Page: Symmetric Cipher Model
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“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

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