Cryptanalysis: A Study of Ciphers and Their Solution by Helen F. Gaines

Author:Helen F. Gaines [Gaines, Helen F.]
Language: eng
Format: azw3, pdf
Publisher: Dover Publications
Published: 2014-11-18T05:00:00+00:00

(b) Deciphering with the MESSAGE:

Here, the cryptogram, or a substantial portion of it, would be written across a sheet of quadrille paper, and the probable word would be written at one side, where each of its letters will govern one row of decipherments. The first letter, 5 in the figure, has been used to decipher the whole row of cryptogram-letters, giving every possible key-letter which can produce S. The second letter, U, has been used to decipher them all again (except the very first letter; we do not expect a word UPPLIES). The third letter, P, has been used to decipher them all a third time; and so on. The resulting rows of decipherment include all key-letters which could have produced S, then U, then P, and so on. To read them consecutively, beginning at any cryptogram letter, start immediately below that letter, and read diagonally downward to the right. The first diagonal gives key CYK… , the second gives AFS… , and so on to the fifth diagonal, showing the key as T C O M E T C O. (If it is desired that these possible keys should come out standing in a horizontal position, then the decipherments may be made diagonally.) F. R. Carter, the originator of this scheme, does not necessarily make all of the decipherments which are included in the figure. He begins with the assumption that his key will be a recognizable word; having deciphered in full the first three rows, he abandons all of those diagonals which cannot develop into words. If, in the end, he is forced to conclude that his key was incoherent, no decipherments have been erased; he may still go back and develop the rest of his diagonals, in the hope that one will begin repeating.

The more difficult of our two cases, that in which we have no probable words other than the, and, which, that, have, but, etc., can follow exactly the routine out lined in Fig. 90; but in this case there must be two separate work-sheets. Here, it is usually better to forget words and start at once with the list of normally frequent trigrams, THE, AND, THA, ENT, ION, TIO, etc. The key-fragments which are deciphered by these will be very short, and very numerous; a great many of them will be very good usable sequences, and perhaps the correct key-sequence will not look quite so inviting as others which are incorrect. It becomes necessary, then, to have a second work-sheet on which we may take these fragments one by one and try them as keys. If any one of them is a fragment of the original key, it must bring out fragments of plaintext, and must bring them out at some regular interval. If the scheme of Fig. 90 is the one preferred, the second work-sheet may be prepared exactly like the first, and used in the same way. The only difference is as follows: On the first work-sheet, where the figure shows the word SUPPLIES, a supposed trigram (THE, AND, etc.

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