12. | 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

Table of Contents

In a varying substitution cipher, the replacement of plaintext by ciphertext results in a very high probability that repeating plaintext characters are replaced by different ciphertext characters. To accomplish this requires the use of more than one “mapping” alphabet to convert plaintext to ciphertext, a term referred to as polyalphabetic substitution. As an alternative, a random or pseudo-random sequence can be used to generate a variable ciphertext replacement. Varying substitutions through the use of a pseudo-random sequence generated by a key is the basis for many commercially developed enciphering systems as well as the well-known data encryption standard (DES). In separate chapters in this book we will examine the construction of monoalphabetic, polyalphabetic, and pseudo-random number based cipher systems.

Prior to the start of World War II, the U.S. Army Signal Corps successfully constructed a cipher machine called PURPLE that duplicated the cryptographic operations of a Japanese machine. Intercepted Japanese transmissions were fed into PURPLE, producing plaintext information that was then translated into English and distributed under the code name MAGIC to President Roosevelt and top-level civilian and military officials within his administration. Although a series of urgent messages from Tokyo to the Japanese ambassador in Washington was intercepted and deciphered, this then-secret effort did not prevent the Pearl Harbor attack simply because Japan never transmitted a message stating they would attack Pearl Harbor! Whether intentional or not, the lack of an explicit attack message resulted in a degree of complacency within the U.S. government.
The ability to read enciphered messages, while important, does not necessarily mean you will always be able to understand the intentions of others. While effective in many situations, the ability to read enciphered messages is no substitute for the analysis of the contents of those messages. For example, in June of 1942, U.S. naval intelligence decoding of intercepted Japanese transmissions noted that one Japanese unit gave Midway as its post office address. By concentrating U.S. naval forces to meet the Japanese threat, the battle of Midway represented the turning point of the war in the Pacific.

Transposition Cipher Systems

As its name implies, a transposition cipher system rearranges the characters in a message. An elementary transposition system could simply swap plaintext character positions so that position n is mapped to n+1 and position n+1 is mapped to position n. Then, the message KILL ALL THE LAWYERS would become IKLL LAT LEH ALYWRES. In the preceding example, note that a visual observation immediately provides a good clue as to the meaning of the plaintext.

As an alternative to the transposition of plaintext characters, some cipher systems are based upon the use of an algorithm which transposes the characters of a mapping alphabet or series of mapping alphabets. A variety of transposition systems are covered in a later chapter of this book.

Electronic Mail Transmission Considerations

Until now, I have simply glossed over the contents of the plaintext alphabet and the resulting ciphertext alphabet. The plaintext alphabet represents all possible characters in the character set used to develop a plaintext message. The ciphertext alphabet represents all possible enciphered characters resulting from the enciphering process. It is the plaintext character set and the enciphering process that generates the ciphertext character set you must consider to successfully transmit an enciphered message through many electronic mail systems. The reason for this is the fact that most computer alphabets use eight bits to represent a character. This results in 2⁸, or 256, unique characters that can be used to represent both plaintext and ciphertext alphabet sets. Unfortunately, several electronic mail systems are restricted to transmitting seven-bit characters, with the eighth bit in a transmitted byte used for parity. Such electronic mail systems are restricted to transmitting 2⁷, or 128, characters. This means that many cipher systems, such as the DES algorithm, that are not restricted with respect to the ciphertext alphabet set it generates cannot be used to directly transfer data across certain electronic mail systems.

Transmitting randomly generated ciphertext that can represent any of the 256 unique characters in an 8-bit character set over a 7-bit electronic mail system requires a conversion of each 8-bit character. One of the earliest programs developed to perform this conversion is UUENCODE, which converts three 8-bit characters into four 7-bit characters. Another program known as UUDECODE is used to reconvert each group of four 7-bit characters transmitted via a 7-bit electronic mail system back into their original three 8-bit characters. Although you can use the UUENCODE/UUDECODE program pair or similar software products to transmit a message created using an 8-bit character set via a 7-bit electronic mail system, all recipients must have the decoding program associated with the encoding program.

To obtain the capability to transmit ciphertext via certain electronic mail systems without requiring recipients to have a 7-bit to 8-bit conversion program, you need to place limits on both the plaintext alphabet set as well as the algorithm used to create ciphertext. Several methods you can use to limit the plaintext alphabet set and the algorithm used to generate a ciphertext alphabet are discussed at appropriate points in succeeding chapters.

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