What is 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. By definition, "Cryptography" is the process of converting recognizable data into an encrypted code for transmitting it over a network ( may be trusted or untrusted). Data is encrypted at the source and decrypted at the destination.

As WIKIPEDIA says,

Cryptography or cryptology (from Greek κρυπτός kryptós, "hidden, secret"; and γράφειν graphein, "to write", or -λογία -logia, "study", respectively) is the practice and study of techniques for secure communication in the presence of third parties called adversaries.
Objectives of Cryptograph.

A Brief History ->

The origin of cryptography is usually dated from about 2000 BC, with the Egyptian practice of hieroglyphics. These consisted of complex pictograms, the full meaning of which was only known to an elite few. The first known use of a modern cipher was by Julius Caesar (100 BC to 44 BC), who did not trust his messengers when communicating with his governors and officers. For this reason, he created a system in which each character in his messages was replaced by a character three positions ahead of it in the Roman alphabet.

Mainly there are four branches which is concerned by Cryptography.

Authentication
Confidentiality / Privacy
Integrity
Non - repudiation

Authentication

The sender and receiver can confirm each other's identity and the origin/destination of the information or simply "The process of proving one's identity".

Confidentiality / Privacy

The information cannot be understood by anyone for whom it was unintended or
Ensuring that no one can read the message except the intended receiver.

Integrity

The information cannot be altered in storage or transit between sender and intended receiver without the alteration being detected or Assuring the receiver that the received message has not been altered in any way from the original.

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 or A mechanism to prove that the sender really sent this message.

How it works?

In general, cryptographic algorithms are classified into three categories as follow :

Secret Key Cryptography (SKC)
Public Key Cryptography (PKC)
Hash Functions

Secret Key Cryptography

In 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.

SKC Algorithms

1. Data Encryption Standard (DES) ->
This is a block-cipher employing a 56-bit key that operates on 64-bit blocks. This method uses a key of only 56 bits, and thus it is now susceptible to "brute force" attacks.

2. Advanced Encryption Standard (AES) ->
The algorithm can use a variable block length and key length.

3. International Data Encryption Algorithm (IDEA) ->
A 64-bit SKC block cipher using a 128-bit key

4. Rivest Ciphers ->
A series of SKC algorithms.

5. Blowfish ->
A symmetric 64-bit block cipher invented by Bruce Schneier and optimized for 32-bit processors with large data caches, it is significantly faster than DES.

Public Key Cryptography

Generic PKC employs two keys that are mathematically related although knowledge of one key does not allow someone to easily determine the other key. One key is used to encrypt the plaintext and the other key is used to decrypt the ciphertext. No matter which key is applied first, but both the keys are required for the process to work. Because a pair of keys are required, this approach is also called asymmetric cryptography.

In PKC, one of the keys is designated the public key and may be advertised as widely as the owner wants. The other key is designated the private key and is never revealed to another party.

Here we talk about RSA(Rivest–Shamir–Adleman) algorithm only.

This is a one of the most popular encryption algorithm, invented in 1977 by three MIT scientists (Ronald Rivest, Adi Shamir, and Leonard Adleman).

The key-pair is derived from a very large number, n, that is the product of two prime numbers chosen according to special rules; these primes may be 100 or more digits in length each, yielding an n with roughly twice as many digits as the prime factors. The public key information includes n and a derivative of one of the factors of n; an attacker cannot determine the prime factors of n (and, therefore, the private key) from this information alone and that is what makes the RSA algorithm so secure.

Hash Functions

Hash functions, also called message digests and one-way encryption, are algorithms that, use no key. Instead, a fixed-length hash value is computed based upon the plaintext that makes it impossible for either the contents or length of the plaintext to be recovered. Hash algorithms are used to provide a digital fingerprint of a file's contents, often used to ensure that the file has not been altered by an intruder or virus. Hash functions are also commonly employed by many operating systems to encrypt passwords.

Hash Algorithms

1.Message Digest (MD) algorithms ->
A series of byte-oriented algorithms that produce a 128-bit hash value from an arbitrary-length message

2.Secure Hash Algorithm (SHA)

3.RIPEMD ->
A series of message digests that initially came from the RIPE (RACE Integrity Primitives Evaluation) project

4.HAVAL (HAsh of VAriable Length) - >
a hash algorithm with many levels of security

5.Whirlpool ->
Whirlpool operates on messages less than 2256 bits in length, and produces a message digest of 512 bits

6.Tiger ->
Designed by Ross Anderson and Eli Biham, Tiger is designed to be secure, run efficiently on 64-bit processors, and easily replace MD4, MD5, SHA and SHA-1 in other applications

Uses of Cryptography

1.Secrecy in transmission
2.Secrecy in storage
3.Integrity in transmission
4.Integrity in storage
5.Authentication of identity
6.Credentialing systems
7.Digital signatures
8.Electronic money
9.Threshold cryptosystem
10.Secure multi-party computation