Encryption is the process of encoding a message so that its meaning is not obvious. Decryption is the reverse process, transforming an encrypted message back into its normal, original form. Alternatively the terms encode and decode or encipher and decipher are used instead of encrypt and decrypt. That is we say that we encode encrypt or encipher the original message to hide its meaning. Then we decode, decrypt or decipher it to reveal the original message. A system for encryption and decryption is called a cryptosystem.

The original form of a message is known as plaintext, and the encrypted form is called cipher text. For convenience in explanation, we denote a plaintext message p as a sequence of individual characters p={p1, p2, …..pn}. Similarly cipher text is written as C={c1, c2,…..cm}. for instance, the plaintext message “ I want cookies” can be through  of as the message string { I, w ,a, n, t, c, o, o, k, I, e, s}. it may be transformed into cipher text { c1, c2, ……c14} and the encryption algorithm tells us how the transformation is done.