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Cryptography By definition cryptography is the process of converting recognisable data into an encrypted code for transmitting it over a network (either trusted or untrusted). Data is encrypted at the source, i.e. sender's end and decrypted at the destination, i.e. receiver's end.

In all cases, the initial unencrypted data is referred to as plaintext. It is encrypted into ciphertext, which will in turn (usually) be decrypted into usable plaintext using different encryption algorithms.

The Purpose :-
* Authentication : The process of proving one's identity.
* Privacy/confidentiality : Ensuring that no one can read the message except the intended receiver.
* Integrity : Assuring the receiver that the received message has not been altered in any way from the original.
* Non-repudiation : A mechanism to prove that the sender really sent this message.

In general cryptographic algorithms are classified into three categories as follows :

1) Secret Key Cryptography (SKC) : Uses a single key for both encryption and decryption.
2) Public Key Cryptography (PKC) : Uses one key for encryption and another for decryption.
3) Hash Functions : Uses a mathematical transformation to irreversibly "encrypt" information.

Secret Key Cryptography :- With secret key cryptography, a single key is used for both encryption and decryption. Because a single key is used for both functions, secret key cryptography is also called symmetric encryption.

Secret key cryptography algorithms that are in use today include :

1) Data Encryption Standard (DES) : DES is a block-cipher employing a 56-bit key that operates on 64-bit blocks. DES uses a key of only 56 bits, and thus it is now susceptible to "brute force" attacks.
Triple-DES (3DES) and DESX are the two important variants that strengthen DES.

2) Advanced Encryption Standard (AES ) : The algorithm can use a variable block length and key length; the latest specification allowed any combination of keys lengths of 128, 192, or 256 bits and blocks of length 128, 192, or 256 bits.

3 ) International Data Encryption Algorithm (IDEA) : Secret-key cryptosystem written by Xuejia Lai and James Massey, in 1992 and patented by Ascom; a 64-bit SKC block cipher using a 128-bit key. Also available internationally.

4) Rivest Ciphers : Named for Ron Rivest, a series of SKC algorithms.

RC1 : Designed on paper but never implemented.
RC2 : A 64-bit block cipher using variable-sized keys designed to replace DES. It's code has not been made public although many companies have licensed RC2 for use in their products. Described in RFC 2268.
RC3 : Found to be breakable during development.
RC4 : A stream cipher using variable-sized keys; it is widely used in commercial cryptography products, although it can only be exported using keys that are 40 bits or less in length.
RC5 : A block-cipher supporting a variety of block sizes, key sizes, and number of encryption passes over the data. Described in RFC 2040.
RC6 : An improvement over RC5, RC6 was one of the AES Round 2 algorithms.

5) Blowfish : A symmetric 64-bit block cipher invented by Bruce Schneier; optimized for 32-bit processors with large data caches, it is significantly faster than DES on a Pentium/PowerPC-class machine. Key lengths can vary from 32 to 448 bits in length. Blowfish, available freely and intended as a substitute for DES or IDEA, is in use in over 80 products.

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