imported>Matt Crypto: Reverted edits by 59.99.134.164 (talk) to last version by Kevyn

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Reverted edits by 59.99.134.164 (talk) to last version by Kevyn

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'''Cryptography''' (from [[Greek language|Greek]] ''kryptós'', "hidden", and ''gráphein'', "to write") is, traditionally, the practice of rendering [[information]] in an obscured form in order to prevent others from understanding it. In recent decades, cryptography has expanded its remit and now provides mechanisms for a wide range of [[security]] goals, in addition to the ever-important task of ensuring [[secrecy]] in [[communication]]s. Cryptography is an [[interdisciplinary]] subject, drawing from several fields: it can be viewed as an application of [[information theory]], and yet it makes extensive use of other areas of [[mathematics]] too, notably [[number theory]]. It is also a branch of [[engineering]], but an unusual one as it must deal with active, intelligent and malevolent opposition (see [[cryptographic engineering]] and [[security engineering]]).

Closely related fields are [[steganography]], which is distinguished from cryptography as the study of ways of obscuring the very ''existence'' of a [[message]], and not the contents of the message itself; and [[traffic analysis]], which is the analysis of patterns of communication in order to gain knowledge.

==Terminology==
The original information which is to be protected by cryptography is called the ''[[plaintext]]''. ''[[Encryption]]'' is the process of converting plaintext into an unreadable form, termed ''[[ciphertext]]'', or, occasionally, a ''[[cryptogram]]''. ''[[Decryption]]'' is the reverse process, recovering the plaintext back from the ciphertext. ''Enciphering'' and ''deciphering'' are alternative terms. A ''[[cipher]]'' is an [[algorithm]] for encryption and decryption. The exact operation of a cipher is almost always controlled by one or more ''[[Cryptographic key|keys]]'' &mdash pieces of information that customise the encryption and decryption. ''[[cryptographic protocol|Protocol]]s'' specify the details of how ciphers (and other cryptographic primitives) are to be used to achieve specific tasks. Protocols, cryptographic algorithms, prescribed actions and other specifics are combined together and implemented in a ''[[crypto system|cryptosystem]]''.

In ordinary parlance, a (secret) "[[code]]" is synonymous with "cipher". In cryptography, however, the term has a specialised, [[code|technical meaning]], and should be avoided unless this is intended.

The study of how to circumvent the use of cryptography is called ''[[cryptanalysis]]''; an ''attack'' is a specific attempt at or method for cryptanalysis. Cryptography and cryptanalysis are sometimes linked together under the umbrella term "cryptology", though "cryptography" also has this wider meaning, and both are acceptable terms for the entire field. "Crypto" is a common informal abbreviation for cryptography

"Cipher" is alternatively spelt "cypher"; similarly "ciphertext" and "cyphertext", and so forth. Although both spellings have long histories in English, there is occasional tension between their adherents.

==Cryptanalysis==
''Main article'': [[Cryptanalysis]]

A cryptanalyst might appear to be the natural adversary of a cryptographer, and to an extent this is true: one can view this contest all through the [[history of cryptography]]. However, it is also possible to interpret the two roles as complementary: a thorough understanding of cryptanalysis is necessary to create secure cryptography.

A variety of different assumptions are considered regarding what an attacker can know and do in order to learn secret information. One important distinction is whether the cryptanalyst has access only to ciphertext, whether he also knows or can guess some corresponding plaintexts, or even if he can ''choose'' plaintexts to be encrypted; (see [[ciphertext-only attack|ciphertext only]], [[known-plaintext attack|known plaintext]] and [[chosen plaintext attack|chosen plaintext]]). These scenarios all view the cipher as a [[black box]]. Some attacks, however, are based on the [[implementation]] of the cipher. If a cryptanalyst has access to, for example, [[Timing attack|timing]] or power consumption, he may be able to deduce secret information.

Notable methods of cryptanalysis include [[Brute force attack|exhaustive search]] for the key, which is a risk for all ciphers. [[Linear cryptanalysis]] and [[differential cryptanalysis]] are general methods for [[symmetric key algorithm|symmetric key cryptography]]. When cryptography relies on [[NP-hard|hard]] mathematical problems, as is usually the case in [[asymmetric key algorithm|asymmetric cryptography]], algorithms for tasks such as [[factoring]] become potential tools for cryptanalysis.

==History of cryptography==
''Main article'': [[History of cryptography]]

Cryptography has had a long and colourful [[history of cryptography|history]].
The earliest forms of secret writing required only pen and paper, and are now collectively termed ''classical'' cryptography. The two main categories are [[transposition cipher]]s, which rearrange the order of letters in a message, and [[substitution cipher]]s, which systematically replace groups of letters with others. Classical ciphers tend to leak varying amounts of information about the [[statistics]] of the plaintext, and because of this they are easily broken, for example by [[frequency analysis]]. Classical ciphers still enjoy popularity today, though mostly as [[puzzle]]s (see [[cryptogram]]).

Early in the 20th century, several mechanical devices were invented for performing encryption, including [[rotor machine]]s &mdash most famously the [[Enigma]] cipher used in [[World War II]]. The ciphers implemented by these machines brought about a significant increase in the complexity of cryptanalysis. The various attacks on Enigma, for example, succeeded only after considerable effort.

With the advent of digital [[computers]] and [[electronics]], very complex ciphers could be 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.

Extensive academic research into modern cryptography is relatively recent &mdash it only began in the open community during the 1970s with the specification of [[DES]] and the invention of [[RSA]]. It is well that much progress has been made in a short time; popular applications such as the [[Internet]] and [[mobile phone]]s have repositioned cryptography, historically the sole province of a few groups with exceptional needs for secrecy, into a mainstream technology on which millions rely.

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 [[Cryptographic_key_length|key-lengths]], and the potential effects of [[Quantum_Cryptography#Quantum_Computing_applications_for_Cryptanalysis|quantum computing]] have already been considered. Note also [[quantum cryptography]].

==Secure communications==
''See also:'' [[Information security]]

Cryptography is commonly used for [[Information_security|securing]] [[communication]]s. Four desirable properties are:

# [[confidentiality]], also known as [[secrecy]]: only an authorised recipient should be able to extract the contents of the message from its encrypted form. Otherwise, it should not be possible to obtain any significant information about the message contents.
# [[data integrity|integrity]]: the recipient should be able to determine if the message has been altered during transmission.
# [[authentication]]: the recipient should be able to identify the sender, and verify that the purported sender actually did send the message.
# [[non-repudiation]]: the sender should not be able to deny sending the message.

Cryptography can provide mechanisms to help achieve all of the above. However, some goals aren't always necessary, practical or even desirable in some contexts. For example, the sender of a message may wish to remain anonymous; clearly non-repudiation would be inappropriate.

==Symmetric key cryptography==
''Main article'': [[Symmetric key algorithm]]

Symmetric key ciphers use the same key for encryption and decryption, or a little more precisely, the key used for decryption is "easy" to calculate from the key used for encryption. Other terms include "private-key", "one-key" and "single-key" cryptography.

Symmetric key ciphers can be broadly grouped into [[block cipher]]s and [[stream cipher]]s. Stream ciphers encrypt one bit at a time, in contrast to a block cipher, which operates on a group of bits (a "block") of a certain length all in one go. The block ciphers [[DES]], [[International Data Encryption Algorithm|IDEA]] and [[AES]], and the stream cipher [[RC4 (cipher)|RC4]], are among the most well-known symmetric key ciphers.

Other cryptographic primitives are sometimes classified as symmetric cryptography:
* [[Cryptographic hash function]]s produce a [[hash function|hash]] of a message. While it should be easy to compute, it must be very difficult to invert ([[one way function|one-way]]-ness, though other properties are usually needed as well). [[MD5]] and [[SHA-1]] are well-known hash functions.
* [[Message authentication code]]s (MACs), also known as ''keyed-hash functions'', are similar to hash functions, except that a key is needed to compute the hash. As the name suggests, they are commonly used for message authentication. They are often constructed from other primitives, such as block ciphers, unkeyed-hash functions or stream ciphers.

==Public key cryptography==
''Main article'': [[Public key cryptography]] / [[Asymmetric key algorithm]]

Symmetric key encryption has a troublesome drawback &mdash two people who wish to exchange confidential messages must share a secret key. The key must be exchanged in a secure way, and not by the means they would normally communicate. This is usually inconvenient, and public-key (or aysmmetric) cryptography provides an alternative. In public key encryption there are two keys used, a ''public'' and a ''private'' key, for encryption and decryption respectively. It must be "difficult" to derive the private key from the public key. This means that someone can freely send their public key out over an insecure [[channel]] and yet be sure that only they can decrypt messages encrypted with it.

Public key algorithms are usually based on [[NP-hard|hard]] mathematical problems. [[RSA]], for example, relies on the (conjectured) difficulty of [[factorisation]]. For efficiency reasons, ''hybrid'' encryption systems are usually used in practice; a key is exchanged using a public-key cipher, and the rest of the communication is encrypted using a symmetric-key algorithm (which is typically much faster). [[Elliptic curve cryptography]] is a type of public-key algorithm that may offer efficiency gains over other schemes.

Assymetric cryptography also provides mechanisms for [[digital signature]]s, which are a digital equivalent of physical signatures for paper documents, for example [[DSA]] and [[ElGamal]].

==Other Topics==
''See also'': [[Topics in cryptography]]

The security of all practical encryption schemes remains unproven, both for symmetric and asymmetric schemes. For symmetric ciphers, confidence gained in an algorithm is usually anecdotal &mdash e.g. no successful attack has been reported on an algorithm for several years despite intensive analysis. Such a cipher might also have provable security against a limited class of attacks. For asymmetric schemes, it is common to rely on the difficulty of the associated mathematical problem, but this, too, is not provably secure.

Surprisingly, cryptography does have provably secure ciphers &mdash the [[one time pad]], for example. However, such schemes require keys as long as the plaintext, so are almost always too cumbersome to use.

When the security of a system fails, it is rare that a weakness in the cryptographic algorithms is exploited. More often, it is a mistake in the implementation, the protocols used or some other human error. The study of how best to implement and integrate cryptography is a field in itself, see: [[cryptographic engineering]], [[security engineering]] and [[crypto system|cryptosystem]].

Cryptography can be used to implement some remarkable protocols: [[zero-knowledge proof]], [[secure multiparty computation]] and [[secret sharing]], for example.

==Warnings==
Myths and misunderstandings about cryptography are widespread, and there is an abundance of insecure cryptographic software on the market, sometimes referred to (perjoratively) as [[snake oil]]. Readers, buyers, and users should therefore exercise substantial caution when selecting books and products; self-education is advised.

==Further reading==
* [[Topics in cryptography]] &mdash an analytical list of articles and terms.
* [[List of cryptology topics]] &mdash an alphabetical list of article and terms.
* [[Books on cryptography]] &mdash an annotated list of suggested readings.
* [[List of cryptographers]] &mdash an annotated list of cryptographers.
* [[List of important publications in computer science#Cryptography|Important publications in cryptography]] &mdash some cryptography papers in computer science.

==External links==
* [http://www.tcs.hut.fi/~helger/crypto/ Helger's cryptography pointers]
* [http://www.rsasecurity.com/rsalabs/faq/index.html RSA Laboratories' FAQ About today's cryptography ]
* [http://www.faqs.org/faqs/by-newsgroup/sci/sci.crypt.html The sci.crypt FAQ]
* [http://www.mindspring.com/~schlafly/crypto/faq.htm sci.crypt mini-FAQ (more recent)]
* [http://groups.google.com/groups?q=sci.crypt The sci.crypt newsgroup]
* [http://home.ecn.ab.ca/~jsavard/crypto/jscrypt.htm Savard's glossary]
* [http://www.murky.org/cryptography/index.shtml The Beginner's Guide to Cryptography] - An elementary overview of a few basic areas of cryptography.
* [http://www.hermetic.ch/crypto/intro.htm An Introduction to the Use of Encryption] - A fairly non-technical introduction to the subject.
* [http://www.mycrypto.net Encryption and Privacy]

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User:Matt Crypto/Simplified Cryptography - Revision history

imported>Matt Crypto: Reverted edits by 59.99.134.164 (talk) to last version by Kevyn