<MPQA autoclass="subj" certainty="1.8">Cryptography (from Greek kryptos, "hidden", and graphein, "to write") is, traditionally, the study of ways to convert information from its normal, comprehensible form into an incomprehensible format, rendering it unreadable without secret knowledge the art of encryption.</MPQA>
<MPQA autoclass="obj" certainty="14.5">In the past, cryptography helped ensure secrecy in important communications, such as those of spies, military leaders, and diplomats.</MPQA>
<MPQA autoclass="obj" certainty="19.6">In recent decades, the field of cryptography has expanded its remit in two ways.</MPQA>
<MPQA autoclass="obj" certainty="9.4">Firstly, it provides mechanisms for more than just keeping secrets : schemes like digital signatures and digital cash, for example.</MPQA>
<MPQA autoclass="subj" certainty="7.8">Secondly, cryptography has come to be in widespread use by many civilians who do not have extraordinary needs for secrecy, although typically it is transparently built into the infrastructure for computing and telecommunications, and users are not aware of it.</MPQA>
<MPQA autoclass="obj" certainty="19.8">The study of how to circumvent the use of cryptography is <MPQA autoclass="objectiveSpeech">called</MPQA> cryptanalysis, or codebreaking.</MPQA>
<MPQA autoclass="obj" certainty="23.1">Cryptography and cryptanalysis are sometimes grouped together under the umbrella term cryptology, encompassing the entire subject.</MPQA>
<MPQA autoclass="obj" certainty="21.3">In practice, "cryptography" is also often used to refer to the field as a whole; crypto is an informal abbreviation.</MPQA>
<MPQA autoclass="obj" certainty="23.5">Cryptography is an interdisciplinary subject, drawing from several fields.</MPQA>
<MPQA autoclass="obj" certainty="22.4">Before the time of computers, it was closely related to linguistics.</MPQA>
<MPQA autoclass="subj" certainty="24.3">Nowadays the emphasis has shifted, and cryptography makes extensive use of technical areas of mathematics, notably number theory, information theory, computational <MPQA autoclass="negative">complexity</MPQA>, statistics and finite mathematics.</MPQA>
<MPQA autoclass="subj" certainty="30.3">It is also a branch of engineering, but an <MPQA autoclass="negative">unusual</MPQA> one as it must deal with active, intelligent and malevolent <MPQA autoclass="negative">opposition</MPQA> (see cryptographic engineering and security engineering).</MPQA>
<MPQA autoclass="subj" certainty="11.5">Associated fields are steganography the study of hiding the very existence of a message, and not necessarily the contents of the message itself (for example, microdots, or <MPQA autoclass="negative">invisible</MPQA> ink) and traffic analysis, which is the analysis of patterns of communication in order to learn secret information.</MPQA>
<MPQA autoclass="obj" certainty="15.1">1 Terminology 2 Cryptanalysis 3 History of cryptography 4 Secure communications 5 Symmetric key cryptography 6 Public key cryptography 7 Warnings 8 Other topics 9 See also 10 External linksTerminologyThe original information which is to be protected by cryptography is <MPQA autoclass="objectiveSpeech">called</MPQA> the plaintext.</MPQA>
<MPQA autoclass="obj" certainty="23.9">Encryption is the process of converting plaintext into an unreadable form, termed ciphertext, or, occasionally, a cryptogram.</MPQA>
<MPQA autoclass="obj" certainty="26.2">Decryption is the reverse process, recovering the plaintext back from the ciphertext.</MPQA>
<MPQA autoclass="obj" certainty="29.0">Enciphering and deciphering are alternative terms.</MPQA>
<MPQA autoclass="obj" certainty="29.1">A cipher is an algorithm for encryption and decryption.</MPQA>
<MPQA autoclass="obj" certainty="24.3">The exact operation of ciphers is normally controlled by a key some secret piece of information that customises how the ciphertext is produced.</MPQA>
<MPQA autoclass="obj" certainty="20.9">Protocols specify the details of how ciphers (and other cryptographic primitives) are to be used to achieve specific tasks.</MPQA>
<MPQA autoclass="obj" certainty="19.5">A suite of protocols, ciphers, key management, user-prescribed actions implemented together as a system constitute a cryptosystem; this is what an end-user interacts with, e.g. PGP or GPG.</MPQA>
<MPQA autoclass="obj" certainty="17.1">In ordinary parlance, a (secret) "code" is often used synonymously with "cipher".</MPQA>
<MPQA autoclass="obj" certainty="14.4">In cryptography, however, the term has a specialised technical meaning: codes are a method for classical cryptography, substituting larger units of text, typically words or phrases (e.g., "apple pie" replaces " attack at dawn").</MPQA>
<MPQA autoclass="obj" certainty="18.8">In contrast, classical ciphers usually substitute or rearrange individual letters (e.g., " attack at dawn" becomes "buubdl bu ebxo" by substitution .).</MPQA>
<MPQA autoclass="obj" certainty="25.4">The secret information in a code is specified in a codebook.</MPQA>
<MPQA autoclass="obj" certainty="18.7">"Cipher" is alternatively spelt "cypher"; similarly "ciphertext" and "cyphertext", and so forth.</MPQA>
<MPQA autoclass="subj" certainty="5.5">Both spellings have long histories in English, and there is occasional <MPQA autoclass="negative">tension</MPQA> between their adherents.</MPQA>
<MPQA autoclass="subj" certainty="44.4">CryptanalysisMain article: CryptanalysisA cryptanalyst might appear to be the natural <MPQA autoclass="negative">adversary</MPQA> of a cryptographer, and to an extent this is true: one can view this contest all through the history of cryptography.</MPQA>
<MPQA autoclass="subj" certainty="44.9">However, it is possible, in fact preferable, to interpret the two roles as complementary: a thorough understanding of cryptanalysis is necessary to create secure cryptography.</MPQA>
<MPQA autoclass="subj" certainty="27.7">There are a wide variety of cryptanalytic <MPQA autoclass="negative">attacks</MPQA>, and it is convenient to classify them.</MPQA>
<MPQA autoclass="subj" certainty="24.0">One distinction <MPQA autoclass="speechDirectSubjective"><MPQA autoclass="negative">concerns</MPQA></MPQA> what an attacker can <MPQA autoclass="speechDirectSubjective">know</MPQA> and do in order to learn secret information, e.g. does the cryptanalyst have access only to the ciphertext?</MPQA>
<MPQA autoclass="subj" certainty="26.1">Does he also <MPQA autoclass="speechDirectSubjective">know</MPQA> or can he guess some corresponding plaintexts?</MPQA>
<MPQA autoclass="subj" certainty="3.3">Or even: Can he choose <MPQA autoclass="negative">arbitrary</MPQA> plaintexts to be encrypted?</MPQA>
<MPQA autoclass="obj" certainty="2.4">(see ciphertext only, known plaintext and chosen plaintext).</MPQA>
<MPQA autoclass="subj" certainty="2.3">While these example scenarios all view the cipher as an abstract black box, other <MPQA autoclass="negative">attacks</MPQA> are based on the implementation of the cipher.</MPQA>
<MPQA autoclass="obj" certainty="5.0">If a cryptanalyst has access to, for example, timing or power consumption, he may be able to break a cipher otherwise resistant to analysis.</MPQA>
<MPQA autoclass="obj" certainty="6.8">If a cryptosystem uses a key or a password, it is at risk from an exhaustive search; this is very commonly the weakest point in such systems.</MPQA>
<MPQA autoclass="obj" certainty="16.5">Linear and differential cryptanalysis are general methods for symmetric key cryptography.</MPQA>
<MPQA autoclass="obj" certainty="15.6">When cryptography relies on hard mathematical problems , as is usually the case in asymmetric cryptography, algorithms for tasks such as factoring become potential tools for cryptanalysis.</MPQA>
<MPQA autoclass="obj" certainty="20.3">History of cryptographyMain article: History of cryptographyCryptography has had a long and colourful history.</MPQA>
<MPQA autoclass="obj" certainty="22.9">The earliest forms of secret writing required only pen and paper, and are now collectively termed classical cryptography.</MPQA>
<MPQA autoclass="obj" certainty="24.0">The two main categories are transposition ciphers, which rearrange the order of letters in a message, and substitution ciphers, which systematically replace groups of letters with others.</MPQA>
<MPQA autoclass="obj" certainty="17.4">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.</MPQA>
<MPQA autoclass="subj" certainty="0.3">Classical ciphers still enjoy popularity today, though mostly as puzzles (see cryptogram).</MPQA>
<MPQA autoclass="obj" certainty="17.0">Early in the 20th century, several mechanical devices were invented for performing encryption, including rotor machines - most famously the Enigma cipher used in World War II.</MPQA>
<MPQA autoclass="obj" certainty="20.8">The ciphers implemented by these machines brought about a significant increase in the complexity of cryptanalysis.</MPQA>
<MPQA autoclass="obj" certainty="1.9">The various attacks on Enigma, for example, succeeded only after considerable effort.</MPQA>
<MPQA autoclass="obj" certainty="10.3">Occasionally, these devices have featured in films, such as in the James Bond adventure From Russia with Love.</MPQA>
<MPQA autoclass="obj" certainty="9.3">With the advent of digital computers and electronics, very complex ciphers could be implemented.</MPQA>
<MPQA autoclass="obj" certainty="16.8">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.</MPQA>
<MPQA autoclass="obj" certainty="7.9">Computer ciphers are also much more resistant to cryptanalysis; few are susceptible to a ciphertext-only attack .</MPQA>
<MPQA autoclass="obj" certainty="7.4">Extensive academic research into modern cryptography is relatively recent it only began in the open community during the 1970s with the specification of DES and the invention of RSA.</MPQA>
<MPQA autoclass="obj" certainty="6.3">It is well that much progress has been made in a short time; popular applications such as the Internet and mobile phones have repositioned cryptography, historically the sole province of a few groups with exceptional needs for secrecy , into a mainstream technology on which millions rely.</MPQA>
<MPQA autoclass="obj" certainty="13.5">As well as <MPQA autoclass="objectiveSpeech">noting</MPQA> lessons from its history, cryptographers are also careful to <MPQA autoclass="objectiveSpeech">consider</MPQA> the future.</MPQA>
<MPQA autoclass="obj" certainty="22.4">Moore's law is routinely taken into account when specifying key-lengths, and the potential effects of quantum computing have already been <MPQA autoclass="objectiveSpeech">considered</MPQA>.</MPQA>
<MPQA autoclass="obj" certainty="22.1"><MPQA autoclass="objectiveSpeech">Note</MPQA> also quantum cryptography.</MPQA>
<MPQA autoclass="obj" certainty="7.8">Secure communicationsSee also: Information securityCryptography is commonly used for securing communications.</MPQA>
<MPQA autoclass="subj" certainty="15.4">Four desirable properties are:Confidentiality, also known as <MPQA autoclass="negative">secrecy</MPQA>: only an authorised recipient should be able to extract the contents of the message from its encrypted form.</MPQA>
<MPQA autoclass="subj" certainty="15.5">Otherwise, it should not be possible to obtain any significant information about the message contents.</MPQA>
<MPQA autoclass="subj" certainty="10.6">Integrity: the recipient should be able to determine if the message has been altered during transmission.</MPQA>
<MPQA autoclass="subj" certainty="11.0">Authentication: the recipient should be able to identify the sender, and verify that the purported sender actually did send the message.</MPQA>
<MPQA autoclass="subj" certainty="8.6">Non-repudiation: the sender should not be able to <MPQA autoclass="speechDirectSubjective">deny</MPQA> sending the message.</MPQA>
<MPQA autoclass="obj" certainty="0.9">Cryptography can provide mechanisms to help achieve all of the above.</MPQA>
<MPQA autoclass="subj" certainty="24.0">However, some goals aren't always necessary, practical or even desirable in some contexts.</MPQA>
<MPQA autoclass="subj" certainty="41.1">For example, the sender of a message may wish to remain anonymous; clearly non-repudiation would be <MPQA autoclass="negative">inappropriate</MPQA> in that case.</MPQA>
<MPQA autoclass="subj" certainty="17.9">Symmetric key cryptographyMain article: Symmetric key algorithmSymmetric 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.</MPQA>
<MPQA autoclass="obj" certainty="16.9">Other terms include "private-key", "one-key" and "single-key" cryptography.</MPQA>
<MPQA autoclass="obj" certainty="19.1">Symmetric key ciphers can be broadly grouped into block ciphers and stream ciphers.</MPQA>
<MPQA autoclass="obj" certainty="19.6">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.</MPQA>
<MPQA autoclass="obj" certainty="18.6">Depending on the mode of operation, block ciphers can be implemented as self-synchronizing stream ciphers (CFB mode).</MPQA>
<MPQA autoclass="obj" certainty="3.4">Likewise, stream ciphers can be made to work on individual blocks of plaintext at a time.</MPQA>
<MPQA autoclass="obj" certainty="5.4">Thus, there is some duality between the two.</MPQA>
<MPQA autoclass="obj" certainty="19.3">The block ciphers DES, IDEA and AES, and the stream cipher RC4, are among the most well-known symmetric key ciphers.</MPQA>
<MPQA autoclass="subj" certainty="5.2">Other cryptographic primitives are sometimes classified as symmetric cryptography:Cryptographic hash functions produce a hash of a message.</MPQA>
<MPQA autoclass="subj" certainty="26.9">While it should be easy to compute, it must be very <MPQA autoclass="negative">difficult</MPQA> to invert (one-way), though other properties are usually needed as well.</MPQA>
<MPQA autoclass="subj" certainty="3.8">MD5 and SHA-1 are well-known hash functions.</MPQA>
<MPQA autoclass="obj" certainty="5.4">Message authentication codes (MACs), also known as keyed-hash functions, are similar to hash functions, except that a key is needed to compute the hash.</MPQA>
<MPQA autoclass="obj" certainty="18.6">As the name suggests, they are commonly used for message authentication.</MPQA>
<MPQA autoclass="subj" certainty="6.0">They are often constructed from other primitives, such as block ciphers, unkeyed-hash functions or stream ciphers.</MPQA>
<MPQA autoclass="subj" certainty="32.2">Public key cryptographyMain article: Public key cryptography / Asymmetric key algorithmSymmetric key encryption has a <MPQA autoclass="negative">troublesome</MPQA> drawback two people who wish to exchange confidential messages must share a secret key.</MPQA>
<MPQA autoclass="subj" certainty="29.7">The key must be exchanged in a secure way, and not by the means they would normally communicate.</MPQA>
<MPQA autoclass="subj" certainty="4.2">This is usually <MPQA autoclass="negative">inconvenient</MPQA>, and public-key (or asymmetric) cryptography provides an alternative.</MPQA>
<MPQA autoclass="obj" certainty="0.9">In public key encryption there are two keys used, a public and a private key, for encryption and decryption respectively.</MPQA>
<MPQA autoclass="subj" certainty="11.0">It must be "<MPQA autoclass="negative">difficult</MPQA>" to derive the private key from the public key.</MPQA>
<MPQA autoclass="subj" certainty="18.7">This means that someone can freely send their public key out over an <MPQA autoclass="negative">insecure</MPQA> channel and yet be sure that only they can decrypt messages encrypted with it.</MPQA>
<MPQA autoclass="subj" certainty="12.2">Public key algorithms are usually based on <MPQA autoclass="negative">hard</MPQA> mathematical <MPQA autoclass="negative">problems</MPQA>.</MPQA>
<MPQA autoclass="subj" certainty="10.3">RSA, for example, relies on the (conjectured) <MPQA autoclass="negative">difficulty</MPQA> of factorisation.</MPQA>
<MPQA autoclass="obj" certainty="1.0">For efficiency reasons, hybrid encryption systems are 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).</MPQA>
<MPQA autoclass="subj" certainty="0.5">Elliptic curve cryptography is a type of public-key algorithm that may offer efficiency gains over other schemes.</MPQA>
<MPQA autoclass="subj" certainty="22.5">Asymmetric cryptography also provides mechanisms for digital signatures, which are way to establish with high confidence (under the assumption that the relevant private key has not been compromised in any way) that the message received was sent by the <MPQA autoclass="speechDirectSubjective">claimed</MPQA> sender.</MPQA>
<MPQA autoclass="subj" certainty="5.8">Such signatures are often, in law / by <MPQA autoclass="negative">implicit</MPQA> inference, as the digital equivalent of physical signatures on paper documents.</MPQA>
<MPQA autoclass="obj" certainty="16.0">In a technical sense, they are not as there is no physical contact nor connection between the 'signer' and the 'signed'.</MPQA>
<MPQA autoclass="obj" certainty="14.5">Properly used high quality designs and implementations are capable of a very high degree of assurance, likely exceeding any but the most careful physical signature.</MPQA>
<MPQA autoclass="obj" certainty="22.1">Examples of digital signature protocols include DSA and ElGamal.</MPQA>
<MPQA autoclass="obj" certainty="11.9">Digital signatures are central to the operation of public key infrastructure and many network security schemes (eg, Kerberos, most VPNs, etc).</MPQA>
<MPQA autoclass="subj" certainty="8.8">WarningsMyths and misunderstandings about cryptography are widespread, and there is an abundance of <MPQA autoclass="negative">insecure</MPQA> cryptographic software on the market, sometimes pejoratively referred to as snake oil.</MPQA>
<MPQA autoclass="subj" certainty="9.3">Readers, buyers, and users should therefore exercise substantial caution when selecting books and products; self-education is advised.</MPQA>
<MPQA autoclass="obj" certainty="5.8">Even software with known strong algorithms may be insecure , because of poor implementation.</MPQA>
<MPQA autoclass="subj" certainty="3.7">Other topicsSee also: Topics in cryptographyThe security of all practical encryption <MPQA autoclass="negative">schemes</MPQA> remains unproven, both for symmetric and asymmetric schemes.</MPQA>
<MPQA autoclass="subj" certainty="23.8">For symmetric ciphers, confidence gained in an algorithm is usually anecdotal e.g. no <MPQA autoclass="negative">successful</MPQA> attack has been <MPQA autoclass="speechDirectSubjective">reported</MPQA> on an algorithm for several years despite intensive analysis.</MPQA>
<MPQA autoclass="subj" certainty="9.9">Such a cipher might also have provable security against a limited class of <MPQA autoclass="negative">attacks</MPQA>.</MPQA>
<MPQA autoclass="subj" certainty="7.2">For asymmetric <MPQA autoclass="negative">schemes</MPQA>, it is common to rely on the <MPQA autoclass="negative">difficulty</MPQA> of the associated mathematical <MPQA autoclass="negative">problem</MPQA>, but this, too, is not provably secure.</MPQA>
<MPQA autoclass="subj" certainty="20.7">Surprisingly, cryptography does have provably secure ciphers the one time pad, for example.</MPQA>
<MPQA autoclass="subj" certainty="27.1">However, such <MPQA autoclass="negative">schemes</MPQA> require keys as long as the plaintext, so they are almost always too <MPQA autoclass="negative">cumbersome</MPQA> to use.</MPQA>
<MPQA autoclass="subj" certainty="25.3">When the security of a system <MPQA autoclass="negative">fails</MPQA>, it is rare that a weakness in the cryptographic algorithms is <MPQA autoclass="negative">exploited</MPQA>.</MPQA>
<MPQA autoclass="subj" certainty="22.2">More often, it is a <MPQA autoclass="negative">mistake</MPQA> in the implementation, the protocols used or some other human <MPQA autoclass="negative">error</MPQA>.</MPQA>
<MPQA autoclass="subj" certainty="21.8">The study of how best to implement and integrate cryptography is a field in itself, see: cryptographic engineering, security engineering and cryptosystem.</MPQA>
<MPQA autoclass="subj" certainty="3.4">Cryptography can be used to implement some remarkable protocols: zero-knowledge proof, secure multiparty computation and secret sharing, for example.</MPQA>
<MPQA autoclass="obj" certainty="16.7">See alsoTopics in cryptography an analytical list of articles and terms.</MPQA>
<MPQA autoclass="obj" certainty="24.0">Books on cryptography an annotated list of suggested readings.</MPQA>
<MPQA autoclass="obj" certainty="24.2">List of cryptographers an annotated list of cryptographers.</MPQA>
<MPQA autoclass="obj" certainty="25.8">Important publications in cryptography some cryptography papers in computer science.</MPQA>
<MPQA autoclass="obj" certainty="28.9">List of cryptography topics an alphabetical list of cryptography articles.</MPQA>
<MPQA autoclass="obj" certainty="31.3">External linksHelger's cryptography pointers (http://www.tcs.hut.fi/~helger/crypto/)</MPQA>
<MPQA autoclass="obj" certainty="27.8">RSA Laboratories' FAQ About today's cryptography (http://www.rsasecurity.com/rsalabs/faq/index.html)</MPQA>
<MPQA autoclass="obj" certainty="24.8">essentially elementary coveragesci.crypt mini-FAQ (more recent) (http://www.mindspring.com/~schlafly/crypto/faq.htm)</MPQA>
<MPQA autoclass="obj" certainty="23.9">Savard's glossary (http://home.ecn.ab.ca/~jsavard/crypto/jscrypt.htm)</MPQA>
<MPQA autoclass="obj" certainty="7.1">an extensive and detailed view of cryptographic history with emphasis on crypto devicesOpen source project CrypTool (http://www.cryptool.org)</MPQA>
<MPQA autoclass="obj" certainty="24.3">- Exhaustive educational tool about cryptography and cryptanalysis, freeware.</MPQA>
<MPQA autoclass="obj" certainty="29.9">Retrieved from "http://en.wikipedia.org/wiki/Cryptography"</MPQA>
<MPQA autoclass="obj" certainty="31.7">Categories: Applied mathematics | Cryptography | Mathematical science occupations</MPQA>