# Cryptography

Industrial espionage among highly competitive businesses often requires that extensive security measures be put into place. And, those who wish to exercise their personal freedom, outside of the oppressive nature of governments, may also wish to encrypt certain information to avoid suffering the penalties of going against the wishes of those who attempt to control. .

Encryption is the process of transforming text into an unintelligible form called cipher. Data encryption is the process used to hide the true meaning of data. Reversing the process of encryption is called decryption. Encryption and decryption comprise the science of cryptography as it is applied to the modern computer. Data encryption is achieved through the use of an algorithm that transforms data from its intelligible form to cipher. An algorithm is a set of rules or steps for performing a desired operation. An algorithm can be performed by anything that can be taught or programmed to follow a specific and unambiguous set of instructions

INTRODUCTION

encryption is the process of encoding messages (or information) in such a way that eavesdroppers or hackers cannot read it, but that authorized parties can. In anencryption scheme, the message or information (referred to as plaintext) is encrypted using an encryption algorithm, turning it into an unreadable ciphertext. This is usually done with the use of an encryption key, which specifies how the message is to be encoded. Any adversary that can see the ciphertext, should not be able to determine anything about the original message. An authorized party, however, is able to decode the ciphertext using a decryption algorithm, that usually requires a secret decryption key, that adversaries do not have access to. For technical reasons, an encryption scheme usually needs a key-generation algorithm, to randomly produce keys. Methods Of Encryption

There are two types of cryptosystems: secret key and public key.

In secret-key cryptography, also referred to as symmetric cryptography, the same key is used for both encryption and decryption. The most popular secret-key cryptosystem in use today is known as DES, the Data Encryption Standard.

In public-key cryptography, each user has a public key and a private key. The public key is made public whilst the private key remains secret. Encryption is performed with the public key, while the private key is used for decryption. The RSA publickey cryptosystem is the most popular form of public-key cryptography. RSA stands for Rivest, Shamir, and Adleman, the inventors of the RSA cryptosystem.

In both cases the encryption can be applied in either a block or stream cipher. Block Cipher

A block cipher is a type of symmetric-key encryption algorithm that transforms a fixed-length block of plaintext (unencrypted text) data into a block of ciphertext (encrypted text) data of the same length. This transformation takes place under the action of a user-provided secret key.

Decryption is performed by applying the reverse procedure to the ciphertext block, whilst using the same secret key. The fixed length is called the block size, and for many block ciphers, the block size is 64 bits. In the coming years the block size will increase to 128 bits as processors become more sophisticated. Stream Cipher

A stream cipher is a type of symmetric encryption algorithm. Stream ciphers can be designed to be exceptionally fast, much faster than any block cipher. While block ciphers operate on large blocks of data, stream ciphers typically operate on smaller units of plaintext, usually bits.

The encryption of any particular plaintext with a block cipher will result in the same ciphertext when the same key is used. With a stream cipher, the transformation of these smaller plaintext units will vary, depending on when they are encountered during the encryption process. A stream cipher generates what is called a keystream, which is a sequence of bits used as a key. Encryption is...

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