PART FOUR

Polyalphabetic Substitution Systems

CHAPTER 8

PERIODIC POLYALPHABETIC
SUBSTITUTION SYSTEMS

Section I
Characteristics of Periodic Systems

Types of Polyalphabetic Systems
8-1.
All the substitution systems explained up to this point are monoalphabetic systems.
Whether they deal with one letter at a time or several, whether they have one cipher
equivalent for each plaintext letter or more than one, they are still systems with only
one alphabet. The constant feature that makes a system monoalphabetic is that a
given ciphertext value always translates into the same plaintext value. In
polyalphabetic systems, a given ciphertext value changes its plaintext meaning.

a. Most polyalphabetic systems are monographic; they encipher a single letter at a
time. Polygraphic polyalphabetics are possible, but have little practical military
value.
b. A typical polyalphabetic system will use from 2 to 26 different alphabets.
Polyalphabetic systems which repeat the same set of alphabets over and over again
in the same sequence are known as periodic systems. Polyalphabetic systems which
do not keep repeating the same alphabets in the same order are known as aperiodic
systems. Periodic systems, because of their regular repeating keys, are generally
less secure than aperiodic systems. Aperiodic systems, on the other hand, are
generally more difficult to use, unless the encipherment is done automatically by a
cipher machine or computer.

c. The classic types of polyalphabetic systems use a set of alphabets, such as the 26
alphabets pictured in Figure 8-1. Figure 8-1, known as a Vigenere square, includes
all possible alignments of a direct standard alphabet. Mixed alphabets can also be
used in such a square. If all 26 alphabets are used, any letter can equal any other
letter. There are necessarily three elements to the encryption process with
polyalphabetic ciphers, which the square and the accompanying examples
illustrate. The plaintext letters are listed across the top of the square. The cipher
equivalents are found in the 26 sequences below. The final element is the key that
designates which alphabet is used at any given time. The key letter is found on the

8-1
left side of the square. The first example in Figure 8-1 shows the use of a repeating
key based on a keyword. Since the same key is repeated over and over again, the
resulting system is periodic. The second example uses a nonrepeating key based on
a quotation. Since this key does not repeat, it is an aperiodic system. Note that the
reuse of the same alphabets does not constitute a repeating key. For the system to
be classified as periodic, the same alphabets must be reused over and over again in
the same sequence.

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d. Another way to picture the same system as the first example in Figure 8-1 is shown
below. In this case, instead of using the complete alphabet square, only the
alphabets actually used are shown. These alphabets are used repeatedly to produce
the same results. In this example, the key is expressed in terms of the number of
the cipher sequence used, instead of by the repeating key letters.

e. Another type of polyalphabetic system does not use multiple alphabets in the
classic sense, but instead enciphersa message in a single alphabet. Then it applies
either a repeating key or nonrepeating key to the first encipherment to create a
polyalphabetic. One method of applying a polyalphabetic key to a monoalphabetic
encipherment is to use a numeric system and arithmetically add a key to it. For
example, here is a dinomic system, which has been further enciphered by a
repeating numeric additive. The first encipherment is labeled I, for intermediate
cipher, and the second encipherment is labeled C. The 8-digit repeating key is
labeled K. Modulo 10 arithmetic is used (paragraph 5-3f(1)).

f. Another approach to applying a polyalphabetic key begins with the built-in
encoding system used by teleprinters or computers. Paragraph 8-2 shows examples
of these.

Machine Based Polyalphabetics
8-2.
When text is sent electronically by radio or wire, some form of coding must be used.
The earliest system of coding for electronic transmission was Morse code, which is still
used widely today. When teleprinters took their place in communications, a new

8-3
binary type of coding system was devised, which can be handled by machine more
easily than Morse code can. Any binary coding system uses only two characters, which
can be represented electronically as a signal pulse or no signal pulse, high voltage or
low voltage, or one frequency or another frequency. Which of these approaches is used
depends on the equipment in use and is not our concern here. We are concerned with
how the two binary characters, whatever their electronic origin, are combined to repre-
sent alphabetic, numeric, and special characters, and how they may further be en-
crypted. Various notations have been used to represent the two binary charactersâ€”Xs
and 0s, 1s and 0s, +s and -s, or Ms (for marks) and Ss (for spaces). We will use 1s and
0s in this text, but you should be aware that you may see other notations elsewhere,
particularly in older literature.

a. The Baudot Code. Teleprinter systems generally use a 5-digit binary code known
originally as the Baudot code. There are 32 possible combinations of 5 digits, which
are not enough for the letters, numbers, and printer control characters needed for
communications. The number of possible characters is approximately doubled by
the use of upper and lower shift characters, similar to the shift key on a typewriter,
giving all characters two alternate meanings except the shift characters themselves
and the space character. There are still not enough characters for upper and lower
case letters, so all traffic passed by such teleprinter systems use capital letters only.
The standard international teleprinter code is shown in Figure 8-2. Each dot repre-
sents a 1 and each space represents a 0. Other codes are also used besides the one
shown.

The binary digits themselves are known as baudsâ€”a term derived from the Baudot
code. The terminology has carried over into modern computer. systems as well.
Polyalphabetic keys, also in 5-digit binary form, are easily applied to coded text

8-4
electronically by baud addition. An example of this process is shown below.
Although other rules are also possible, the addition of key and plaintext bauds is
usually accomplished by the rule, Like values sum to 0; unlikes sum to 1. (In com-
puter logic, this would be called an exclusive OR, or XOR operation.)

One advantage of this rule of addition is that adding the same key to the ciphertext
produces the plaintext again.

b. Computer Codes. Communications between computers use more than 5 digits.
Typical computer codes use either 7- or 8-binary digits (bits), giving a range of
128 characters or 256 characters. These permit upper and lower case letters, a full
range of punctuation marks and special characters, and a number of codes to con-
trol printers and communications devices as well. With the 8-bit, 256 character set,
graphics may also be enabled to permit transmitting pictures as well as text. The
most common standard for the first 128 characters, whether 7-bit or 8-bit, is the
American standard code for information interchange (ASCII) standard, which you
can find in many computer manuals. Encipherment and decipherment can be
accomplished in 7- and 8-bit operation just as was shown for 5-digit teleprinter
operations. The more complex systems are far beyond the scope of this manual, but
simple repeating key systems can be solved using the techniques discussed here.
One problem that computer codes present is that less than half of the possible 7-bit
characters are letters and numbers, and many of them stand for printer control
codes that do not print out as characters normally. Working with binary numbers
themselves is unwieldy, but any 7- or 8-bit value can be represented by two hex-
adecimal (base 16) arithmetic digits. Hexadecimal arithmetic is not explained
here, but explanations are available in many computer manuals and texts, if
needed. Hexadecimal and binary numbers are also explained in Army Correspon-
dence Course Program Subcourse SA0709.

Section II
Identifying Periodic Systems

Analysis of Repeated Ciphertext
8-3.
Polyalphabetic systems normally have very flat frequency counts. The phi IC is
normally close to the random expectation of 1.00. Since other systems, including

8-5
variant multiliterals and aperiodic systems, also can produce flat frequency counts,
this is not enough to identify a system as periodic. The key to identifying a system as
periodic is to recognize through repeated ciphertext that a repeating key is used.

a. Repeated ciphertext can occur in two ways. Whenever the same plaintext is
enciphered by the same keys, the ciphertext will also repeat. Such repeats are
called causal repeats. The second way that ciphertext can repeat is by pure chance.
Different plaintext enciphered with different keys will sometimes produce short
ciphertext repeats. Causal repeats are much more likely to occur than accidental
repeats, particularly if they are longer than two or three characters. The example
below, repeated from Section I, shows how causal repeats occur.

The plaintext words ZERO and TWO both occur twice. The repeated ZEROs lined
up with the same alphabets, producing a ciphertext repeat. The repeated TWOs
lined up with different alphabets and did not produce a ciphertext repeat.
b. Whenever causal repeats occur, the distance between them must be a multiple of
the period length. In the example above, the two ZEROs occurred 10 letters apart.
Note that the distances are counted from the first letter of one repeat to, but not in-
cluding, the first letter of the second repeat. If the distance was not a multiple of the
period five, the ciphertext repeat would not have occurred.

c. The distance between causal repeats is a multiple of the period length. Given a
cryptogram of unknown period that includes ciphertext repeats, the period can be
determined, or at least narrowed down, by analyzing the distances between repeats.
The period must be a factor of the distance. The factors of a number are all the
numbers which divide evenly into that number. When there is more than one
repeat, the period must be a common factor of all such distances. For example, if a
cryptogram has repeats that are 28, 35, and 42 letters apart, the only number that
evenly divides all the distances is 7. The period must be 7. Utility tables showing
common factor numbers are in Appendix E.

d. Here is a more complex example. Suppose a cryptogram suspected of being periodic
includes the following repeats.

8-6
The next step after determining the distances is to list the factors for each repeat, as
shown below.

No numbers evenly divide the distances between all the repeats. In such cases,
either the system was not a periodic system, or one or more of the repeats is
accidental. In this problem, the SRM repeat is probably accidental, because it is
the shortest. Discarding the SRM repeat from consideration, the remaining repeats
all have common factors of 2, 3, and 6. Where more than one factor is possible, it is
generally safest to assume the largest. If the period is actually 3, for example, it will
reveal itself by repeated alphabets as the cryptogram is solved.

Analysis by Frequency Counts
8-4.
Periodic systems can be identified even when there are no repeated words in the text.
Causal single-letter ciphertext repeats will still occur and significantly outnumber the
accidental single-letter repeats.
a. To find the causal single-letter repeats, take frequency counts for each alphabet
according to its position in the suspected repeating cycle. If the period is incorrect,
the separate frequency counts will remain flat. If the period is correct, the separate
frequency counts will be as rough as plaintext on the average. Recognizing when a
count is rough or flat is difficult by eye, particularly with anything but very long
cryptograms, but the phi test performed on each separate alphabet gives a reliable
indication. Taking separate frequency counts by position for each suspected period
and then calculating phi tests on each is a laborious and time-consuming process by
hand. It can be done when necessary, but it is best performed by computer support.
Figures 8-3, 8-4, and 8-5 show computer generated output for suspected periods of 6,
7, and 8 for the following cryptogram.

8-7
b. The average ICs for each period in Figure 8-3 and 8-4 are flat, The average IC for a
period of 8 in Figure 8-5 is much higher than the other two. This clearly shows that
the period of 8 is more likely correct than periods of 6 and 7.

c. The computer program used to generate these examples is listed in Appendix F. It is
written in GW BASIC, and is readily adaptable to many different computers.

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