Information Technology Reference
In-Depth Information
storing any data and decode all information before it is displayed to
an authorized user.
The Caesar Cipher: With encoding and decoding, of course, the
level of security depends upon the ciphering system actually used.
During the Gallic Wars, Julius Caesar encoded messages to his troops
by replacing each letter by the third letter after it in the alphabet.
Applying this idea to the modern English alphabet, “a” would be en
coded as “d”, “b” as “e”, and so forth. At the end of the alphabet,
“w” would be coded as “z”, “x” as “a”, “y” as “b”, and “z” as “c”.
Although this system, now called a
Caesar Cipher
, seems very simple,
the code was never broken by Caesar's enemies, and it served as a se
cure form of communication.
Today, the art and science of cryptanalysis has become very sophis
ticated, and simple codes such as a Caesar Cipher can be broken very
quickly and easily.
1
Secure codes must be much more sophisticated.
The Caesar Cipher, for example, has at least two major weak
nesses. Every letter is coded by the third (or fourth or
n
th) letter after
it, so once the code for one letter is determined, the codes for every
other letter also are known. To illustrate, consider the coded message
Igkygx iovnkxy gxk kgye zu ixgiq
and suppose you know (or guess) that this has been sent using a
type of Caesar cipher, but you do not know if the shift is one letter,
two letters, three, or more.
To decipher the message, you could simply try each of the
26 possible shifts of the alphabet and see which line makes sense.
In the case of the previous message, the choices are shown in
Table 10.1.
With this approach, a simple trialanderror process produces the
actual message with little difficulty. (In this case, A was coded G.)
A second weakness inherent in the Caesar Cipher is that every
letter in the actual message is encoded each time by the same letter in
the cipher alphabet. (In this example, A always appears as G, etc.)
This allows people trying to break a code to take advantage of statis
tical properties of English. Although there can be variations among
different texts, E, for example, is usually the most frequently used
1 For an interesting account of a variety of simple techniques for breaking codes, see
Abraham Sinkov,
Elementary Cryptanalysis, A Mathematical Approach
, The New
Mathematical Library, New York: Random House and the L. W. Singer Company, 1968.
