Cryptography

Cryptography is an interdisciplinary subject, drawing from several fields. Older forms of cryptography were chiefly concerned with patterns in language. More recently, the emphasis has shifted, and cryptography makes extensive use of mathematics, particularly discrete mathematics, including topics from number theory, information theory, computational complexity, statistics and combinatorics. Cryptography is also considered a branch of engineering, but it is considered to be an unusual one as it deals with active, intelligent and malevolent opposition (see cryptographic engineering and security engineering). Cryptography is a tool used within computer and network security.

History of cryptography

Main article: History of cryptography

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Cryptography has had a long and colourful history. Generally, the earliest forms of secret writing (now collectively termed classical cryptography) required only pen and paper. The two main categories of classical ciphers are transposition ciphers, which rearrange the order of letters in a message, and substitution ciphers, which systematically replace letters or groups of letters with other letters or groups of letters. Text encrypted by classical ciphers tends to reveal a certain statistical information about the plaintext. By using this information, classical ciphers are easily broken (e.g. by frequency analysis). Classical ciphers still enjoy popularity today, though mostly as puzzles (see cryptogram).

Related Topics:
Transposition cipher - Substitution cipher - Frequency analysis - Puzzle - Cryptogram

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Various devices and aids have been used for encryption. One of the earliest may have been the scytale. Early in the 20th century, several mechanical devices were invented for performing encryption, including rotor machines — most famously the Enigma machine used by Germany in World War II. The ciphers implemented by these machines brought about a significant increase in the complexity of cryptanalysis.

Related Topics:
Scytale - Rotor machine - Enigma machine - World War II

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With the advent of digital computers and electronics, very complex ciphers could been implemented. A characteristic of computer ciphers is that they operate on binary strings, unlike classical and mechanical schemes, which use an alphabet of around 26 letters (depending on the language). Computer ciphers are also much more resistant to cryptanalysis; few are susceptible to a ciphertext-only attack.

Related Topics:
Electronics - Binary

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Extensive academic research into modern cryptography is relatively recent — it only began in the open community during the 1970s with the specification of the Data Encryption Standard (DES) and the invention of RSA. Since then, cryptography has become a widely-used tool in communications and computer security.

Related Topics:
Data Encryption Standard - RSA

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As well as noting lessons from its history, cryptographers are also careful to consider the future. Moore's law is routinely taken into account when specifying key lengths, and the potential effects of quantum computing have already been considered.

Related Topics:
Moore's law - Key length - Quantum computing

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