CODES AND CIPHERS.
A code is a list or collection of arbitrary words or groups of letters
to each of which some ordinary word, proper name, phrase, or sentence
is assigned for meaning.
Ciphers embrace all means whereby writings may be transcribed
into occult terms. All ciphers employ some distinct method for
transcription, which method is termed a key. In practice the key is
usually applied directly in enciphering and reversed in deciphering
messages.
CODES IN USE.
The codes of the Western Union and Postal Telegraph companies are
examples of well-known codes suited to general commercial use. Besides
these, many special codes have been formulated, so as to embody
technical expressions especially adapted to use in particular lines of
industry. The War Department Code is a military code adapted to the
special needs of the military establishment in peace and war.
EMPLOYMENT OF CODES.
Codes are primarily intended for economy, but they may also be readily
employed to secure secrecy. When used solely for economy, the coded
message is said to be plain code; that is, the word or phrases of
the message are coded by direct reference to their respective code
equivalents. Thus plain code is readily translatable to anyone in
possession of a code book. When secrecy is desired, some method of
enciphering or key is employed in such a way that only persons in
possession of it can in conjunction with the code book decipher it. In
such case the message is said to be in cipher code.
CIPHER CODE.
In all codes each expression and its equivalent in plain language is
assigned a number. These numbers usually commence at unity and increase
consecutively to any desired figure. Messages may be enciphered by
means of a key number or series of numbers. An additive number, say
55 additive, requires that in enciphering a message, the fifty-fifth
word numerically greater than the proper code word shall be used; if 55
subtractive is used, the fifty-fifth word numerically smaller than the
proper code word is to be used. By agreement a single key number can
be used alternately additive and subtractive, that is, first additive,
second subtractive, third additive, etc.
The key numbers are used over and over until the entire message is
enciphered. The key number can sometimes be expressed by a single
word, as, for instance, "Grant," each letter having a value of tens in
accordance with its position in the alphabet; that is, G, the seventh
letter equals 70; R equals 180; A equals 10; N equals 140; and T
equals 200. Or by preconcerted arrangement letters may represent units
or hundreds. Security from translation by persons not having the key
number is greater when the key numbers are used alternately additive
and subtractive. If a cipher key word is used, it should be one of
an odd number of letters, as, for instance, "Jones," the numbers
corresponding to the positions of the letters in the alphabet. The
first number should be additive, the second subtractive, etc. By this
means the first letter of the key word is additive the first time it is
used, subtractive the second, additive the third, and so on. In some
instances the key number, when added to or subtracted from the code
number, gives a resulting number exceeding the highest code number
or less than unity. In cases of this kind it should be remembered in
enciphering that unity follows the highest code number in addition,
and that the highest code number follows unity in subtraction. In
deciphering a message the process of enciphering is reversed.
THE WAR DEPARTMENT CODE.
As previously stated, the War Department Code is the technical military
code and contains expressions numbered consecutively from 1 to 62,000.
All the code words are composed of 6 letters, which are so arranged
that the vowels and consonants invariably alternate. In the formation
of code words the following 13 letters only are used, viz, A, B, D, E,
F, G, I, K, M, N, S, U, and X. The body of the code book is arranged as
follows:
(_a_) Army list, containing the name of every commissioned officer
in the regular establishment.
(_b_) Military organizations, giving all batteries, companies,
troops, etc.
(_c_) Military posts and stations, covering Alaska, Hawaii,
Philippine Islands, Porto Rico, and the United States.
(_d_) United States naval stations and vessels.
(_e_) Geographical names.
(_f_) Miscellaneous tables as follows:
Numerals.
Arrivals and departures.
Dates.
Indorsements.
Letter acknowledgments.
Requisitions.
Telegram acknowledgments.
Mails, shipments, and transports.
Blanks for future additions as they may be needed.
Ranks and grades of officers and men in the Army.
Wireless stations of the Army and Navy.
(_g_) Alphabetical list of code expressions arranged conveniently
for use.
When it is desired to transmit some word or expression not to be
found in the code and no suitable synonym can be discovered the word
or expression should be sent in plain language or spelled out by the
equivalents for letters and endings to be found on page 589.
Complete instructions for the use of the code either as a code or
cipher are contained in the introductory pages of the book.
CIPHER CODE IN FIELD WORK.
The use of cipher code in enciphering field messages will usually be
practicable only between the several headquarters and other large
stations supplied with code books. This method, too, is prohibitive
for urgent messages when the time of enciphering and deciphering is an
important factor connected with delivery.
FIELD CIPHERS.
_Description and use._--Field ciphers include all systems and the
apparatus connected therewith which are ordinarily employed in
enciphering and deciphering field messages. Field ciphers are intended
for use when code books are not available, and hence the employment of
cipher code is precluded. Some methods of field cipher employ simple
forms of apparatus, while others require the use of no apparatus at all.
_Forms of field cipher._--There are two general classes of field
cipher. The first class employs the transposition or reversal of the
letters or words of a message according to some preconcerted rule as a
means of secrecy. The route cipher hereafter described is an example
of this class. The method used in ciphers of the second class consists
in the substitution of certain letters or symbols for each of the
individual letters composing the words of the message. Both classes of
cipher can be rendered more efficient by a judicious use of inversions
and by the concealment of terminations.
_Inversions._--By the inversions of the whole or certain parts of
messages, according to some preconcerted arrangement, the complications
of cipher can be greatly increased. If a message is to be inverted,
either as a whole or by clauses, it should be inverted before the
cipher letters are written over it. Messages may be further complicated
by sending the letters of each word backward in various other
prearranged combinations.
_Concealment of terminations._--To evade the discovery of the key or
keys employed, it is most important that the termination of the words
of a message should be concealed. The best method to conceal the
beginning, and at the same time the termination of words, is to divide
them into arbitrary groups of four or five letters each. This procedure
will add immeasurably to the strength of the cipher and should in no
way confuse one in possession of the key. For instance, the words
"sufficient time" would be divided "suff" "icie" "ntti" "me," and such
blind letters as may be agreed upon to fill the last two spaces of the
last group. All such artifices as this will surely delay a translator
not in possession of the key.
CIPHER APPARATUS.
_The cipher disk._--The cipher disk is composed of two disks of
cardboard, leather, or other material joined concentrically, the upper
disk revolving upon the lower. The alphabet, reading from left to
right, and such other signals, numerals, or combinations of letters, as
may be desired, are printed around the circumference of the lower disk.
On the upper disk are printed the alphabet and such other signals,
numerals, or combinations of letters as are printed on the lower disk.
On the lower disk they are printed from left to right, while on the
upper disk they are printed from right to left. If it is desired to
encipher a message, the key letter or the first letter of the key word
or words is set opposite "A." Let us assume it to be "J." The cipher
letters to be written are those opposite the text letter when the
letter "a" on the upper disk is set opposite "J" on the lower disk. For
example, "Send powder" would be written "rfwg uvngfs."
Having a cipher disk as above described, this mere transposition of
letters would delay but a short time the deciphering of a message by
one not knowing the key letter, as it would be necessary only to place,
in turn, opposite "a" each of the letters of the alphabet beginning
with "b" and noting the letters opposite the enciphered letters. But
this simple disk can be used with a cipher word, or preferably, cipher
words known only to the correspondents, and it is entirely improbable
that a message so enciphered could be deciphered in time to be of any
value to the enemy. Using the key words "permanent body" to encipher
the message "Reenforcements will reach you at daylight," we would
proceed as follows: Write out the message to be enciphered and above it
write the key word or key words, letter over letter, thus:
PERMANENTBODYPERMANENTBODYPERMANENTB
Reenforcementswillreachyouatdaylight
yanzvznlppkqfxijbBpwanruqpeplomccwhmi
Now bring the "a" of the upper disk under the first letter of the
key word on the lower disk, in this case "P." The first letter of
the message to be enciphered is "R." "Y" is found to be the letter
connected with "R" and it is put down as the first cipher letter.
The letter "a" is then brought under "E," which is the second letter
of the key word. "E" is to be enciphered and "a" is found to be the
second cipher letter. Then bring "a" to "R" and the cipher letter
will represent "e," the third text letter of the message. Proceed in
this manner until the last letter of the cipher words is used, and,
beginning again with the letter "P," so continue until all letters of
the message have been enciphered. Divided into groups of four letters,
it will be as follows: "yanz vznl ppkq fxij bpwa nruq pepl omcc whmi."
[Illustration: FIG. 19.--Cipher disk.]
To decipher the message, reverse the proceedings above described; thus
the letter "a" on the upper disk is brought under the first letter
of the key word "P." Following these instructions, we find the first
cipher letter of the message; "a" is then brought to the next letter of
the key word. In this case "E" is, of course, the next letter of the
text. "R" is the next letter in the key and "a" is brought over it.
The cipher letter "n" gives us the next text letter, which is "e," and
so on until the completion of the message. If the letters of the key
word or phrase are exhausted, begin again with the first letter and so
continue until the entire message is deciphered.
With a key word, or, preferably, a key phrase of three or four words,
the deciphering of a message is extremely difficult.
In a military cipher message, it may be desired to transmit numerals,
the spelling out of which would require considerable time. This can
be done by an arrangement of the cipher disk so that the numerals of
which will appear in the same order as and follow the letters of the
alphabet. Thus on the lower disk 1 is placed opposite A; 2 opposite B;
3 opposite C; 4 opposite D; 5, 6, 7, 8, 9, and 0 opposite E, F, G, H,
I, and J, respectively.
On the upper disk the above numerals also appear, beginning numeral 1
opposite A; 2 opposite B, etc., 0 being opposite J.
The arbitrary sign XX will be used to indicate "numerals follow" and
"numerals end." Supposing then we wish to send the following message:
"Send 6,000 cavalry at once," and that the key word was "Washington."
Following the instructions heretofore given for enciphering, we would
place the words as follows:
WASHINGTONWASHINGTONWASHI
SENDXX6000XXCAVALRYATONCE
EWFELQBKFEZDQHNNYCQNDMFFE
In place of a disk means may be extemporized by taking two strips of
paper, on one of which the alphabet, numerals, etc., are twice written
in succession. On the other, with equal spacing, the alphabet, etc.,
are written once, but in reverse order. By sliding these strips in
juxtaposition with each other they will replace the disk.
Cipher disks should never be allowed to fall into the hands of the
enemy or of anyone unauthorized to have and use them; to insure this,
special instructions should be issued for their care and keeping.
THE MATHEMATICAL CIPHER.
This cipher is a highly efficient one for the purpose of secrecy and at
the same time requires no apparatus whatever attendant upon its use.
The cipher is constructed as follows: Commit to memory the alphabet
by numbers, viz, A, 1; B, 2; etc. Take any key word, phrase, or
sentence desired; for example, "A discovery." Suppose the message to be
enciphered is "Send me powder tonight." The enciphering of the message
using the key given above will be as follows:
To encipher, first write out the key, letter by letter, placing the
message letter by letter beneath it. Then reduce the letters of the
key and the message to the numeral alphabetical equivalents. Add the
individual columns and subtract unity from each. From any result thus
found, which exceeds the number of letters in the alphabet, the number
26 must be subtracted. The final totals reduced to letters by numerical
alphabetical equivalents will then give the cipher.
ADISCOVERYADISCOVER
sendmepowdertonight
which reduced to numerical equivalents according to alphabetical
position of letters becomes:
1 4 9 19 3 15 22 5 18 25 1 4 9 19 3 15 22 5 18
19 5 14 4 13 5 16 15 23 4 5 18 20 15 14 9 7 8 20
Now add the columns and subtract unity from each. If any result
so found exceeds the number of letters in the alphabet 26 must be
subtracted from it.
In the example given the numerical totals are as follows:
20 9 23 23 16 20 38 20 41 29 6 22 29 34 17 24 29 13 38
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
-------------------------------------------------------
19 8 22 22 15 19 37 19 40 28 5 21 28 33 16 23 28 12 37
26 26 26 26 26 26 26
-------------------------------------------------------
19 8 22 22 15 19 11 19 14 2 5 21 2 7 16 23 2 12 11
which connected to letters gives:
SHVVOSKSNBEUBGOWBLK
the cipher required.
Translation of cipher is had by reversing the processes described.
THE ROUTE CIPHER.
This is a cipher in which the words or a message are retained
unchanged, but are so disarranged by preconcerted rules that the
sense becomes unintelligible. The message as received seems to be a
number of disconnected words and without meaning, but by arrangement
in proper order in accordance with certain rules can be easily read.
Messages enciphered in this manner may be translated by persons not
in possession of the key, and therefore the information contained
therein should only be of such a character as to be of little value to
the enemy unless acted upon immediately. The usual method employed in
arranging a message for this cipher is to write the words in vertical
columns. The number of words in each column should always equal the
number of columns, being made so, if necessary, by the addition of
sufficient "blind" words. A preconcerted route is agreed upon, as up to
the first column, down the third, up the second, etc. The message is
then transmitted without reference to the columns, but is deciphered
at the receiving station by column arrangement and perusal along the
original route.
For example, to encipher the message "Move daylight. Enemy approaching
from north. Prisoners say strength one hundred thousand. Meet him as
planned," arrange as follows:
Move strength planned say
daylight one as prisoners
enemy hundred him north
approaching thousand meet from
Here the route is down the first column, up the fourth, down the
second, and up the third.
CIPHER DETECTION.
_General instructions._--In deciphering a message in which the same
cipher letter or symbol is uniformly used to represent the same text
letter, the following data will be of assistance.
The proportion of occurrence of letters of the alphabet in English
words is as follows: For every 2 of the letter Q there are 4 of the
letter X, 8 of K, 16 of B, 13 of C, 80 of I, N, O, and S; 85 of A, 90
of T, and 120 of letter E.
The compounds most frequently met with are NG EE LL MM TT DD and NN.
The order of frequency in which the letters of the alphabet occur as
initial letters in words is as follows:
S, C, P, A, D, I, F, B, L, T.
_Employment of Cipher Disk._
If messages are enciphered by a mere transposition of the letters of
the alphabet, the cipher disk can be used to quickly decipher the
message, as the following example will show: Assuming that F is used to
represent A, G to represent B, H to represent C, I to represent D, J
to represent E, etc., in regular sequence, and that the message to be
enciphered is: "We are short of rifle ammunition; send 30,000 rounds at
once."
This would be enciphered if divided into groups of four letters as
follows:
jbfo bnyr omra oxub fuls xmxr snbs cmjb smhm yrln
fsco rlsc nfmr sdb.
Place "a" of the upper cipher disk under B of the lower disk and notice
whether the cipher letters jbfo--the first group--are intelligible.
They give "sawn," continue this for "saw," the first three letters, may
be the text word. Now the next group is B N Y R and these give A O D K.
We know that A does not represent B because the first 8 cipher letters
give the meaningless letters "sawnaodk." Turn "a" to C and we have for
the first group T B X O, which is without meaning. Turning "a" to D we
get U C Y P, a meaningless jumble. Turn "a" to E and we get V D Z Q,
which is meaningless. Now turn "a" under F and we find that JBFO mean
"Wear," which, so far at least, gives us a part of a word, or the word
"We" and part of another word. We continue to the next group B N Y R,
which gives us "esho." We now have these letters "Wearesho," which at a
glance we read "We are sho;" continuing to the next group O M R A the
cipher disk gives us "rtof," and we read "We are short of" and know we
have found the key letter, and the information hidden in the cipher
is ours. Continue deciphering with "a" under F until the end of the
message. Sometimes the key letter is changed after two, three, or four
letters.
It is a matter of minutes only to run through the alphabet and learn
the meaning of a message so enciphered.