## Cryptography

Cryptography is a technique by which we can safely send our data from one place to another or we can say from sender to receiver computer. Or we can say it is an art of writing the data into secret codes so that it cannot be understand by any other third party or intruder.

As we can send data from one computer to another computer without using cryptography technique it will be easily hack by any another third party any miss use it and it may also possible to change the data to any other form which would be a wrong information to receiver user. As shown in below figure (1).

In the above figure sender want to send information to receiver of pay $ 100 which is original information when it is goes from sender to receiver it is hacked by intruder and intruder transform it to pay $ 200 and send it to the receiver and receiver will received a wrong piece of information which would be harmful for receiver.

But if we use cryptography techniques we use encryption method to transform the data into encrypted form before send to receiver so if it is hacked by any intruder it cannot be understand by it and he will not be able to transform it. As shown in figure (2).

As show in above figure when a sender send a particular data to receiver before send it encrypted in another format which cannot be understand by any third a party or we can say intruder . so at the receiver end it first decrypt the data and the get a original data.

Generally it is based on mathematical algorithms and these algorithms used a secret key for secure transactions.

**Cryptography conditions **

In Cryptography each person receives a pair of keys, called the public-key, or private-key. Through encryption and decryption will be done with the help of public key and private key both the receiver end and sender end. Each user public key is published but private key kept secret which is only known to user. Any user can send the information or any data using public key encryption but can only decrypted it with private key. The original data known as plane text and after encryption known as cipher text.

**Classical Cryptography techniques**

Classical cipher’s compromise two basic components:

- Substitution ciphers.
- Transposition ciphers.
- Mon alphabetic.
- Poly alphabetic.

Several of these ciphers are group together to form a product ciphers.

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