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Cryptography is the study of hiding information.In all the recent definitions, cryptography refers to encryption, which is the process of converting ordinary information (plaintext) into unintelligible gibberish (ciphertext).Decryption movines the message from the unintelligible ciphertext back to plaintext.A cipher is a pair of algorithms that create the encryption and the reversing decryption. The operation of a cipher is controlled by the algorithm and in each instance by a key.ciphers without keys can be considered broken with only the knowledge of the cipher used and are not useful for most purposes. Keys are hence important. The key is a secret parameter known only to the communicants for a particular message exchange. A code in cryptography means the replacement of a unit of plaintext with a code word.

The main purpose of cryptography was to make it unreadable by interceptors or eavesdroppers without secret knowledge of the key. Modern developments have brought in message integrity checking, , digital signatures ,sender/receiver identity authentication, interactive proofs and secure computation.

Classical cryptography
In olden days, major cipher types were transposition ciphers, which rearrange the order of letters in a message and substitution ciphers, which replace letters or groups of letters with other letters or groups of letters. Cryptography is recommended in the Kama Sutra as a way for lovers to communicate without inconvenient discovery. Steganography is an extension which hides the existence of a message too. Modern steganography includes the use of invisible ink, microdots, and digital watermarks to conceal information.

The computer era
Computers made possible much more complex ciphers.any kind of data representable in any binary format could be encrypted by computers. computer ciphers are characterized by their operation on binary bit sequences.good modern ciphers have stayed ahead of cryptanalysis. The use of a quality cipher is very efficient .IBM personnel designed the algorithm that became the Federal Data Encryption Standard in the US. the RSA algorithm was then published .Modern methods keep their keys secret if certain mathematical problems are intractable, such as the discrete logarithm or the integer factorisation problems, so there are connections with abstract mathematics.
Increases in computer processing power have increased the scope of brute-force attacks, thus the required key lengths are advancing.quantum computing are already being considered by some cryptographic system designers.

Modern cryptography
Symmetric-key cryptography
This refers to encryption methods in which both the sender and receiver share the same key.block ciphers and stream ciphers and applications are important areas.block ciphers take as input a block of plaintext and a key, and output a block of ciphertext of the same size.The Data Encryption Standard (DES) and the Advanced Encryption Standard (AES) are block cipher designs which have been designated as the cryptography standards by the United States government.ATM encryption, e-mail privacy and secure remote access are applications of DES.
Stream ciphers, create an arbitrarily long stream of key material, which is combined with the plaintext bit-by-bit or character-by-character.
Cryptographic hash functions take a message of any length as input, and output a short, fixed length hash which can be used in a digital signature.Message authentication codes (MACs) are similar to cryptographic hash functions,but a secret key is used to authenticate the hash value on recieving it.

Public-key cryptography
Symmetric systems use the same key for encryption and decryption of a message, though a message or group of messages may have a different key than others.In private key cryptography sytems, cryptography in which two different but mathematically related keys are used ” a public key and a private key. This system is so constructed that calculation of one key is computationally infeasible from the other even though they are related.This system was proposed by Whitfield Diffie and Martin Hellman.public-key cryptography can also be used to implement digital signature schemes. there are two algorithms: one for signing, in which a secret key is used to process the message and one for verification, in which the matching public key is used to verify.

Its aim is to find some weakness or insecurity in a cryptographic scheme, thus permitting its subversion.Most ciphers, except one-time pad, can be broken with enough computational effort by brute force attack.In a ciphertext-only attack, the cryptanalyst has access only to the ciphertext.In a known-plaintext attack, the cryptanalyst has access to a ciphertext and its corresponding plaintext. In a chosen-plaintext attack, the cryptanalyst may choose a plaintext and learn its corresponding ciphertext. Finally, in a chosen-ciphertext attack, the cryptanalyst may be able to choose ciphertexts and learn their corresponding plaintexts.

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