4.1 RUNNING KEYS
A monoalphabetic substitution on plaintext1
uses a single rule θ to encipher each letter
A polyalphabetic substitution uses more than one rule
to encipher the plaintext letters.
A running key
is a simple polyalphabetic generalization of Caesar encipherment Ck of plaintext, which polyalphabetically enciphers the plaintext x = (x0, x1, …, xn−1) according to the rule
A book cipher derives the running key from the text in some (secret) book; the key is composed of the letters starting on some specified page, line, and word in the book. Ken Follet's novel The Key To Rebecca relates the adventures of Cicero, a World War II German spy who uses a book cipher based on Rebecca of Sunnybrook Farm to encipher messages.
An alternative method to obtain a running key is to extend a key word k = (k0, k1, …, kr−1) of length r by periodicity
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