CRYPTOGRAPHY

	Cryptography is the art of secret writing. It involves transforming information into 
apparently unintelligible garbage so that unwanted eyes will be unable to comprehend it. This 
transformation, however, must be done so that it is reversible, so that individuals intended 
to view the information may do so. This is the traditional use of cryptography. 

A HISTORY OF CRYPTOGRAPHY

	This topic provides an overview of some of the classical methods of cryptography and 
some idea of how they evolved. None of the methods described here is used today, because they 
are considered either insecure or impractical. We begin with some definitions:

	Definition One
	A cipher, or cryptosystem, is a pair of invertible functions:
fk  ( fk known as the enciphering function), which maps from a set S to a set T, based
    on a quantity k called an enciphering key.
gk' (gk' known as the deciphering function), the inverse of fk. k’ is known as the 
    deciphering key.
 
	The function fk maps an element x in S to an element fk(x) in T so that determining
the inverse mapping is extremely difficult without knowledge of k’.
An element of S is called plaintext, whereas an element of T is called ciphertext.
Some ciphers are better at satisfying this definition than others. The terms encipher and 
encrypt are synonymous, as are the terms decipher and decrypt.

	Definition Two
	
	If, for some cipher k=k’, or if k’ is easily computable given k, such a cipher is
called a secret key cipher. However, if k’ is extremely difficult to obtain even with 
knowledge of k, such a cipher is called a public key cipher. In this case k is called a 
public key, whereas k’ is called a private key.

	Definition Three

	Cryptology consists of two disciplines: cryptography and cryptanalysis.
	Cryptography refers to the study of concealing information with the use of 
	mathematical transformations. One who studies and/or practices cryptography
	is called a cryptographer.
	Cryptanalysis refers to the practice of revealing information hidden by 
	cryptography using analytical and mathematical techniques, without the consent of 
	cryptographer. One who practices cryptanalysis is called a cryptanalyst.

    
Note:A cryptanalyst uses mathematical means to break ciphers. One can also break a cipher 
by spying on the users of a system, stealing the decryption key, bribing the cryptographer,
injecting him with truth serum, pointing a light (or a gun) in his face,bashing him over
the head with a rock, as well as various other tactics. This refers to a rather different 
discipline known as intelligence.

	
Extract of “Introduction to CRYPTOGRAPHY with Java Applets” (Author David BISHOP)    

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