**CRYPTOGRAPHY**

Cryptography is a method of storing and transmitting data in a particular form so that only those for whom it is intended can read and process it.

Cryptography, or cryptology, is the practice and study of hiding information. It is sometimes called code, but this is not really a correct name. It is the science used to try to keep information secret and safe. Modern cryptography is a mix of mathematics, computer science, and electrical engineering. Cryptography is used in ATM (bank) cards, computer passwords, and shopping on the internet.

Different types of cryptography can be easier or harder to use and can hide the secret message better or worse. Ciphers use a “key” which is a secret that hides the secret messages. The cryptographic method needn’t be secret. Various people can use the same method but different keys, so they cannot read each other’s messages. Since the Caesar cipher has only as many keys as the number of letters in the alphabet, it is easily cracked by trying all the keys. Ciphers that allow billions of keys are cracked by more complex methods.

Modern cryptography concerns itself with the following four objectives:-

**(1) Confidentiality:- **The information cannot be understood by anyone for whom it was unintended.

**(2) Integrity:-** The information cannot be altered in storage or transit between sender and intended receiver without the alteration being detected.

**(3) Non-repudiation:-** The creator/sender of the information cannot deny at a later stage his or her intentions in the creation or transmission of the information.

**(4) Authentication:-** The sender and receiver can confirm each other?s identity and the origin/destination of the information.

**TYPES OF CRYPTOGRAPHY**

**(1) ****Public Key Cryptography**

Public key cryptography, or asymmetrical cryptography, is any cryptographic system that uses pairs of keys: public keys which may be disseminated widely, and private keys which are known only to the owner.

This accomplishes two functions: authentication, where the public key verifies a holder of the paired private key sent the message, and encryption, where only the paired private key holder can decrypt the message encrypted with the public key.

In a public key encryption system, any person can encrypt a message using the receiver’s public key. That encrypted message can only be decrypted with the receiver’s private key. To be practical, the generation of a public and private key -pair must be computationally economical. The strength of a public key cryptography system relies on the computational effort (work factor in cryptography) required to find the private key from its paired public key. If so, effective security only requires keeping the private key private; the public key can be openly distributed without compromising security.

**(2) Private key cryptography**

In cryptography, a private key (secret key) is a variable that is used with an algorithm to encrypt and decrypt code. Quality encryption always follows a fundamental rule: the algorithm doesn’t need to be kept secret, but the key does. Private keys play important roles in both symmetric and asymmetric cryptography.

Most cryptographic processes use symmetric encryption to encrypt data transmissions but use asymmetric encryption to encrypt and exchange the secret key. Symmetric encryption, also known as private key encryption, uses the same private key for both encryption and decryption. The risk in this system is that if either party loses the key or the key is intercepted, the system is broken and messages cannot be exchanged securely.

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