Cryptography can de defined as practice and study of Hiding information. Modern cryptography intersects the disciplines of mathematics, computer science and engineering. Applications of Cryptography include ATM cards, Computer Passwords and Electronic Commerce.
Cryptology prior to the modern age was almost synonymous with encryption, the conversion of information from a readable state to nonsense. The sender retained the ability to decrypt the information and therefore avoid unwanted persons being able to read it. Since WWI and the advent of the computer, the methods used to carry out cryptology have become increasingly complex and its application more widespread.
For Authentication – Cryptography can be used for Authentication of Data or a Message.
Integrity – Using Cryptography, One can make out if the Data or Message has been Tampered or Altered.
Non-repudiation – Cryptography also ensures Responsibility and Accountability from the Sender.
Cryptography can also be used for:
Symmetric-key cryptography refers to encryption methods in which both the sender and receiver share the same key (or, less commonly, in which their keys are different, but related in an easily computable way).
Symmetric-Key Ciphers are of 2 Types:
A block cipher is, in a sense, a modern embodiment of Alberti's polyalphabetic cipher: block ciphers take as input a block of plaintext and a key, and output a block of ciphertext of the same size. Since messages are almost always longer than a single block, some method of knitting together successive blocks is required. Several have been developed, some with better security in one aspect or another than others. They are the modes of operation and must be carefully considered when using a block cipher in a cryptosystem.
Examples of Block Ciphers:
Many other block ciphers have been designed and released, with considerable variation in quality. Many have been thoroughly broken.
Stream ciphers, in contrast to the 'block' type, create an arbitrarily long stream of key material, which is combined with the plaintext bit-by-bit or character-by-character, somewhat like the one-time pad. In a stream cipher, the output stream is created based on a hidden internal state which changes as the cipher operates. That internal state is initially set up using the secret key material.
Examples of Steam Ciphers:
Asymmetric algorithms use pairs of keys. One is used for encryption and the other one for decryption. The decryption key is typically kept secret, therefore called “private key”, while the encryption key is spread to all who might want to send encrypted messages, therefore called “public key”. Everybody having the public key is able to send encrypted messages to the owner of the secret key. The secret key can't be reconstructed from the public key. The idea of asymmetric algorithms was first published 1976 by Diffie and Hellmann.
In a groundbreaking 1976 paper, Whitfield Diffie and Martin Hellman proposed the notion of public-key (also, more generally, called asymmetric key) cryptography in which two different but mathematically related keys are used—a public key and a private key.
In addition to encryption, public-key cryptography can be used to implement digital signature schemes. A digital signature is reminiscent of an ordinary signature; they both have the characteristic that they are easy for a user to produce, but difficult for anyone else to forge.
Diffie–Hellman Key Exchange
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