The use of co-ordinates, in those cases where row and column are interchangeable as to order, shows up very plainly when the pair-count has been made on a chart; as previously mentioned for a case of digits, the letters will divide automatically into two groups, neither of which ever forms any combination within itself. With the other case, where an order must be observed, there are not so many substitutes per letter. But in either case, it is possible to pair the letters which belong together. Here, for instance, are the letters E and F. The frequent combinations of both E and F are always formed with the same letters; and both have avoided the same letters; these two must have been paired. Their combinations with G and H are much more frequent than their combinations with I and J; thus G and H must have been paired, and I and J must have been paired. This combination
| Figure 84 An Example of BOOK CIPHER 4-1 1-5 3-16 4-11 1-3 1-6 2-2 6-21 1-4 3-2 4-25 4-2 3-3 l-l 2-12 5-22 4-10 6-7 6-2 5-6 5-7 2-7 1-2 1-8 6-1 3-7 5-4 3-6. (Key "volume": 23d Psalm). |
EG (and its equivalents), has been frequently followed by this other combination (and its equivalents) and so on. When a great many pairs can be considered equivalent to one another, it is possible to begin setting up the checkerboard. Some such devices, of course, are safer than others. But the mere fact that they double the lengths of the cryptograms renders them unfit for any purpose where speed is a requirement; nor can the added time and expense of transmission be tolerated for any purpose whatever unless there is some very definite gain in secrecy.
One great objection to any device offering optional substitutes is that the encipherer himself seems unable to take full advantage of his system. Even having at his disposal five different substitutes for E, he falls into the habit of using one of these in preference to the other four; or, determined to avoid this, he uses them meticulously according to rotation, so that when a frequency chart is prepared from his cryptograms, this chart, which is, after all, a graph, will show the five uniform frequencies sticking out like a sore thumb. Even book cipher, notably secure, however unwieldy in use, has been decrypted because of the encipherer’s very human tendency to use a substitute more than once rather than search for a new one among the hundreds at his disposal.
In book cipher, any agreed book, or other written or printed document, will serve as a key-volume, so long as it is one that is sure to be at hand when wanted. Words, or letters, can then be represented by a series of numbers usually indicating: page, (column), line, serial position. One letter or one word may thus have a substitute such as 20-1-4-32. An example is provided in Fig. 84, which the interested student may puzzle out for himself. The particular key-volume was issued in 1848, but we think this should cause no trouble. When the key-volume selected happens to be the ordinary dictionary, identically the same cipher becomes known as dictionary cipher, which is, to all intents and purposes, a very insecure form of code. Perhaps the two names together, book cipher and dictionary cipher, might be said to represent the maximum and minimum degree of safety found in the code family.
We leave undiscussed the subject of those alphabets which are based on phonetics, with digraphs TH, SH, CH, having their individual symbols, and each vowel capable of having several. The student who desires to prepare one may find the necessary suggestions in any shorthand manual; his substitutes can be two-digit numbers, and his encipherment may be any one of those intended for the normal alphabet. Having made mention of several processes which, to the younger student, may present frightening possibilities, we hasten to add that the four appended examples are all of a type which he should be able to solve without a great deal of difficulty.
100. By PICCOLA. (Key-Phrase Cipher - intercepted by a "Royalist" spy).
N H H K O H W A E H M A U I H U U H S T U S A S T U N H U
M H N I W A H T. N H H H S A D T H H I A I I E I A M H K M
U W A H O L W N W H T M A M D S T H A J T E S U T O T K
N W I E W A O O. O U H K M W A H M N U I H U U H S T N W T O I K
H K M W A H A H O A N W T O I K A W O.
101. By PICCOLA. (Probable words: CIPHER, SUBSTITUTION, ALPHABET, etc).
D K I U O C Z P V C L U Z I Q U W Y V B V I N C D U U L C U K U Z I I
U O C Z P V C L U Z P Y N U S Q S C Z I U L Q T U K H I C Z I K L U Z
P Y N N Y J Q Y L U P L U Z I Q J S C U L U S U E Y G U Z I Q I U T Q
N U F S U Z F L U I C V F Q S W Q I I S U Y S U G S Q N B L U G U O V
V Y F S Q Y I H I Y I K O H K U V P T Y K Q I J U Y V P P C E S Y U O
F Q S U L C Z N Y Z F K E S Y Z I U Z I R Q V V C.
102. By PICCOLA. (Probable words: COLLECTION, GALLERIES, FIGURINE, etc).
Y C G U T H M P Y B X S K R M G X U F P C M I B C J G R M K X L X S Z
N Q V V U N I X Q S Q E E X Z H M X S R L E Q M L C V U D Y C G R N Q
S E X J U S K X C V X S E Q M T C I X Q S K Y C I I Y R K C S C Z M C
L I Q V U S E M Q T K Y R T Q M Z C S Z C V V U M X R L F C L L U R S
M U N R S I V B X S C N U M K C X S W H C M I R M D Y C L L X S N U J
X L C P P R C M U J D T C B O R X S K Y X L E U V V Q F L N Q V V R N
I X Q S S Q F X X X.
103. By EFSEE. (Probable words: PEOPLE, PERSON, CIRCUMSTANCES, etc).
B E C O M I C I Q U E X P A Y O T I A N S I Z I P I A N D O A B U M Y
O R E A N U S Q U I M O N I P M A M A M I F O X E G A O K A Z U K I S
G O V I X A W A Z A I T H I N A I L M O S U I S H E A T R U A L E M O
F A T I C A G I D O Y E M B E Y O L E N A C O S E K E E L S O G I Z A
C O O L S I D I O R Q U A Z O W A G E S D I B U S I V I P U A Z A M E
S I D A R T A C O O Y A P E S L I A R S E W O A L O N I K O L O M B A
R I L A Z A L O W I A V U M A K A T L O F I C I N A I M I L N A Q U I
M O N I P S A W O G A P A V I H I S U E C A N O S M O L E T A M E K O
W A I V S I A R T E Z E I R S I L A Z E G A S A M I V E E P.