220 lines
12 KiB
Plaintext
220 lines
12 KiB
Plaintext
Copyright - 1992 Grolier Electronic Publishing, Inc.
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cryptology
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Cryptology, the branch of knowledge that concerns secret writing
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or communications in code or cipher, originated in human desire
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to communicate secretly and is as old as writing itself. The word
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derives from the Greek kryptos ("hidden") and logos ("word").
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EARLY HISTORY OF SECRET WRITING
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Methods of secret communication were developed by many ancient
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societies, including those of Egypt, Mesopotamia, India, and
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China, but details regarding the origins and early evolution of
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cryptology are unknown. About 400 BC the Spartans used a system
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of secret writing, the scytale, a cylindrical rod around which
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the sender wrapped a length of parchment or papyrus in a spiral.
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Words were then written lengthwise along the rod, one letter on
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each revolution of the strip. Once unrolled, the strip showed
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nothing but a succession of meaningless letters; to be read, the
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strip had to be wrapped around a rod of exactly the same diameter
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as the first.
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Julius Caesar is said to have used a simple letter substitution
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method of secret writing in his correspondence. Caesar's method
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consisted of writing the ordinary alphabet from left to right,
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and beneath, another normal alphabet shifting three letters. The
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letter A was replaced by D, the letter B by E, and so on. Thus
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the Latin word omnia appeared as RPQLD. This method is still
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called the Julius Caesar cipher, regardless of how many letters
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the lower alphabet is shifted.
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In the latter part of the Middle Ages the use of secret writing
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increased. For example, The Equatorie of the Planetris (c.1390),
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a work attributed to Geoffrey Chaucer, contains passages in
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cipher.
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In 1470, Leon Battista Alberti published Trattati in cifra, in
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which he described a cipher disk capable of enciphering a small
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code. Most authorities, however, consider Johannes Trithemius,
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abbot of Spanheim in Germany, to be the father of modern
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cryptography. In 1510, Trithemius wrote Polygraphia, the first
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printed work on cryptology. He introduced for the first time the
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concept of a square table, or tableau, in which the normal
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alphabet was successively shifted.
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Each alphabet in turn was used to encipher successive letters.
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For example, if the first letter is enciphered with the first
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alphabet, the second letter with the second alphabet, and so on,
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the word secret would be enciphered as SFEUIY.
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TECHNICAL ASPECTS OF CRYPTOLOGY
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Cryptology is divided into two general fields, cryptography and
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cryptanalysis. Cryptography concerns the methods of converting
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plaintext (also known as cleartext) into ciphertext. Ciphertext
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messages are called cryptograms. Cryptanalysis concerns the
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methods of solving or reading cryptograms without their keys.
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Today, experienced and knowledgeable cryptologists agree that a
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number of cryptographic systems are unsolvable by analytic
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techniques. Cryptographic systems in which a key is used only
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once, known as holocryptic systems, can be mathematically proven
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to be analytically unsolvable. Other cryptographic systems,
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especially those using electrical devices, can often be
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completely secure from a practical viewpoint against
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cryptanalytic attack. Even so-called paper and pencil systems
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can be constructed in which analytic solutions are virtually
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impossible. Nonetheless, the most theoretically secure
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cryptographic system can be vulnerable to solution if the system
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is incorrectly used in some manner or if there is a partial or
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complete physical compromise of the system.
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Cryptographic systems invented by amateurs or nonexperts will
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almost always be either nonpractical or cryptographically weak.
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The amateur usually overlooks the problems inherent in electrical
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or telegraphic transmission, such as whether messages received
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with many erroneous letters, or even with missing letters, can
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still be read by recipients. With any new cryptographic system,
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it must be assumed that the enemy, or adversary cryptanalyst,
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knows everything about the general system. Only specific keys
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can be presumed unknown.
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Codes
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When cryptographic treatment is applied to plaintext elements of
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irregular length, the cryptographic system is called a code. The
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letters or digits that replace the irregular length plaintext
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elements in a code are termed code groups. The plaintext
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elements with their accompanying code groups are found in a code
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book. If both the plaintext elements and the code groups run
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simultaneously in alphabetic or numerical order in the code book,
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the code is said to be a one-part code. If, however, the
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plaintext elements are in alphabetic order, and the code groups
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are not in order, or vice versa, the code is said to be a
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two-part code. In a one-part code the same book is used for both
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encoding and decoding. In a two-part code, two sections are
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required, one for encoding and one for decoding. A two-part code
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is normally more secure than a one-part code.
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Ciphers
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When cryptographic treatment is applied to plaintext elements of
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regular length, usually single letters or pairs of letters
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(digraphs), the cryptographic system is called a cipher. In a
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transposition cipher the plaintext letters are transposed
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following a prearranged plan decided upon by the correspondents.
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To facilitate transmission, the ciphertext is usually written in
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five-letter groups: TIIAR NPSTO CPEHS STASO IINIH R. This kind
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of a transposition is a railfence cipher. Transposition ciphers
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may use geometrical figures of all types; the rectangle is used
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most often. Thus, writing the plaintext normally into a
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rectangle, then reading the ciphertext down the columns from left
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to right.
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The ciphertext is TNXFP NHOAA OCITM TSISH PRIPI ELATH SRENI EAEOS
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OR. In a substitution cipher the plaintext letters are replaced
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by other, usually different, letters. In the Julius Caesar
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cipher the letters follow a normal progression, D for A, E for B,
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and so on. If the symmetry is broken and plaintext letters are
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replaced by mixed letters, the increased security is apparent.
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Such a system is called a monoalphabetic substitution cipher or
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simple substitution cipher.
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A message may be enciphered with more than one ciphertext
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alphabet, using perhaps a cipher square or tableau, such as the
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square table of Trithemius. Such a system is called a
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polyalphabetic substitution cipher.
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Cryptanalysis
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Cryptanalysis is the analytic solution of cryptographic systems
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without knowledge of the key. Most governments attempt to read
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the secret messages of their enemies or potential enemies because
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the "reading" of such messages provides a wealth of intelligence
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information. Cryptanalytic successes are rarely revealed because
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to do so would cause the enemies to change their cryptographic
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systems. Perhaps one of the most important cryptanalytic
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successes ever revealed was that of the British naval
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intelligence, which in early 1917 transmitted to the United
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States the text of a German message known as the Zimmermann
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telegram. In this message, the German ambassador in Mexico City
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was asked to approach the Mexican government with an offer of an
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alliance, the reward for which was Mexican possession of Texas,
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New Mexico, and Arizona. The Zimmermann telegram was possibly
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one of the most significant events leading to U.S. entry into
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World War I.
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Enigma, the cryptographic machine used by the Germans during
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World War II, was broken by means of cryptanalysis. The code
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word "Ultra" was used by the Allies to designate information
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derived from German secret messages. In addition, the success of
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the United States in reading Japanese codes during World War II
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helped shorten the war and save American lives.
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Cryptanalysis is successful principally because plaintext is not
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random. Not only do individual letters and words occur with
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definite frequencies, but certain letters and words appear
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together with predictable frequencies.
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As cryptographic systems become more complicated, however,
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sophisticated cryptanalytic techniques are required. Today the
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computer's ability to store millions of pieces of information is
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both an invaluable aid in cryptanalysis and itself an incentive
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to the development of high complex cryptographic systems, because
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of the wide range of sensitive information that now exists in
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computer databanks and is transmitted through computer networks.
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Such data are stored in ciphers so complex that only other
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computers can decipher them. Governments, banks, and
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manufacturers primarily make use of encryption systems that are
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based on the difficulty involved in factoring large numbers, as
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compared with the difficulty in finding out whether those numbers
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are primes (see PRIME NUMBER). Primes are used in coding systems
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by computer networks, which encrypt their data so that only those
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authorized users who have the proper "key" can decode the
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transmitted information. A "key," which determines the
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relationship between the plaintext and the ciphertext, is made up
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of a certain number of binary digits, or BITS--the basic units of
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digital computer data.
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The DES (data encryption standard) system developed by IBM and
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approved in 1976 by the U.S. National Bureau of Standards for
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governmental use employs a variable 56-bit "key." In DES, which
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has been widely adopted commercially, plaintext is converted into
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ciphertext by the encrypting operations of substitution and
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transposition, repeating the operations several times by means of
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special techniques that make the codes particularly hard to
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break. DES, however, shares with earlier systems the
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vulnerability inherent in a key exchange between a sender and a
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receiver. Other new systems, such as the so-called public-key
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systems, bypass the problem by making use of both a public
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encryption key and a secret decryption key that can be generated
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locally by the authorized receiver of the data. The public-key
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systems also depend upon large complex numbers for coding.
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In 1988 a group of U.S. researchers using hundreds of computers
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was able to factor a 100-digit number in just 26 days, a feat
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thought to be impossible a decade earlier. The ever-increasing
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power of computers and the development of more sophisticated
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factoring methods are forcing cryptographers to choose even
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larger and more cumbersome numbers on which to base code keys.
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Wayne G.
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Barker
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Bibliography: Barker, Wayne G., Manual of Cryptography (1981);
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Danning, Dorothy E., Cryptography and Protection (1982);
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Friedman, W. F., Elements of Cryptanalysis (1976); Gardner,
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Martin, Codes, Ciphers, and Secret Writing (1984); Kahn, David,
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Kahn on Codes (1983); Konheim, A. G., Cryptography: A Primer
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(1981); Mayer, Carl, and Matyas, Stephen, Cryptography: New
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Dimensions in Computer Security (1982); Meyer, C., and Matyas,
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S., Cryptography (1982); Pierce, C. C., Crypto-privacy (1988);
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Sinkov, Abraham, Cryptanalysis: A Mathematical Approach (1980);
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Winterbotham, F. W., The Ultra Secret (1978); Wolfe, James R.,
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Secret Writing: The Craft of the Cryptographer (1970).
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