“The process of covering from plaintext to ciphertext is called *Encryption*, restoring the plaintext from ciphertext is known as *Decryption* "

**Symmetric Cipher Model:** (uses a single secret key for both encryption & decryption)

Where,

K= Secret Key

X = Plaintext/Message

Ciphertext Y = E(X,K)

Decrypted/Plaintext X = D(Y,K)

A symmetric encryption scheme has five components:

**Plaintext:**This is the original intelligible message or data that is fed into the algorithm as input.**Encryption algorithm:**The encryption algorithm performs various substitutions and transformations on the plaintext.**Secret key:**The secret key is also input to the encryption algorithm.The key is a value independent of the plaintext and of the algorithm. The algorithm will produce a different output depending on the specific key being used at the time.The exact substitutions and transformations performed by the algorithm depend on the key.**Ciphertext:**This is the scrambled message produced as output. It depends on the plaintext and the secret key. For a given message, two different keys will produce two different ciphertexts. The ciphertext is an apparently random stream of data and, as it stands, is unintelligible.**Decryption algorithm:**This is essentially the encryption algorithm run in reverse. It takes the ciphertext and the secret key and produces the original plaintext.

E.g.:

Original text X $\rightarrow$ plain text eg. CNS

Encryption Algorithm = replacing each letter of the alphabet with the letter standing three places further down the alphabet. i,e (X+3) So,

Coded text Y $\rightarrow$ Cipher text for CNS is *Fqv*

Decryption Algorithm = reverse of encryption i,e ( Y - 3)

Here key = 3