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Chapter 5
Monoalphabetic Ciphers
Here is an example of a plaintext – ciphertext alphabet pair for each type of
cipher we have seen thus far.
1. A Caesar cipher with key 5:
plaintext alphabet
ciphertext alphabet
a b c d e f g h i j k l m n o p q r s t u v w x y z
F G H I J K L M N O P Q R S T U V W X Y Z A B C D E
2. A Decimation Cipher modulo 26 with key 21:
plaintext alphabet
ciphertext alphabet
a b c d e f g h i j k l m n o p q r s t u v w x y z
U P K F A V Q L G B W R M H C X S N I D Y T O J E Z
3. A Linear Cipher modulo 26 with key 7m + 9:
plaintext alphabet
ciphertext alphabet
a b c d e f g h i j k l m n o p q r s t u v w x y z
P W D K R Y F M T A H O V C J Q X E L S Z G N U B I
These are all monoalphabetic ciphers, ciphers in which the same plaintext
letters are always replaced by the same ciphertext letters. Mono, meaning one,
indicates that each letter has a single substitute. In this chapter we look at
other ways of creating monoalphabetic ciphers. 1
To construct a monoalphabetic cipher, we need to create some ordering of
the alphabet, such as S O M E R D I N G X H B V L T U J W K Y Z F A C P Q, and
pair it with a plaintext alphabet,
plaintext alphabet
ciphertext alphabet
a b c d e f g h i j k l m n o p q r s t u v w x y z
S O M E R D I N G X H B V L T U J W K Y Z F A C P Q
1 When the ciphertext alphabet is in the usual order just shifted, as in (1), it is said to be
a regular or direct substitution alphabet. When the ciphertext alphabet is mixed up, as in
(2) and (3), it is said to be a mixed substitution alphabet. A reversed alphabet is another
possibility, and is exactly what it sounds like.
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