27. By DAN SURR.
O L T L A L I G E R T I V H E L L E R K I E A E I J F E I Y Y O O U U
S T H E A V A S G Y A S A W C K E P L U E Z T I Z O S I T.
28. By PICCOLA. (This is not a grille. It's a serious matter!)
H S O E S N P T A E T O H I S T W L E D T T F A I B T Y U Y O T C E O
I I T R C Y S B T R A H B T E I D O U S C I U O K R O Q N.
CHAPTER VI
Irregular Types — Columnar Transposition
Square units, in actual use, are less convenient than those rectangular encipherments in which only one dimension of the block is restricted, thus permitting that a single key govern messages of many different lengths. We have a more practical cipher in the columnar transposition of [Chapter IV], and this can be rendered somewhat safer if care be taken to avoid completing the rectangle. The preparation of such a block is illustrated in Fig. 28, where the key-word PARADISE is being used to encipher the following text: REGRET CHANGE IN SYSTEMS BUT THOUGHT ADVISABLE ACCOUNT INCREASED VOLUME SENT BY AIR.
| Figure 28 P A R A D I S E 6 1 7 2 3 5 8 4 R E G R E T C H A N G E I N S Y S T E M S B U T T H O U G H T A D V I S A B L E A C C O U N T I N C R E A S E D V O L U M E S E N T B Y A I R X X X X . . . . . |
First, let us understand the purpose of the four nulls. It is customary, when cryptograms are to be transmitted by wire or radio, to make them evenly divisible into five-letter groups. This usually means the addition of from one to four nulls, and since the nature of the cipher makes it inadvisable that additional letters be added to the enciphered cryptogram, any desired nulls must be added in the block before the columns are taken off. Another precaution usually recommended is the avoidance altogether of key-lengths which are divisible by 5, so that an encipherer is practically never compelled to add a complete five-letter group in order to leave his rectangle incomplete. It might be added that our use of letters XXXX is for emphasis only; a better series would be one of the nature AAEO.
The decipherer’s only problem is illustrated in Fig. 29. Knowing the key, the decipherer knows that there must be eight columns. The number of letters, 75, divided by 8, results in 9, with remainder 3; thus, the short columns are to contain nine letters, and there will be three which contain ten letters. He lays out an 8 x 10 block, cancels the last five cells, writes his key-numbers across the tops of the columns, and then begins to copy letters, filling the column numbered 1, then the column numbered 2, and so on, finally reading his message by straight horizontals.
| Figure 29 P A R A D I S E 6 1 7 2 3 5 8 4 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ............... ............... _ _ _ _ _ _ _ _ _ _ _ x x x x x |
The cryptogram from this block is shown as Fig. 30, and illustrates the manner in which the decryptor will number the letters of practically all cryptograms in order that he may quickly locate any desired letter, or learn, by subtraction, the distance apart of any two letters. The decryptor, of course, does not know how many columns the cryptogram contains, and even after he finds out the key-length, he still does not know exactly the point at which any one column ends and another begins.