Since we lack that most powerful of decrypting tools, a probable word, we are forced to begin with probable letter-sequence. If the magic letter Q were present, we should look for a companion U, and after that for a vowel to follow QU. But this, too, is lacking.
Familiarity with English digrams (or, in the case of the beginner, an inspection of the digram chart or the list of digrams) shows that TH is by far the most frequent combination used in the language, and that HE and HA, also including an H, are very prominent among the leaders. Further than this, the list of trigrams informs us that both THE and THA are of outstanding frequency. Of the four letters included, three are so frequent, and appear in so many different combinations, as to be confusing; but H, though belonging to the high-frequency group, does not appear in many different combinations, and is less frequent than the other three.
Looking, then, for H, we find it twice in our present cryptogram, once on the second row and once on the seventh; and, since the seventh row shows two T’s and the second only one T, suppose we try the second row, placing together the two columns (strips) which are headed by the numbers 6-5 in order to set up a digram TH on the second row, as shown at (b).
| Figure 16 (a) (b) 1 2 3 4 5 6 7 8 6 5 I W O F G N O L N G Y B A D H T E R T H B T X G I R D E R I R I E O T S M V S T T S O U Y O O T O Y S T L A T I F L I T A T F H E K T E K E X X C D I V A U V I (c) 6 5 7 ........ 6 5 7 4 ... 1 6 5 7 4 N G O N G O F I N G O F T H E T H E D Y T H E D R I D R I D G B R I D G S T M S T M O R S T M O O Y O O Y O U T O Y O U I T F I T F A S I T F A K E T K E T H A K E T H V I A V I A D X V I A D (d) 6 5 3 ........ 6 5 3 4 ... N G O N G O F (Abandoned in T H A T H A D R I X R I X G favor of c.) S T E S T E O O Y O O Y O U I T L I T L A K E F K E F H V I C V I C D |
The formation of this digram TH on the second row has automatically set up a digram NG on the top row, a digram RI on the third row, and so on; and we find, upon examining these newly-formed digrams, that the whole series is made up of good English combinations. Thus, it looks as if our combination 6-5 is correct, and we will proceed with a possible HE or HA, attempting to complete a trigram THE or THA on the second row.
Both E and A are present on the second row, and we may observe at the steps marked (c) and (d) in the figure just what would be the result of adding strip 7 or strip 3. At first glance, it appears that combinations 6-5-7 and 6-5-3 are about equally probable. But it so happens that both set-ups have formed a sequence YO on the fifth line, suggesting YOU; and when the only U on that line is tried in both places, it becomes evident that combination 6-5-7-4 is going to give better results than combination 6-5-3-4, where we find poor sequences like KEFH. At this point, or earlier, a decryptor will probably proceed on the left side of his set-up, completing the syllable ING and the series of column-numbers 1-6-5-7-4, as shown. When this setting together of columns automatically brings out on the third row a sequence BRIDG, we have our first suggestion of a probable word, since the man who had this cryptogram on his person had just attempted to blow up a BRIDGE. After this, all is plain sailing; the necessary E happens to be on the same line, and even if it were not, we have only three strips left, and these may be placed by trial. Thus our eight paper strips arrive at the stage indicated on the left-hand side of Fig. 17.
| Figure 17 Strips in order Adjustment of rows 2 1 6 5 7 4 8 3 1 W I N G O F L O 2.... B Y T H E D R A 2 B Y T H E D R A 1.... W I N G O F L O 3 T B R I D G E X 6.... T S I T F A L L 4 I R S T M O V E 5.... S T O Y O U T O 5 S T O Y O U T O 7.... T A K E T H E F 6 T S I T F A L L 4.... I R S T M O V E 7 T A K E T H E F 8.... X X V I A D U C 8 X X V I A D U C 3.... T B R I D G E X "Taking-out" Key: 2 1 6 5 7 4 8 3 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 "Writing-in" Key: 2 1 8 6 4 3 5 7 |
If we have previously met the Nihilist transposition, we can see now what the cipher is, and, if it is a true Nihilist, we can finish the reconstruction by decipherment with the key. To do this, we simply number the rows from 1 to 8 and then disarrange these rows so that their numbers will reproduce the series of column numbers. This is shown on the right-hand side of Fig. 17, where the plaintext is easily read: “By the drawing of lots, it falls to you to take the first move. Viaduct bridge.” The gentleman required three nulls, and thriftily made use of them as punctuation. If we have not previously met the Nihilist encipherment, or if this cryptogram is of a kindred type but governed by two separate keys, one for columns and another for rows, the only difference is that we may have to experiment a little with rows before finding their correct order.
In completing our solution, we have obtained a key, 2 1 6 5 7 4 8 3, shown in the series of column-numbers, and should other cryptograms be intercepted having the same key as the first, we need merely decipher them with our key. It is, however, a “taking out” key, while the Nihilist, as we have seen, is ordinarily written in. Having either of the keys, we may find the other easily enough as suggested in the figure. Simply “number the numbers” and put them back in serial order. The new set of numbers, now disarranged, will show you the other key. It would not be impossible for the student who is a good guesser to find the keyword on which our present writing-in key was based. This kind of work, with paper strips, is much more rapid than it probably seems, and is often done at random. The keen eye needs no digram list for the spotting of HT, merely reversed, with GN above it.
Speaking now of the ordinary columnars (Fig. 11), one minor point should perhaps be brought to the attention of the very new student. Quite often, a digram, such as the QU of Fig. 18, is not written on a single line, and it may be necessary to match this valuable digram in the manner shown at (b) of that figure, coming out in the end as at (c). In such event, we can later on transfer columns 5-6-7 to the other side of the block, raising them all by one position. (Column numbers, in this case, are for reference only.) The same would not apply to a Nihilist block in which the whereabouts of the “next” row is unknown; the digram QU would have to be abandoned in favor of something else.