Cryptography

"Cryptography, a word with Greek origin means Secret  writing. However, we use the term to refer to the science and art of transforming messages to make them secure and immiane to attacks." 

  Following figure shows the components involved in cryptography.






Plain Text and Cipher Text

The original message, before being transformed is called plain text. After the message is transformed, it is called cipher text. An encryption algorithm transform the plain text into cipher text; a description algorithm transformed the cipher text into plain text. The sender uses an encryption algorithm and the receiver uses a decryption algorithm.


Ciphers

We refer to encryption and decryption algorithm as ciphers. The term cipher is also used to refer to different categories of algorithm in cryptography. This is not o say that every sender-receiver pair needs their very own unique cipher for a secure communication on the contrary, one cipher can serve millions of communicating pairs.


Key

A key is a number ( or a set of numbers), that the ciphers, as an algorithm, co-operates on to encrypt a message. We need an encryption algorithm, an encryption key and the plain text. These creates a cipher text. To decrypt a message, we need a decryption algorithm,a decryption key and a cipher text.

Two Categories

We can divide all the cryptography algorithms (ciphers) into two categories, namely symmetric key cryptography (also called secret key cryptography)  algorithm and asymmetric key cryptography (also called public key cryptography) algorithm.




Symmetric key cryptography : In symmetric key cryptography the same key is used by both parties ( i.e. sender and receiver ). The sender uses this key and encryption algorithm to encrypt data, the receiver uses the same key and the corresponding decryption algorithm to decrypt the data. In symmetric key cryptography, the same key is used by the sender ( for encryption ) and the receiver ( for decryption ). The key is shared. The following block diagram illustrates the basic concept of symmetric key cryptography.

Asymmetric key cryptography : In asymmetric key cryptography or public key cryptography, there are two different keys namely - a private key and public key. The private key is kept by the receiver and the public key is announced to the public. Imagine Alice wants to send a message to Bob. Alice uses the public key to encrypt the message,when the message is received by Bob the private key is used for decrypt the message. In public key encryption/decryption the public key used for encryption is differ from the private key used for decryption. The public key is available to public but the private key is available for individual.


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