Linear Ciphers - Introduction to Cryptography with Java Applets - page 104

CHAPTER 5

Linear Ciphers

The earliest cryptosystems were simple character substitution ciphers; that is, ciphers
which mapped individual characters to characters. These were the predecessors of
stream ciphers. Stream ciphers may encipher characters, or they may encipher quantities as
small as a single bit. What characterizes most modern stream ciphers is that the encipher-
ing transformation enciphers quantities differently based on their position in the stream.
The cipher that follows actually does not fit this definition, since its enciphering transfor-
mation always maps any particular character to the same character.

5.1

THE CAESAR CIPHER
The earliest known cipher is the Caesar cipher. This cipher simply replaces each letter in the
plaintext with the letter three characters down. That is, if we are using the alphabet A thru
Z, A is replaced with D, B is replaced with E, and so on, with the substitution wrapping
around for letters near the end of the alphabet. (See Chapter 1, “A History of Cryptography.”)
For convenience, each letter-number pair of the ordinary alphabet is shown in Table 5.1.
We can represent the enciphering transformation as

C P + 3 (mod 26) 0 ≤ C < 26

where C represents the ciphertext character, and P represents the plaintext character. To
decipher, it is obvious that we solve the above congruence for P to get

P C 3 (mod 26) 0 ≤ P < 26.

ABCDE F GHI J KL MNOP QRS T UVWXYZ
0123456789 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

TABLE 5.1