OTHER SUBSTITUTION METHODS
The foregoing cases by no means exhaust the possibilities of the
substitution cipher but they cover practically all methods which
are satisfactory for military purposes, having in mind conservation
of time, the minimizing of mental strain, and the requirements that
complicated apparatus and rules be avoided, and that the resulting
cipher should be adapted to telegraphic correspondence.
A message may be re-enciphered two or more times using a different key
word each time or it may be enciphered by one method and re-enciphered
by another method, using the same or a different key word. Complicated
cipher systems requiring the memorizing of, or reference to, numerous
rules have been devised for special purposes. Such systems usually
fail utterly if there are any errors in transmission and it will be
seen later that such errors are very common.
There are several ingenious cipher machines by which complicated
ciphers can be formed, but if the apparatus is available and fairly
long messages are at hand for examination, it is usually possible to
solve them. Such machines are not, as a rule, simple and small enough
for field use; and it must always be remembered that a machine cipher
operates on certain mechanical cycles, which can be determined if
the machine is available.
A book by Commandant Bazeries, entitled "Etude sur la Cryptographie
Militaire," and a series of articles by A. Collon, entitled "Etude
sur la Cryptographie," which appeared in the Revue de L'Armée Belge,
1899-1902, give illustrations and details of operation of several
of these cipher machines and the latter goes into the methods of
deciphering messages enciphered with them. These methods of analysis
require long messages, and as each one is adapted only to the product
of a certain machine or apparatus, it is not considered advisable to
include a discussion of them here. Those interested in such advanced
cipher work must refer to these and other European authors on the
subject.
The requirement that cipher messages should be adapted to telegraphic
transmission, practically excludes ciphers in which three or more
letters or whole words are substituted for each letter of the plain
text. Such ciphers might be used for the transmission of very short
messages but in no other case.
The cipher of Case 7, with a key word or phrase longer than one-fourth
of the message, the cipher after the method of Case 7, using a
certain page of a book as a key, and the cipher with a running
key, where each letter of the cipher is the key for enciphering
the next letter, all look safe and desirable, theoretically, but,
practically, the work of enciphering and deciphering is hopelessly
slow, and errors in enciphering or transmission make deciphering very
difficult. Incidentally the first and second of these ciphers can
be solved by the special solution for Case 7, and the third can be
solved by trying each of the twenty-six letters of the alphabet as
the first key letter, and then continuing the work for five or six
letters of the cipher. When the proper primary key letter is found,
the solution of the next five or six letters of the cipher will make
sense, and thereafter the cipher offers no difficulty.
There are numerous other methods of preparing what is virtually a
very long, or even an indefinitely long key from a short key word,
but all such cipher methods have the same practical disadvantages
of slowness of operation and difficulty in deciphering, if errors of
enciphering or transmission have been made.
The ciphers of Napoleon were long series of numbers representing
letters, syllables and words. They were really codes; and a code
based on these principles, but using letters instead of numerals,
might be evolved very easily. The War Department Code, the Western
Union Code, and, in fact, all codes are nothing but specialized
substitution ciphers in which each code word represents a letter,
word or phrase of the plain text.
Combined Transposition and Substitution Methods
It is evident that a message can be enciphered by any transposition
method, and the result enciphered again by any substitution method,
or vice versa. But this takes time and leads to errors in the work, so
that, if such a process is employed, the substitution and transposition
ciphers used are likely to be very simple ones which can be operated
with fair rapidity.
On preliminary determination, a cipher prepared by such a combination
of methods will appear to be a substitution cipher to be solved as
such. The frequency table of the result will resemble the normal
frequency table, although the message will still be unintelligible
and we will know at once that it is a transposition cipher for
further solution.
The substitution methods usually found in combination ciphers are
those of Case 4, 5 and 6, and the transposition method is nearly
always Case 1, and particularly the simple varieties of this case
like the fence rail (Case 1-i), reversed writing or vertical writing.
A few examples will show some of the possible combinations.
The first line of the message of Case 4-a is:
OBQFOBPBRP
We might write it BFBBPOQOPR (Case 1-i), or PRBPBOFQBO (Case 1,
reversed writing), or OFQBOPRBPB (Case 1, reversed by groups of five).
The first line of the message of Case 2-b is:
SLCOF WEETN EBRDO ORVYM FFEDI
We might write it TMDPG XFFUO FCSEP PSWZN GGFEJ, or RKBNE VDDSM DAQCN
NQUXL EEDCH (Case 4-a, going forward one letter or back one letter).
These examples give an idea of the use of combination methods. It is
very rare to find both complicated transposition and substitution
methods used in combination. If one is complicated, the other will
usually be very simple; and ordinarily both are simple, the sender
depending on the combination of the two to attain indecipherability. It
is evident how futile this idea is.
Methods of Enciphering Numerals
It is frequently desirable to send numerals in the body of a cipher
message. Several cipher systems prescribe that all numerals in the
body of a message must be spelled out; and, while there is no doubt
but that this insures greater accuracy, it also greatly increases the
length of such messages. In most systems in which it is permissible
to send numerals, the following system is used. An indicator, one of
the little used letters and especially X, is interpolated before and
after the numeral or numerals to be enciphered, and then, for each
numeral, a letter is substituted using this or a similar table:
1 2 3 4 5 6 7 8 9 0
A B C D E F G H I J
The enciphering of the message then proceeds, dealing with the
indicator and substituted letters as if they were the letters of a
word. The decipherer arriving at an X, a series of the letters of the
above table and another X, casts out the X's and substitutes numbers
for the letters.
Sometimes no indicator is used, but the system of substitution of a
certain letter for each numeral is followed. Again, the indicator NR
may be used instead of a single letter.
Conventional letters may also be substituted for special characters
like ?, $, ", -, and periods and commas, but this is rarely done except
for the period and question mark. The context will usually determine
the meaning of such letters when found. In this connection, the use
of X to represent end of a sentence and Q to represent a question
mark is quite common.