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).

