MANUAL
FOR THE SOLUTION OF
MILITARY CIPHERS
BY
PARKER HITT
Captain of Infantry, U. S. A.
PRESS OF
THE ARMY SERVICE SCHOOLS
Fort Leavenworth, Kansas
1916
MANUAL FOR THE SOLUTION
OF MILITARY CIPHERS
BY
PARKER HITT
Captain of Infantry, United States Army
INTRODUCTION
The history of war teems with occasions where the interception of
dispatches and orders written in plain language has resulted in
defeat and disaster for the force whose intentions thus became known
at once to the enemy. For this reason, prudent generals have used
cipher and code messages from time immemorial. The necessity for
exact expression of ideas practically excludes the use of codes for
military work although it is possible that a special tactical code
might be useful for preparation of tactical orders.
It is necessary therefore to fall back on ciphers for general military
work if secrecy of communication is to be fairly well assured. It
may as well be stated here that no practicable military cipher is
mathematically indecipherable if intercepted; the most that can be
expected is to delay for a longer or shorter time the deciphering of
the message by the interceptor.
The capture of messengers is no longer the only means available to
the enemy for gaining information as to the plans of a commander. All
radio messages sent out can be copied at hostile stations within radio
range. If the enemy can get a fine wire within one hundred feet of a
buzzer line or within thirty feet of a telegraph line, the message can
be copied by induction. Messages passing over commercial telegraph
lines, and even over military lines, can be copied by spies in the
offices. On telegraph lines of a permanent nature it is possible to
install high speed automatic sending and receiving machines and thus
prevent surreptitious copying of messages, but nothing but a secure
cipher will serve with other means of communication.
It is not alone the body of the message which should be in cipher. It
is equally important that, during transmission, the preamble, place
from, date, address and signature be enciphered; but this should
be done by the sending operator and these parts must, of course,
be deciphered by the receiving operator before delivery. A special
operators' cipher should be used for this purpose but it is difficult
to prescribe one that would be simple enough for the average operator,
fast and yet reasonably safe. Some form of rotary cipher machine
would seem to be best suited for this special purpose.
It is unnecessary to point out that a cipher which can be deciphered
by the enemy in a few hours is worse than useless. It requires a
surprisingly long time to encipher and decipher a message, using even
the simplest kind of cipher, and errors in transmission of cipher
matter by wire or radio are unfortunately too common.
Kerckhoffs has stated that a military cipher should fulfill the
following requirements:
1st. The system should be materially, if not mathematically,
indecipherable.
2d. It should cause no inconvenience if the apparatus and methods
fall into the hands of the enemy.
3d. The key should be such that it could be communicated and
remembered without the necessity of written notes and should
be changeable at the will of the correspondents.
4th. The system should be applicable to telegraphic correspondence.
5th. The apparatus should be easily carried and a single person
should be able to operate it.
6th. Finally, in view of the circumstances under which it must
be used, the system should be an easy one to operate,
demanding neither mental strain nor knowledge of a long series
of rules.
A brief consideration of these six conditions must lead to the
conclusion that there is no perfect military cipher. The first
requirement is the one most often overlooked by those prescribing
the use of any given cipher and, even if not overlooked, the
indecipherability of any cipher likely to be used for military purposes
is usually vastly overestimated by those prescribing the use of it.
If this were not true, there would have been neither material for,
nor purpose in, the preparation of these notes. Of the hundreds of
actual cipher messages examined by the writer, at least nine-tenths
have been solved by the methods to be set forth. These messages were
prepared by the methods in use by the United States Army, the various
Mexican armies and their secret agents, and by other methods in common
use. The usual failure has been with very short messages. Foreign
works consulted lead to the belief that many European powers have
used, for military purposes, cipher methods which vary from an
extreme simplicity to a complexity which is more apparent than
real. What effect recent events have had on this matter remains to
be seen. It is enough that the cipher experts of practically every
European country have appealed to the military authorities of their
respective countries time and again to do away with these useless
ciphers and to adopt something which offers more security, even at
the expense of other considerations.
The cipher of the amateur, or of the non-expert who makes one up
for some special purpose, is almost sure to fall into one of the
classes whose solution is an easy matter. The human mind works along
the same lines, in spite of an attempt at originality on the part of
the individual, and this is particularly true of cipher work because
there are so few sources of information available. In other words,
the average man, when he sits down to evolve a cipher, has nothing
to improve upon; he invents and there is no one to tell him that his
invention is, in principle, hundreds of years old. The ciphers of the
Abbé Tritheme, 1499, are the basis of most of the modern substitution
ciphers.
In view of these facts, no message should be considered
indecipherable. Very short messages are often very difficult and may
easily be entirely beyond the possibility of analysis and solution,
but it is surprising what can be done, at times, with a message of
only a few words.
In the event of active operations, cipher experts will be in demand
at once. Like all other experts, the cipher expert is not born or
made in a day; and it is only constant work with ciphers, combined
with a thorough knowledge of their underlying principles, that will
make one worthy of the name.
CHAPTER I
EQUIPMENT FOR CIPHER WORK
Success in dealing with unknown ciphers is measured by these four
things in the order named; perseverance, careful methods of analysis,
intuition, luck. The ability at least to read the language of the
original text is very desirable but not essential.
Cipher work will have little permanent attraction for one who expects
results at once, without labor, for there is a vast amount of purely
routine labor in the preparation of frequency tables, the rearrangement
of ciphers for examination, and the trial and fitting of letter to
letter before the message begins to appear.
The methods of analysis given in these notes cover only the simpler
varieties of cipher and it is, of course, impossible to enumerate all
the varieties of these. It is believed that the methods laid down are
sound and several years of successful work along this line would seem
to confirm this belief. For more advanced work there is no recourse
but to study the European authorities whose writings are mostly in
French, German, and Italian and, unfortunately, are rarely available
in English translations.
Under intuition must be included a knowledge of the general situation
and, if possible, the special situation which led to the sending
of the cipher message. The knowledge or guess that a certain cipher
message contains a particular word, often leads to its solution.
As to luck, there is the old miner's proverb: "Gold is where you
find it."
The equipment for an office, where much cipher work is handled,
will now be considered. The casual worker with ciphers can get along
with much less, but the methods of filing and keeping a record of all
messages studied should be followed wherever possible. The interchange
of results between individuals and between offices should be encouraged
and, in time of active operations, should be mandatory. An enemy
may be using the same cipher in widely separated parts of the zone
of operations and it is useless labor to have many cipher offices
working on intercepted messages, all in the same cipher, when one
office may have the solution that will apply to all of them.
Cipher work requires concentration and quiet and often must proceed
without regard to hours. The office should be chosen with these points
in mind. A clerical force is desirable and even necessary if there
is much work to do. The clerk or clerks can soon be trained to do
the routine part of the analysis.
It is believed that each Field Army should have such an office
where all ciphers intercepted by forces under command of the Field
Army Commander should be sent at once for examination. This work
naturally falls to the Intelligence section of the General Staff at
this headquarters. A special radio station, with receiving instruments
only, should be an adjunct to this office and its function should
be to copy all hostile radio messages whether in cipher or plain
text. Such a radio station requires but a small antenna; one of the
pack set type or any amateur's antenna is sufficient, and the station
instruments can be easily carried in a suit case. Three thoroughly
competent operators should be provided, so that the station can be
"listening in" during the entire twenty-four hours.
The office should be provided with tables of frequency of the language
of the enemy, covering single letters and digraphs; a dictionary and
grammar of that language; copies of the War Department Code, Western
Union Code and any other available ones; types of apparatus or, at
least, data on apparatus and cipher methods in use by the enemy; and
a safe filing cabinet and card index for filing messages examined. A
typewriter is also desirable.
The office work on a cipher under examination should be done on paper
of a standard and uniform size. Printed forms containing twenty-six
ruled lines and a vertical alphabet are convenient and save time in
preparation of frequency tables. Any new cipher methods which are
found to be in use by the enemy should, when solved, be communicated
to all similar offices in the Army for their information.
Unless an enemy were exceedingly vigilant and changed keys and methods
frequently, such an office would, in a few days, be in a position
to disclose completely all intercepted cipher communications of the
enemy with practically no delay.