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A typical distribution of letters in English language text. Weak ciphers do not sufficiently mask the distribution, and this might be exploited by a cryptanalyst to read the message. In cryptanalysis, frequency analysis (also known as counting letters) is the study of the frequency of letters or groups of letters in a ciphertext.
The California Job Case was a compartmentalized box for printing in the 19th century, sizes corresponding to the commonality of letters. The frequency of letters in text has been studied for use in cryptanalysis, and frequency analysis in particular, dating back to the Arab mathematician al-Kindi (c. AD 801–873 ), who formally developed the method (the ciphers breakable by this technique go ...
Traditional line length research, limited to print-based text, gave a variety of results, but generally for printed text it is widely accepted that line lengths fall between 45 and 75 characters per line (cpl), though the ideal is 66 cpl (including letters and spaces). [1]
In cryptography, a substitution cipher is a method of encrypting in which units of plaintext are replaced with the ciphertext, in a defined manner, with the help of a key; the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing ...
The chance of drawing that same letter again (without replacement) is (appearances − 1 / text length − 1). The product of these two values gives you the chance of drawing that letter twice in a row. One can find this product for each letter that appears in the text, then sum these products to get a chance of drawing two of a kind.
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The two letters of the digram are considered opposite corners of a rectangle in the key table. To perform the substitution, apply the following 4 rules, in order, to each pair of letters in the plaintext: If both letters are the same (or only one letter is left), add an "X" after the first letter. Encrypt the new pair and continue.
The letter characters were in numerical order of code point, so an alphabetical sort could be achieved simply by sorting the data numerically. The code point for corresponding upper and lower case letters differed only by the value of bit 6, allowing a mix of cases to be sorted alphabetically if this bit was ignored.