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