Cryptography:-

Cryptography, also called cryptology, is a discipline of mathematics and of computer science that relates to information particularly encryption and authentication.

Cryptography is an interdisciplinary subject and has been influenced by several fields. Before the 20th century came, cryptography was concerned with linguistic patterns. Now, however, cryptography makes use of mathematics, information theory, computational complexity etc. Cryptography is basically a branch of engineering.

Cryptography finds widespread application in areas such as the security of ATM cards, computer passwords, and electronic commerce.

Cryptography usually refers to encryption alone but in reality, decryption is also a part of it. Cryptography, as a word, has been derived from the Greek words kryptons, that means "hidden," and graphein, which means "to write". The basic aim behind cryptography is to convert information (plain text) into unreadable form (cipher text). Ciphers are algorithms to encrypt and decrypt data. While some techniques use the same algorithm to encrypt and decrypt data, some advanced ones have different algorithms in which case the cipher is a pair of the two.

History of cryptography:-

Historically, cryptography was mainly used to ‘encrypt’ data to render it unreadable b a certain person. Reading it required knowing what the text was encrypted with (the key). The earliest form of cryptography called Classical cryptography worked either through transposition ciphers or substitution ciphers. Under transposition ciphers, the order of the letters in the word was changed (eg. History could become Ihtsyor). Substitution ciphers, on the other hand replaced the letters or their groups in a word with others (eg. History may be written as Ijupsz). This simple method was employed by Julius Caesar during his military campaigns.

Steganography, another form of cryptography which is different from encryption aimed at making a message impossible to detect. This was done by hiding the information with some other information. It could also be achieved by using invisible ink, microdots, and digital watermarks to convey hidden information.

As seems obvious, classical ciphers are easy to break. An informed person can easily make out a meaning from such messages. However, these techniques went on until the polyalphabetic cipher was invented by Leon Battista Alberti somewhere around about 1467CE were which different parts of the message (often each plaintext letter) are enciphered using a different key. Still, they were at least partially vulnerable.

In modern day world, the Germans, during the World War II, used the Enigma, a special machine that encrypted information, to convey its plans to other places. The Enigma cipher was broken by Polish mathematicians, and the resultant large-scale decryption of Enigma traffic at Bletchley Park, was an important contributor to the Allied victory.

There are various physical devices that have been used to assist cipher operations. One of the earliest was the scytale of ancient Greece which was a rod supposedly used by the Spartans to help in transposition cipher. The cipher grille, invented in medieval times, was used for steganography. Probably, the most famous cryptography tool was the Enigma machine used by the Germans.

Much more complicated cyphers were made possible due to the development of digital computers and electronics. One of the most popular algorithms is known as RNA which completely revolutionized the cryptography scene. The RNA algorithm was unbreakable for years. Mostly, the algorithms for cryptography have been ahead of cryptoanalysis which means to ‘break the code’ in order to gain access to the information that was encrypted.

Modern cryptographic techniques:-

The modern field of cryptography can be divided into several areas of study as follows:

(1) Symmetric-key cryptography

Symmetric-key cryptography refers to encryption methods in which both the sender and receiver share the same. Today, it is related to study of block ciphers and stream ciphers and to their applications. A block cipher takes a block of plaintext and a key as input and outputs a block of ciphertext of the same size. Stream ciphers, on the other hand, operate on a continuous stream of plaintext, and produce an encrypted output stream based on an internal state which changes as the cipher operates.

As contrary to this, cryptographic hash functions do not use keys, but are a related class of cryptographic algorithms. They take input data (often a entire message) and output a short, fixed length (160 bits is common), hash, and do so as a one-way function, i.e., a function whose only one way calculation is possible but a reverse is not.

(2) Public-key cryptography

Considered the most revolutionary thought in cryptography, public-key cryptosystems use a public key that may be freely distributed, while its paired private key must remain secret. In public-key encryption, the public key is the encryption key, and the private or secret key is the decryption key. The RSA algorithm is one of the best examples of this type of cryptography. It is used to implement digital signature schemes and is based on computational complexity of problems which pertains to number theory.