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).