Here, we have exactly the routine of Fig. 90, except that our search must be made for probable trigrams, and not for a probable word. We have begun with the most likely trigram, THE. But here we do not find it possible to do as we did in Fig. 90; that is, decipher every letter, first as T, then as H, then as E. Of the twelve cryptogram-letters present, only seven can be deciphered as T; the rest are too far away from it in the normal alphabet, and would require keys larger than 9. Of the six letters which immediately follow the possible T’s (the seventh is not shown), only three can be deciphered as H. And of the three letters which immediately follow a possible TH, only two can be deciphered as E or as A. It is often possible, in these ciphers, to investigate simultaneously the trigrams THE and THA. So far as the cryptogram is shown, then, there are only two points at which a trigram THE can be present, while a Vigenère cryptogram of the same length would have presented ten possibilities. Thus, we have no real need for a second work-sheet;
| Figure 93 Decrypting a Known Gronsfeld Cryptogram Fragment: X U I I A Q E U U Y J W....... Trigram tried: T 4 1/ 7/ 1 1 5/ 3 H 1/ 9/ 2/ E 4/ 0/ The sequences U I I and A Q E are the only points at which the trigram T H E could possibly be present, so that only the key-sequences 1 1 4and 7 9 0 are to be tried. The digram T H alone may be present at Y J. |
the only possible key-fragments, 114 and 790, can be tested by any hit-or-miss method which happens to be quickest.
This cipher is often decrypted in much the same way as a “Caesar” simple substitution (shown in Fig. 61). The cryptogram, or a convenient portion of it, is copied on a single line of writing; then, with each letter as a point of beginning, a series of alphabets is extended (written in reverse order), but only for a distance which includes ten letters. That is, the ten possible decipherments for each cryptogram-letter are written in the form of a ten-letter column. The decryptor may then inspect the ten rows of decipherment to see what he can find. At any point where it is possible to find T, H, and E in three consecutive columns, the correctness of this possible THE can be checked by finding out whether or not it has a series of companion-trigrams standing at some regular interval on exactly the same three rows.
In Fig. 94, we have the tableau of Giovanni Battista della Porta, adjusted to suit the modern 26-letter alphabet. Here we have only thirteen cipher alphabets,
Figure 94
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each of which may be governed by either of two key-letters; these pairs of keys may be seen at the left of their respective alphabets. In all thirteen of these cipher alphabets, the encipherment is reciprocal. In the AB-alphabet, for instance, which is the first one on the chart, the substitute for A is N, and the substitute for N is A. The Porta cipher, the oldest known of its kind, employs a key-word, applied as in Vigenère. If the key-letter in use is either A or B, the topmost alphabet is the one to be used; if the key-letter is either C or D, the second alphabet must be used; and so on. Where this encipherment is illustrated in Fig. 95, it may be of some interest to observe that it is not totally impossible for two different key-words to produce identical cryptograms. As to decipherment, we have already mentioned the fact of reciprocal substitution. Whenever the alphabets are reciprocal (in any cipher), the decipherment is identically the same process as encipherment.
The Porta tableau, being smaller than the Vigenère, is not at all inconvenient to prepare and use as it stands. It can be made still more compact: The upper half being alike for all thirteen cipher alphabets, this half can be written once only, at the top of the chart. The lower halves can be written below this on thirteen parallel lines, with their pairs of keys at the left. A ruler may then be used, as suggested for Vigenère, to point out any given lower half. But when it is noticed that these lower halves are identically the same series of letters, with its
| Figure 95 Porta Encipherment Keyword: E A S T E A S T Plaintext: S E N D S U P P... Cipher: D R E Z D H G G... (Compare:) Keyword: F A T S F A T S Plaintext: S E N D S U P P... Cipher: D R E Z D H G G... |
point of beginning shifted one letter at a time, it is promptly seen that a slide is possible, on which the N-to-Z half of the normal alphabet, if written twice in succession, could be placed in 13 different positions with reference to the A-to-M half; and a slide is more convenient still. The slide shown in Fig. 96 is another of Ohaver’s devices. The only new feature in connection with the Porta slide lies in the handling of the key-letters, which, in this cipher, are no longer the first letters of their cipher alphabets. Mr. Ohaver has added them on the sliding portion of the device, each pair of keys being placed directly below the letter which must stand beneath the index (A) whenever one or the other of the pair is the key-letter in use.