Simple Ciphers - Modern Cryptanalysis: Techniques for Advanced Code Breaking - page 1

Chapter 1
Simple Ciphers
As long as there has been communication, there has been an interest in keeping some of this information con-
fidential. As written messages became more widespread, especially over distances, people learned how suscept-
ible this particular medium is to being somehow compromised: The messages can be easily intercepted, read,
destroyed, or modified. Some protective methods were employed, such as sealing a message with a wax seal,
which serves to show the communicating parties that the message is genuine and had not been intercepted. This,
however, did nothing to actually conceal the contents.
This chapter explores some of the simplest methods for obfuscating the contents of communications. Any
piece of written communication has some set of symbols that constitute allowed constructs, typically, words,
syllables, or other meaningful ideas. Some of the simple methods first used involved simply manipulating this
symbol set, which the cryptologic community often calls an alphabet regardless of the origin of the language.
Other older tricks involved jumbling up the ordering of the presentation of these symbols. Many of these tech-
niques were in regular use up until a little more than a century ago; it is interesting to note that even though these
techniques aren't sophisticated, newspapers often publish puzzles called cryptograms or cryptoquips employ-
ing these cryptographic techniques for readers to solve.
Many topics havebeenpublishedthatcovertheuse,history,andcryptanalysisofsimplesubstitutionandtrans-
position ciphers, which we discuss in this chapter. (For example, some of the resources for this chapter are Refer-
ences [2] and [4].) This chapter is not meant to replace a rigorous study of these techniques, such as is contained
in many of these topics, but merely to expose the reader to the contrast between older methods of cryptanalysis
and newer methods.
1.1 Monoalphabetic Ciphers
It's certain that, as long as people have been writing, people have been using codes to communicate — some
form of writing known only to the communicating parties. For example, the two people writing each other secret
letters might agree to write the first letter of each word last, or to exchange some letters for alternate symbols.
Even many children experiment with systems and games of writing based on similar ideas.
The most basic kind of cipher is one in which a piece of text is replaced with another — these are called sub-
stitution ciphers. These can be single-letter substitutions, in which each letter in each word is exchanged one at
a time, or whole-block substitutions, in which whole blocks of text or data are exchanged for other whole blocks
( block ciphers, which are discussed in detail in Chapter 4).
One family of simple substitution ciphers related to the above is the family of monoalphabetic ciphers —
ciphers that take the original message and encrypt it, one letter (or symbol) at a time, using only a single new
alphabet to replace the old. This means that each character is encrypted independently of the previous letter, fol-