Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In cryptanalysis, frequency analysis is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking classical ciphers. Frequency analysis is based on the fact that, in any given stretch of written language, certain letters and combinations of letters occur with varying frequencies. Moreover, there is a characteristic distribution of letters that is roughly the same for almost all samples of that language. For instance, given a section of English language, E, T, A and O are the most common, while Z, Q and X are rare. Likewise, TH, ER, ON, and AN are the most common common pairs of letters (termed bigrams or digraphs), and SS , EE , TT , and FF are the most common repeats.

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