116. By NEMO. (A Vigenère? Or a snare and a delusion?)
W L P C V M O G K E E I F M U R W W F H V M F F W E Y X A V U B I C Z
O J M L C H V X Y F K S C U S X I L M G B Q I D B W I F G B I Q Z G Z
H F J Y P M K I G V P T W Y K W Z H W M Z H W I F A P S D N W F H E D
S C X A V O E B Y Y O K C O Y U I H U J L H U D X P P W V V H P F W Y
L G F B V E J M A A G B P I E B A V U V Q L Z N L P W A J W.
117. By NEON. (U.S.Army Cipher Disk. Surely not an advertisement?)
D J T X J M H L M K O M F D T F N E U I G D D N A A U S N S A C F G Y
M Z Y A Q A N M W U W S R B R F J J Q S K A Y B A N B L T O J E R K S
N W X A G T J L Z Y S T V A R B X L K N R L V D U U F O F A K Z L W Y T E E W.
118. By TITOGI. (What! Another Vigenère? Some collusion here!)
D W P W Z T C G H H Z B B V W F B H I F W Q B L L J D Z R G U M M E S
W B D W L J K X I F Y Z D G K Y I O I K D W P M F H C M S F Q G C E L
J I I H W A M I W L J Z I W S W K V W E.
119. By THE ADMIRAL. (Vigenère).
N S R V K D K S I W J W Y C E C E G K C E B D K N Q Y S J U L X Z O L
X P S U V U T F B S O I N P C R R E U Y O N U F K H K Z D D O J P Q Z
C K J I E N A F J D W B U S J U R C L C J C E P C O K T V F A F P Y X
G K K Y Z V.
120. By THE ADMIRAL. (Beaufort).
Z N J L N Y H C Z D A U D D Z I N H R C Z Y Z K H G B P E C L M L W Y
R O I J Q D T L Q O Z H Q S N D V E S E P E J O Y L S Z O J U P G T K
J F K C U W N S H G W F D T M G K K D W E H L Z R N S B G V E S R A U
K K U M J Z M T K N K F Q L G K C U P Z U S D L W D E Z U B D Y F O D.
121. By DOR. (Another "Aristocrat." - Not hard. No keyword).
A B C D E F G C H G I J A K G F D J F B L M E D M M I M G B A N F L C
L O G J P N F D R F C L N. O G P I M S D A N T D L I F U. F C B G
N B P J E G J F C L E F, K C G A I E D V B F.
CHAPTER XV
Miscellaneous Phases of Vigenère Decryptment
When a Vigenère cryptogram is very short, its alphabets are no longer readily identified by their graphic appearance. But its period, in the majority of cases, can still be determined, and it still remains true that the identification of one letter identifies a whole alphabet. The example of Fig. 111 contains only 30 letters. With this cryptogram in the form shown at (a), we are still dependent upon the search for trigrams and short words, but the case is modified by the presence of a repeated trigram. Unless this repetition, ZIL, is accidental, it indicates a maximum period of 12, and the cryptogram is long enough to provide another interval 12, with another trigram, EUK, upon which any key-fragment brought out at ZIL can be tested in order to see whether or not it will bring out another good sequence. When it finally does, the intermediate trigrams (those at intervals 6 or 4) can be tried, in the hope of finding a shorter period.
| Figure 111 (a) (b) Z I L T F R U I Y T J R Z I L 1 2 3 4 5 6 x x x Z I L T F R U I Y T J R K A R O I E A O A E U K L W K. Z I L K A R x O I E A O A E U K L W K |
But assuming a case in which we have no repeated sequences at all, we almost never meet with a Vigenère cryptogram in which there are no repeated single letters belonging to a same cipher alphabet. These repeated single letters can be tabulated with their separating intervals, and these intervals factored in exactly the same way as intervals between repeated sequences. The evidence, perhaps, will be less clear, and less reliable, than that obtained through repeated sequences; as with sequences, the less frequent letters will usually be more informative than those which are leaders. To illustrate, with our given example, the single letter I has shown the interval 6 three times, the single letter R has shown it twice, and the single letters L and Z have shown its multiple. In the average case, the period will not be so clearly evident as here; however, the example was not in any way manipulated in order to produce this evidence.
Once the cryptogram of (a) can be rearranged as at (b), we no longer have before us the piecemeal decipherments and piecemeal tests which are necessary where a period is likely to be anything at all. Whatever key-fragments can be brought out at ZIL, or on another trigram, need be tested only on the three columns which contain the trigram. Even presuming that the evidence has been inconclusive between two or more periods, the cryptogram, necessarily a short one, can be written into each of these probable periods, and the two or more resulting blocks, standing side by side, can be considered more or less simultaneously. Here, with our period determined as 6, the columns of (b) are very short, and the number of trials and erasures should not be many.
For this kind of case, however, many solvers have a preference for the purely mechanical method which is detailed in Fig. 112. Sheet 1 of this figure has been prepared from the first column of our cryptogram, which included the letters Z U Z O E. Sheet 2 has been prepared from the second column, which included the letters I I I I U; and sheet 3 has been prepared from the third column, which included the letters L Y L E K. In each case, the column of cryptogram letters, as it first stands, is also the A-decipherment. With each letter used as a point of beginning, a series of normal alphabets may be laid out, as in the figure, and the resulting 25 new columns on every sheet will show the other 25 possible decipherments. But if these decipherments have been caused to progress in the normal alphabetical direction,
| Figure 112 Sheet No. 1 (For Column 1 of b, preceding figure) KEYS: a z y x w v u t s r q p o n m l k j i h g f e d c b Z A B C D E F G H I J K L M N O P Q R S T U V W X Y U V W X Y Z A B C D E F G H I J K L M N O P Q R S T Z A B C D E F G H I J K L M N O P Q R S T U V W X Y O P Q R S T U V W X Y Z A B C D E F G H I J K L M N E F G H I J K L M N O P Q R S T U V W X Y Z A B C D Sheet No. 2 (For Column 2 of b, preceding figure) KEYS: a z y x w v u t s r q p o n m l k j i h g f e d c b I J K L M N O P Q R S T U V W X Y Z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A B C D E F G H U V W X Y Z A B C D E F G H I J K L M N O P Q R S T Sheet No. 3 (For Column 3 of b, preceding figure) KEYS: a z y x w v u t s r q p o n m l k j i h g f e d c b L M N O P Q R S T U V W X Y Z A B C D E F G H I J K Y Z A B C D E F G H I J K L M N O P Q R S T U V W X L M N O P Q R S T U V W X Y Z A B C D E F G H I J K E F G H I J K L M N O P Q R S T U V W X Y Z A B C D K L M N O P Q R S T U V W X Y Z A B C D E F G H I J |