  • Two Trigraphic Ciphers, and a Heptagraphic One
• Polyalphabetic Substitution

Next
Skip to Next Section
Chapter Start
Table of Contents
Main Page
Home Page