**Asymmetric Key Cryptography (Public Key Cryptography)**

Different
keys are used for encryption and decryption, it is called **asymmetric key
cryptography. **Public key used for encryption, private key used for decryption
and vice versa. It is also known as **public key cryptography**.** **

**Elements of Asymmetric Key Cryptography**

**Plaintext** is the
original message or data that is fed into the algorithm as input.

**Encryption
algorithm** performs various substitutions and transformations on the
plaintext.

**Public
& Private keys:** This is a pair of keys that have been selected so
that if one is used for encryption, the other is used for decryption.

**Ciphertext** is the
unreadable message produced as output. It depends on the plain text and secret
key.

**Decryption
algorithm** takes the cipher text and the secret key and produces the
original plain text.

*Public
key is distributed to all users and Private key is known to particular user
only.*

There are
two different Scenario of encryption model:

**Public
key**
used for **encryption** and **private key** used for **decryption.**

**Private
key**
used for encryption and **public key** used for **decryption.**

**Scenario
– 1: Encryption with public key**

If Bob
wants to send message to Alice, Bob must have to use public key of Alice. Message
to be transmitted after encryption of message using Alice’s public key.

Alice has
received message and she can decrypt the message using only her private key’s.

Figure: Encryption with Public Key |

Mathematically,
it is represented,

Y =
E(Pu(A), X)

X =
D(Pr(A), Y).

Where,
Pu(A) = Alice public key and Pr(A) = Alice private key.

**Scenario
– 2: Encryption with private key**

If Bob
wants to send message to Alice, Bob must have to use his own private key. Message
to be transmitted after encryption of message using Bob’s public key.

Alice has
received message and she can decrypt the message using Bob’s public key.

Figure: Encryption with Private Key |

Mathematically,
it is represented,

Y =
E(Pr(B), X)

X = D(Pu(B), Y).

Where, Pr(B) = Bob’s private key, Pu(B) = Bob’s public key.

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