37. | Learn Encryption Techniques with BASIC and C++

Learn Encryption Techniques with BASIC and C++
(Publisher: Wordware Publishing, Inc.)
Author(s): Gil Held
ISBN: 1556225989
Publication Date: 10/01/98

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To illustrate the use of two keyword-based mixed alphabets, assume the keyword SANDIEGO is used to form the plaintext alphabet and the keyword BALTIMORE is used to form the ciphertext alphabet. Figure 3.4 illustrates the resulting mixed plaintext and ciphertext alphabets.

Figure 3.4 Dual mixed alphabet relationship.

Suppose you wish to encode BID. To do so, you would perform the same operation as previously discussed. That is, you would first locate B in the mixed plaintext alphabet and note its position in that alphabet. Next, you would extract the character in the ciphertext alphabet that corresponds to the location of B in the plaintext alphabet, resulting in B being replaced by E. Next, the letter I would be located in the mixed plaintext alphabet; the letter I would be extracted from the ciphertext alphabet. Similarly, D would be replaced by T.

Sometimes luck works both ways. During World War I the British War Office developed a cipher device which they planned to distribute to British forces as a field cipher. So highly regarded was the device that one argument against its adoption for field use was its possible capture and use by the Germans—a situation many high-level personnel in the Allied forces thought would preclude the ability of the Allies to decipher enemy messages. To verify the security afforded by their device, the British submitted five short enciphered messages to the only formal cryptologic organization in the United States.
Located on a 500-acre estate named Riverbank at Geneva, IL, the Department of Ciphers at Riverbank was headed by William Frederick Friedman, considered by some as America’s greatest cryptologist. Although Friedman was able to determine the keyword of one of the mixed alphabets, apparently he hit the proverbial “blank wall” in attempting to determine the other keyword. Turning to his wife, he asked her to make her mind a blank and tell him the first word that came to mind when he said a word. When he said the first keyword, “CIPHER,” she replied, “MACHINE.” The intuitive guess of his wife was correct, and Friedman cabled the plaintext solution of the five messages to London, dooming the adoption of the device many persons were afraid would, if captured by the Germans, preclude the Allies from solving enemy messages.

Weaknesses

Note that although two different keywords were used, many characters in the ciphertext alphabet are the same as their plaintext equivalents, especially towards the end of each alphabet. This is one of several weaknesses of mixed alphabets, as the encipherment of a long message may provide numerous hints concerning the contents of a message when ciphertext and plaintext characters coincide. It is more practical to use one keyword-based alphabet and an appropriate alphabetic shift key to eliminate the overlapping of plaintext and ciphertext characters than to simply use two keyword mixed alphabets.

The second major weakness of all keyword-based monoalphabetic substitution techniques is the fact that a continuous one-for-one character replacement forms the basis of this category of enciphering techniques. This means that the techniques are very vulnerable to decipherment by a person performing a frequency analysis of characters in intercepted enciphered messages. For example, as Samuel Morse noted in developing the assignments of dots and dashes to characters in the English language in forming the code that bears his name, the letter E is the most frequently occurring character, followed by T. Thus, Morse assigned a dot to denote E and a dash to denote T prior to assigning sequences of dots and dashes to denote other characters in his code.

Suppose, for example, that you intercepted a message enciphered using the plaintext-ciphertext relationship illustrated in Figure 3.4. Further assume that you performed a frequency analysis of the characters in the enciphered message and discovered that the letter M occurred most frequently, followed by the character S. In this case, your first attempt to reconstruct the plaintext-ciphertext alphabet relationship in order to decipher the message would result in placing the character E in the plaintext alphabet over the character M in the ciphertext alphabet, followed by placing the character T in the plaintext alphabet over the character S in the ciphertext alphabet. Although a frequency analysis of characters may not provide an exact relationship between plaintext and ciphertext characters, the analysis normally provides enough intelligent clues to conduct a successful decipherment if you gain access to one long message or a few short messages enciphered using the same alphabetic relationship.

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