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The encryption of any information in a computer system is done to maintain the privacy or secrecy of the subject. The encrypted file is stored in some location that is not easily identifiable. This is done so that there is no leakage of the file. Even in extreme cases when a file is found and opened by any person then also person should not be able to read to the file.

The contents of the file or data after encryption are not in a readable format. But it is also desired for the person who is encrypting the file to again decrypt it. The file should be decrypted before it can be read. The information regarding the decryption is available only to the person who is authorized for reading the information.

A cipher is the algorithm that is used to encrypt the file. There are many ciphers that are used for encryption of the information. The ciphers can be classified as Classical ciphers, Rotor Machines and finally as Modern Ciphers. The classical ciphers are again classified as Substitution ciphers and Transposition ciphers. The Modern ciphers are classified as Public key and Private Key.

The private key ciphers are again classified as stream and block ciphers. Below mentioned are some of the classical ciphers used in cryptography. These are the algorithms that are employed for the encryption of information. They are Autokey cipher, Playfair cipher by Charles Wheatstone, Permutation cipher, Polyalphabetic substitution - Hill cipher and Vigenere cipher.

The example in case of substitution ciphers includes the following Caesar cipher - ROT13, Affine cipher and Atbash cipher. In case of transposition ciphers the examples are Scytale, Grille cipher, as well as VIC cipher. There are even attacks that are carried out over the classical ciphers like for example the Frequency analysis and Index of coincidence.

In the earlier part of the 20th century; encryption of the text was done using some more sophisticated machines called as rotor machines. These machines were complex than the earlier encryption techniques. The encryption methods can be divided into symmetric key algorithms and asymmetric key algorithms. The examples of symmetric key algorithm include DES – Data Encryption Standard as well as the AES – Advanced Encryption Standard.

In both the cases the sender and receiver are required to have a shared key. The shared key should be known to both of them in advance. It is required for both the parties in communication to keep the information secret from the parties that are involved in the communication. The sender uses this key for providing the encryption. And then the receiver uses the same key for the purpose of decryption and reading of the information that is passed over.

In the case of asymmetric key algorithm, for example RSA there are usually two separate keys public and private. The public key is shared; where as the private key is not shared with others except the two parties. The Symmetric key ciphers there are two different types of encryptions used. They are classified depending on fixed size - block ciphers or in a continuous stream of symbols - stream ciphers.

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