Secret Key Cryptography

17 Codes

This chapter covers

Ideas for constructing a code

Over the centuries, despite advances in ciphers, then cipher machines, and now digital cryptography, the military has always relied on codes. Even today, we can assume that the military still has codes as a backup in case electronic devices fail or power is unavailable.

Most codes replace letters, syllables, words or phrases with groups of a fixed size, usually 3, 4 or 5 decimal digits, or groups of 3 or 4 letters. Variable-length codes are uncommon. Codes generally fall into two types, single codes and double codes. In a single code the words and phrases are listed alphabetically and the code groups are assigned in numeric order, although not consecutively, so words and code groups can ...