daily in many newspapers across the country. In these puzzles, the
player begins by analyzing visually-based linguistic clues, and then,
by deduction, begins guessing which letter replaces the ciphertext
letter in a trial-and-error manner. Examples of clues include the commonness of three-letter words such as the and and. Also, words with
an apostrophe offer strong clues: the letter following the apostrophe
could be an s, t, or m.
Substitution ciphers can be broken down into two subclasses:
monoalphabetic and polyalphabetic. The above-mentioned cryptoquote puzzle is an example of a pure, monoalphabetic scheme. Here,
each occurrence of the letter in the plaintext form is always replaced
by the same letter in the ciphertext, in a one-to-one relationship [ 4].
There are certain inherent weaknesses in monoalphabetic schemes.
One of these is that recurring letters make deciphering the encryption
easier. For example, the word LADDER, with the two adjacent Ds in
the middle, would yield ciphertext with two like, adjacent letters in the
middle. This reduces the mathematical complexity of deciphering the
message [ 4]. Substitution ciphers require some sort of mapping key in
their implementation. Such mapping keys may be rendered in different forms that will be examined shortly.
Monoalphabetic ciphers are sometimes rendered in a “shift” fashion. That is, the message is encrypted by shifting backward or forward
within the alphabet for the cipher-match by a given quantitative difference in alphabetical position. For example, let us say that the shift
key is + 3. Then in this system, A = D, B = E, etc. To decipher, you simply move ahead (in a positive shift) the number of the shift along the
alphabet. One of the more well-known renditions of this system from
history is the Caesar-Shift [ 10]. Obvious weaknesses here include the
ease of breaking the code, the need to have knowledge of the key at
both ends, and transportation of the shift key. Another way to render
a monoalphabetical cipher is to utilize a random set of charac-ter/symbol matches. Again, both sender and receiver need tables of
matches, and transporting the map could present security problems for
such a system. However, a random match cipher is stronger than a
shift-cipher since a simple shift cipher can be broken with relative ease
by repetitive effort until an intelligible result emerges. These types of
ciphers can be thought of as code encryption at the character level.
A more sophisticated and robust means of cipher-based encryption
is found in the polyalphabetic scheme. Here, letters or symbols are replaced by different symbols in the ciphertext by using more than one
replacement alphabet. For example, if the name ABRAHAM were to be
encrypted with a polyalphabetic system, the result could be something
like BFGHURF. Notice that each occurrence of the letter A is replaced
by a different letter. Some of the earliest known polyalphabetic schemes
utilized hand-held, rotating disks and printed tables, including the famous Vigenère’s table, developed by the 16th century French cryptographer Blaise de Vigenère [ 10]. Figure 1 shows one of the more popular versions of Alberti’s disk, a rotating cipher disk from history [ 10].
The cipher disk consists of two disks with a common hub. There is
an inner disk that rotates and a fixed outer disk. Characters on the two
disks will match when aligned. Both sending and receiving parties utilize the same disk, and the setting of the initial alignment is agreed
upon before communicating. This starting base is considered the key.
A rotating cipher disk may be employed in either a mono- or polyalphabetic encryption scheme. In a monoalphabetic implementation,
the disk simply maps one letter to another. A polyalphabetic scheme
Figure 1: Example of a cipher disk, Alberti’s disk.
may also be employed with the cipher disk. Instead of using the same
alignment throughout the entire message, the polyalphabetic method
calls for shifts in disk alignment at predetermined intervals. In this
way, more than one alphabet is used in the process. There are as many
mapped matches to a character as there are shifts. These ciphers are
much harder to break than are the monoalphabetic versions.
The other major class of cipher is the transposition cipher. In this
method, symbols are not replaced by others, but instead are altered
in position within a block or word of plaintext. In a simple transposition cipher, the plaintext FIGHT could be rendered in ciphertext as
TIFHG. Transposition ciphers utilize a mapping key that serves as a
pointer to the cipher symbol. The first step is to set the message into
blocks for mapping, then perform the encryption or decryption
process on them [ 4]. Let us look at a simple mapping scheme:
The encryption algorithm, utilizing this map, or key, would transpose
the plaintext BACD as DCBA and in turn, the decryption system
would map in reverse to expose the plaintext.
As mentioned in the discussion of codes, there are encryption
methods that utilize both code and cipher methods to create a layered encryption that is stronger and more difficult to break. Let us
consider a simple example of a code-and-cipher method using the
missive, “move to the west.” First, we encode the plaintext according
to codebook entries.
to the south
to the west
The encrypted message is gads aeilbret. Notice also that this code
accomplishes some compression in the process by reducing the number of characters to be transported. Many times a compression facility is built into coding schemes. Now this encoded message can be