Tables are then made for kinds of matter that is not ordinary, taken
from various kinds of telegraphic and other documents, which will alter
only slightly the percentage values of the letters as shown in a table
from ordinary English.
Having gone along thus far, the expert figures how many times he
can expect to find two letters occurring together. These are called
digraphs, and one learns that AH will show up once in a thousand
letters, while HA will be found twenty-six times. These double-letter
combinations form a separate table all of their own, and the common
ones are set aside, as TH, ER, ON, OR, etc., so they can be readily
guessed or mathematically figured against any text.
Tables of frequency are figured out for the various languages,
particularly German, and the ciphers are divided into two chief
classes, substitution and transposition. The writer in _The Press_ says:
Now you will remember those percentages of vowels and consonants. Here
is where they come in. When a message is picked up the army expert
counts the times that the vowels recur, and if they do not check with
the 40 per cent for the common vowels, with the consonant figures
tallying within 5 per cent of the key, he knows that he is up against a
substitution cipher. The transposition kind will check to a gnat’s heel.
When the expert knows exactly what he is up against he is ready
to apply the figures and patiently unravel the story. It may take
him hours, and maybe days, but sooner or later he will get it to a
certainty.
If he has picked up a transposition fellow he proceeds to examine it
geometrically, placing the letters so that they form all sorts of
squares and rectangles that come under the heads of simple horizontals,
simple verticals, alternate horizontals, alternate verticals, simple
diagonals, alternate diagonals, spirals reading clockwise, and spirals
reading counter-clockwise. Once one gets the arrangement of the
letters, the reading is simple.
For instance, ILVGIOIAEITSRNMANHMNG comes along the wire. It doesn’t
figure for a substitution cipher and you try the transposition plan.
There are twenty-one letters in it, and the number at once suggests
seven columns of three letters each. Try it on your piano:
I L V G I O I
A E I T S R N
M A N H M N G
And reading down each column in succession you get “I am leaving this
morning.”
After passing over several simple ciphers as not “classy” enough to
engage the reader’s attention, the writer takes up one of a much more
complicated nature, which, however, did not get by Uncle Sam’s code
wizards. Follow the deciphering of this example by Captain Hitt:
He began with an advertisement which appeared in a London newspaper,
which read as follows:
“M. B. Will deposit £27 14_s._ 5_d._ to-morrow.”
The next day this advertisement in cipher appeared:
“M. B. CT OSB UHGI TP IPEWF H CEWIL NSTTLE FJNVX XTYLS FWKKHI BJLSI
SQ VOI BKSM XMKUL SK NVPONPN GSW OL IEAG NPSI HYJISFZ CYY NPUXQG
TPRJA VXMXI AP EHVPPR TH WPPNEL. UVZUA MMYVSF KNTS ZSZ UAJPQ DLMMJXL
JR RA PORTELOGJ CSULTWNI XMKUHW XGLN ELCPOWY OL. ULJTL BVJ TLBWTPZ
XLD K ZISZNK OSY DL RYJUAJSSGK. TLFNS UVD W FQGCYL FJHVSI YJL NEXV PO
WTOL PYYYHSH GQBOH AGZTIQ EYFAX YPMP SQA CI XEYVXNPPAII UV TLFTWMC FU
WBWXGUHIWU. AIIWG HSI YJVTI BJV XMQN SFX DQB LRTY TZ QTXLNISVZ. GIFT
AII UQSJGJ OHZ XFOWFV BXAI CTWY DSWTLTTTPKFRHG IVX QCAFV TP DIIS JBF
ESF JSC MCCF HNGK ESBP DJPQ NLU CTW ROSB CSM.”
Now just off-hand, the average man would shy away from this combination
as a bit of news that he really did not care to read. But to the cipher
fiend it was a thing of joy, and it illustrates one of the many cases
that they are called upon to read, and the methods by which they work.
As a starting-point the cipher-man assumed that the text was in English
because he got it out of an English newspaper, but he did not stop
there. He checked it from a negative view-point by finding the letter
_w_ in it, which does not occur in the Latin languages, and by finding
that the last fifteen words of the message had from two to four
letters each, which would have been impossible in German.
Then he proceeds to analyze. The message has 108 groups that are
presumably words, and there are 473 letters in it. This makes an
average of 4.4 letters to the group, whereas one versed in the art
normally expects about five. There are ninety vowels of the AEIOU group
and seventy-eight letters JKQXZ. Harking back to that first statement
of percentages, it is certain that this is a substitution cipher
because the percentage does not check with the transposition averages.
The canny man with the sharp pencil then looks for recurring groups and
similar groups in his message and he finds that they are:
AIIWG AII BKSM BKAI CT CTWY CTW DLMMJXL DL ESF ESBP FJNVX FJHVSI NPSI
NPUXQG OSB OSY ROSB OL OL PORTELOGJ PO SQ SQA TP TP TLBWTPZ TLFNS
TLFTWMC UVZUA UVD UV SMKUL XMKUHW YJL YJVTI.
Passing along by the elimination route he refers to his frequency
tables to see how often the same letters occur, and he finds that they
are all out of proportion, and he can proceed to hunt the key for
several alphabets.
He factors the recurring groups like a small boy doing a sum in
arithmetic when he wants to find out how many numbers multiplied by
each other will produce a larger one. The number of letters between
recurring groups and words is counted and dissected in this wise:
AII AII 45, which equals 3x3x5
BK BK 345, which equals 23x3x5
CT CT 403, no factors
CTW CTW 60, which equals 2x2x2x5
DL DL 75, which equals 3x5x5
ES ES 14, which equals 2x7
FJ FJ 187, no factors
NP NP 14, which equals 2x7
OL OL 120, which equals 2x2x2x3x5
OS OS 220, which equals 11x2x2x5
OSB OSB 465, which equals 31x3x5
PO PO 105, which equals 7x3x5
SQ SQ 250, which equals 2x5x5x5
TLF TLF 80, which equals 2x2x2x2x5
TP TP 405, which equals 3x3x3x3x5
UV UV 115, which equals 23x5
XMKU XMKU 120, which equals 2x2x2x3x5
UV UV 73, no factors
YJ YJ 85, which equals 17x5
Now the man who is doing the studying takes a squint at this result and
he sees that the dominant factor all through the case is the figure
5, so he is reasonably sure that five alphabets were used, and that
the key-word had, therefore, five letters, so he writes the message in
lines of five letters each and makes a frequency table for each one of
the five columns he has formed, and he gets the following result:
Col. 1. Col. 2. Col. 3. Col. 4. Col. 5.
A 2 A 9 A 1 A 1 A 2
B— B 3 B 3 B— B 7
C 7 C 1 C 3 C 4 C—
D 2 D 2 D 1 D— D 3
E 4 E— E 2 E 7 E—
F 3 F— F 9 F 3 F 5
G 9 G— G 3 G 2 G 2
H 3 H 5 H 3 H 3 H 2
I 2 I 2 I 7 I 17 I 2
J 5 J 1 J 6 J— J 9
K 6 K 5 K— K 1 K 1
L— L 19 L 2 L 5 L 1
M— M— M 7 M 4 M 3
N 7 N 3 N 4 N— N 5
O 5 O— O 9 O 1 O—
P 7 P 7 P 8 P 4 P—
Q 5 Q— Q— Q 2 Q 6
R— R 1 R 1 R 6 R 1
S— S 8 S 6 S 12 S 7
T 7 T 3 T 5 T 1 T 14
U 7 U 3 U 6 U— U 1
V 5 V— V 2 V 5 V—
W 3 W 4 W— W 5 W 7
X 2 X— X 4 X 8 X 6
Y 4 Y 5 Y— Y 3 Y 7
Z— Z 5 Z 3 Z— Z 3
Now, having erected these five enigmatical columns, Captain Hitt
juggles them until he uncovers the hidden message, thus:
“In the table for column 1 the letter G occurs 9 times,” he says with
an air of a man having found something that is perfectly plain. “Let us
consider it tentatively as E.
“Then, if the cipher alphabet runs regularly and in the direction
of the regular alphabet, C (7 times) is equal to A, and the cipher
alphabet bears a close resemblance to the regular frequency table.
Note that TUV (equal to RST) occurring respectively 7, 7, and 5 times
and the non-occurrence of B, L, M, R, S, Z (equal to Z, J, K, P, Q, and
X, respectively).
“In the next table L occurs 19 times, and taking it for E with the
alphabet running the same way, A is equal to H. The first word of our
message, CT, thus becomes AM when deciphered with these two alphabets,
and the first two letters of the key are CH.
“Similarly in the third table we may take either F or O for E, but a
casual examination shows that the former is correct and A is equal to B.
“In the fourth table I is clearly E and A is equal to E.
“The fifth table shows that T is equal to 14 and J is equal to 9. If we
take J as equal to E then T is equal to O, and in view of the many Es
already accounted for in the other columns this may be all right. It
checks as correct if we apply the last three alphabets to the second
word of our message, OSB, which deciphers NOW. Using these alphabets to
decipher the whole message we find it to read:
“‘M. B. Am now safe on board a barge moored below Tower Bridge, where
no one will think of looking for me. Have good friends but little money
owing to action of police. Trust, little girl, you still believe in
my innocence although things seem against me. There are reasons why
I should not be questioned. Shall try to embark before the mast in
some outward-bound vessel. Crews will not be scrutinized as sharply as
passengers. There are those who will let you know my movements. Fear
the police may tamper with your correspondence, but later on, when hue
and cry have died down, will let you know all.’”
It all seems simple to the man who follows the idea closely, but
Captain Hitt proceeds to make further revelations of the art. He adds:
“The key to this message is CHBEF, which is not intelligible as a word,
but if put into figures, indicating that the 2d, 7th, 1st, 4th, and 5th
letter beyond the corresponding letter of the message has been used
as a key it becomes 27145, and we connect it with the personal which
appeared in the same paper the day before reading:
“‘M. B. Will deposit £27 14_s._ 5_d._ tomorrow.’”
This is only one of the many methods for getting under the hide of a
coded message that our bright men of the Army and their cousins of the
State and Navy departments have worked out through years of study and
application.