Cryptanalysis ; a study of ciphers and their solution by Helen F. Gaines

By Helen F. Gaines

Comprises "166 cryptograms."

summary: comprises "166 cryptograms."

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Additional resources for Cryptanalysis ; a study of ciphers and their solution

Sample text

Grilles are made of cardboard, sheet-metal, or other flat material, and are perforated with any desired number, size, and arrangement of openings. The Richelieu grille, of approximately the same size and shape as the paper used for correspondence, could be laid over a sheet of paper so as to reveal only certain portions, and the secret message was written on these. The grille was then removed and the rest of the sheet was filled in with extraneous matter in such a way as to present a seemingly continuous text.

One very useful phase of frequency data is seen in the group percentages. Single letters, especially in short texts, may vary greatly from their normal percentages, while certain classes, taken as a whole, maintain a fairly constant percentage no matter how short the text. Such classes, or groups, listed under the general heading of English Frequency and Sequence Data, can be memorized as having roughly approximate percentages: Vowels, 40%; selected high-frequency consonants, 30%; extreme low-frequency group, 2%; the five most frequent letters, mixed, 45%; the nine most frequent letters, 70%.

This final group of nine letters, ETAONISRH, hardly ever varies appreciably; the shorter groups will sometimes vary as much as 5% one way or the other. Very useful in code decryptment is a list of the commonest words. Trigrams have also been investigated, the favorite positions of individual letters in their own words, average word-length, patterns, and endless other information, some of which is indispensable, and some merely convenient. It will not be possible, in the space at our disposal, to point out all of the uses to which this kind of information can be put; the student is urged to take his cue from the occasional short references made in connection with examples.

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