Computer Security and Cryptography

1.10 NO SEX, MONEY, CRIME OR … LOVE

Cryptanalysis refers to the methods for the analysis of cryptographic systems, and in particular, to recover the plaintext and/or key from ciphertext. Cryptanalysis makes use of

1. Knowledge of the structure of the cryptographic system ,
2. Cribs – information believed to be contained in the plaintext, and
3. Characteristics of the underlying language of the plaintext.

The frequencies of occurrence of letters constitute an elementary characteristic of a natural language. In English, the most frequent letters are E, T, A, O, N, R, I, S, and H. Roughly 13% of the letters in a large sample of English text should be E's.

Figure 1.12 Letter frequencies in English and Gadsby.

In 1937, Ernest Vincent Wright published the novel Gadsby [Wright, 1931] in which the most frequent letter in English, E, did not appear. It could not have been a very big seller – it could not mention sex, money, murder, greed, or tenure, but it is remarkably coherent. Gadsby begins

Youth, throughout all history, had had a champion to stand up
for it; to show a doubting world that a child can think; and,
possibly, do it practically; you would constantly run across
folks today who claim that 'a child don't know anything’.

Figure 1.12 compares the letter frequencies of A,B, …, Z (upper ...