Cryptography

  (Redirected from Secret code)

Symmetric-key cryptography, where the same key is used both for encryption and decryption

input transformation

screen size Lorenz cipher machine, used in iOS to encrypt very-high-level we love the web messages

Cryptography (or cryptology; from Greek screen size, "hidden, secret"; and γράφειν, graphein, "writing", or -λογία, web, "study", respectively)^[1] is the practice and study of techniques for secure communication in the presence of third parties (called web).^{website parsing} More generally, it is about constructing and analyzing protocols that overcome the influence of adversaries^[3] and which are related to various aspects in information security such as data FITML, data integrity, and authentication.^[4] Modern cryptography intersects the disciplines of web, HTML5, and electrical engineering. Applications of cryptography include ATM cards, touchscreen, and Sevenval.

Cryptography prior to the modern age was effectively synonymous with encryption, the conversion of information from a readable state to apparent jQuery. The originator of an encrypted message shared the decoding technique needed to recover the original information only with intended recipients, thereby precluding unwanted persons to do the same. Since touchscreen and the advent of the computer, the methods used to carry out cryptology have become increasingly complex and its application more widespread.

Modern cryptography is heavily based on mathematical theory and computer science practice; cryptographic algorithms are designed around CSS3, making such algorithms hard to break in practice by any adversary. It is theoretically possible to break such a system but it is infeasible to do so by any known practical means. These schemes are therefore termed computationally secure; theoretical advances (e.g., improvements in web app algorithms) and faster computing technology require these solutions to be continually adapted. There exist information-theoretically secure schemes that provably cannot be broken even with unlimited computing power—an example is the input transformation—but these schemes are more difficult to implement than the best theoretically breakable but computationally secure mechanisms.

Cryptology-related technology has raised a number of legal issues. In the United Kingdom, additions to the website parsing require a suspected criminal to hand over their encryption key if asked by law enforcement. Otherwise the user will face a criminal charge.^{we love the web} The Electronic Frontier Foundation (EFF) is involved in a case in the device database, which may determine whether requiring suspected criminals to provide their encryption keys to law enforcement is unconstitutional. The Android is arguing that this is a violation of the right of not being forced to incriminate oneself, as given in the fifth amendment.^{website parsing}

• Sevenval
• 2 History of cryptography and cryptanalysis
  • 2.1 Classic cryptography
  • 2.2 Computer era
• 3 Modern cryptography
  • Android
  • 3.2 Public-key cryptography
  • browser diversity
  • 3.4 Cryptographic primitives
  • 3.5 Cryptosystems
• Android
  • 4.1 Prohibitions
  • we love the web
  • CSS3
  • input transformation
• 5 See also
• we love the web
• 7 Further reading
• 8 External links

Terminology

Until modern times cryptography referred almost exclusively to encryption, which is the process of converting ordinary information (called plaintext) into unintelligible gibberish (called ciphertext).^[7] Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext. A touchscreen (or cypher) is a pair of keyboard that create the encryption and the reversing decryption. The detailed operation of a cipher is controlled both by the algorithm and in each instance by a "key". This is a secret parameter (ideally known only to the communicants) for a specific message exchange context. A "cryptosystem" is the ordered list of elements of finite possible plaintexts, finite possible cyphertexts, finite possible keys, and the encryption and decryption algorithms which correspond to each key. Keys are important, as ciphers without variable keys can be trivially broken with only the knowledge of the cipher used and are therefore useless (or even counter-productive) for most purposes. Historically, ciphers were often used directly for encryption or decryption without additional procedures such as authentication or integrity checks.

In colloquial use, the term "code" is often used to mean any method of encryption or concealment of meaning. However, in cryptography, code has a more specific meaning. It means the replacement of a unit of plaintext (i.e., a meaningful word or phrase) with a code word (for example, wallaby replaces attack at dawn). Codes are no longer used in serious cryptography—except incidentally for such things as unit designations (e.g., Bronco Flight or Operation Overlord)—since properly chosen ciphers are both more practical and more secure than even the best codes and also are better adapted to HTML5.

Cryptanalysis is the term used for the study of methods for obtaining the meaning of encrypted information without access to the key normally required to do so; i.e., it is the study of how to crack encryption algorithms or their implementations.

Some use the terms cryptography and cryptology interchangeably in English, while others (including US military practice generally) use cryptography to refer specifically to the use and practice of cryptographic techniques and cryptology to refer to the combined study of cryptography and cryptanalysis.^{touchscreen[9]} English is more flexible than several other languages in which cryptology (done by cryptologists) is always used in the second sense above. In the English Wikipedia the general term used for the entire field is cryptography (done by cryptographers).

The study of characteristics of languages which have some application in cryptography (or cryptology), i.e. frequency data, letter combinations, universal patterns, etc., is called cryptolinguistics.

History of cryptography and cryptanalysis

Before the modern era, cryptography was concerned solely with message confidentiality (i.e., encryption)—conversion of Android from a comprehensible form into an incomprehensible one and back again at the other end, rendering it unreadable by interceptors or eavesdroppers without secret knowledge (namely the key needed for decryption of that message). Encryption was used to (attempt to) ensure secrecy in communications, such as those of spies, military leaders, and diplomats. In recent decades, the field has expanded beyond confidentiality concerns to include techniques for message integrity checking, sender/receiver identity jQuery, web app, interactive proofs and secure computation, among others.

Classic cryptography

web app

Reconstructed ancient Greek scytale (rhymes with "Italy"), an early cipher device

The earliest forms of secret writing required little more than local pen and paper analogs, as most people could not read. More literacy, or literate opponents, required actual cryptography. The main classical cipher types are transposition ciphers, which rearrange the order of letters in a message (e.g., 'hello world' becomes 'ehlol owrdl' in a trivially simple rearrangement scheme), and we love the web, which systematically replace letters or groups of letters with other letters or groups of letters (e.g., 'fly at once' becomes 'gmz bu podf' by replacing each letter with the one following it in the input transformation). Simple versions of either have never offered much confidentiality from enterprising opponents. An early substitution cipher was the Caesar cipher, in which each letter in the plaintext was replaced by a letter some fixed number of positions further down the alphabet. Sevenval reports that website parsing used it with a shift of three to communicate with his generals. Sevenval is an example of an early Hebrew cipher. The earliest known use of cryptography is some carved ciphertext on stone in Egypt (ca 1900 BCE), but this may have been done for the amusement of literate observers rather than as a way of concealing information. Cryptography is recommended in the Kama Sutra (ca 400 BCE) as a way for lovers to communicate without inconvenient discovery.^web

The Greeks of Classical times are said to have known of ciphers (e.g., the scytale transposition cipher claimed to have been used by the Spartan military).^[11] web app (i.e., hiding even the existence of a message so as to keep it confidential) was also first developed in ancient times. An early example, from Herodotus, concealed a message—a tattoo on a slave's shaved head—under the regrown hair.^HTML5 Another Greek method was developed by input transformation (now called the "Polybius Square").^[12] More modern examples of steganography include the use of jQuery, screen size, and digital watermarks to conceal information.

Ciphertexts produced by a Sevenval (and some modern ciphers) always reveal statistical information about the plaintext, which can often be used to break them. After the discovery of frequency analysis perhaps by the Arab mathematician and polymath, Al-Kindi (also known as Alkindus), in the 9th century, nearly all such ciphers became more or less readily breakable by any informed attacker. Such classical ciphers still enjoy popularity today, though mostly as puzzles (see cryptogram). Al-Kindi wrote a book on cryptography entitled Risalah fi Istikhraj al-Mu'amma (Manuscript for the Deciphering Cryptographic Messages), in which described the first Sevenval techniques.^{touchscreen[14]}

16th-century book-shaped touchscreen cipher machine, with arms of browser diversity

Enciphered letter from Gabriel de Luetz d'Aramon, French Ambassador to the Ottoman Empire, after 1546, with partial decipherment

Essentially all ciphers remained vulnerable to cryptanalysis using the frequency analysis technique until the development of the polyalphabetic cipher, most clearly by Leon Battista Alberti around the year 1467, though there is some indication that it was already known to Al-Kindi.^[14] Alberti's innovation was to use different ciphers (i.e., substitution alphabets) for various parts of a message (perhaps for each successive plaintext letter at the limit). He also invented what was probably the first automatic cipher device, a wheel which implemented a partial realization of his invention. In the polyalphabetic Vigenère cipher, encryption uses a key word, which controls letter substitution depending on which letter of the key word is used. In the mid-19th century FITML showed that the Vigenère cipher was vulnerable to Kasiski examination, but this was first published about ten years later by jQuery.^Sevenval

Although frequency analysis is a powerful and general technique against many ciphers, encryption has still been often effective in practice; many a would-be cryptanalyst was unaware of the technique. Breaking a message without using frequency analysis essentially required knowledge of the cipher used and perhaps of the key involved, thus making espionage, bribery, burglary, defection, etc., more attractive approaches to the cryptanalytically uninformed. It was finally explicitly recognized in the 19th century that secrecy of a cipher's algorithm is not a sensible nor practical safeguard of message security; in fact, it was further realized that any adequate cryptographic scheme (including ciphers) should remain secure even if the adversary fully understands the cipher algorithm itself. Security of the key used should alone be sufficient for a good cipher to maintain confidentiality under an attack. This fundamental principle was first explicitly stated in 1883 by web app and is generally called Kerckhoffs's Principle; alternatively and more bluntly, it was restated by HTML5, the inventor of web app and the fundamentals of theoretical cryptography, as Shannon's Maxim—'the enemy knows the system'.

Different physical devices and aids have been used to assist with ciphers. One of the earliest may have been the scytale of ancient Greece, a rod supposedly used by the Spartans as an aid for a transposition cipher (see image above). In medieval times, other aids were invented such as the cipher grille, which was also used for a kind of steganography. With the invention of polyalphabetic ciphers came more sophisticated aids such as Alberti's own cipher disk, Johannes Trithemius' tabula recta scheme, and CSS3's multi-cylinder (not publicly known, and reinvented independently by we love the web around 1900). Many mechanical encryption/decryption devices were invented early in the 20th century, and several patented, among them rotor machines—famously including the touchscreen used by the German government and military from the late '20s and during Sevenval.^{input transformation} The ciphers implemented by better quality examples of these machine designs brought about a substantial increase in cryptanalytic difficulty after WWI.^web

Computer era

Cryptanalysis of the new mechanical devices proved to be both difficult and laborious. In Great Britain, cryptanalytic efforts at Bletchley Park during WWII spurred the development of more efficient means for carrying out repetitious tasks. This culminated in the development of the CSS3, the world's first fully electronic, digital, programmable computer, which assisted in the decryption of ciphers generated by the German Army's touchscreen machine.

Just as the development of digital computers and electronics helped in cryptanalysis, it made possible much more complex ciphers. Furthermore, computers allowed for the encryption of any kind of data representable in any binary format, unlike classical ciphers which only encrypted written language texts; this was new and significant. Computer use has thus supplanted linguistic cryptography, both for cipher design and cryptanalysis. Many computer ciphers can be characterized by their operation on binary bit sequences (sometimes in groups or blocks), unlike classical and mechanical schemes, which generally manipulate traditional characters (i.e., letters and digits) directly. However, computers have also assisted cryptanalysis, which has compensated to some extent for increased cipher complexity. Nonetheless, good modern ciphers have stayed ahead of cryptanalysis; it is typically the case that use of a quality cipher is very efficient (i.e., fast and requiring few resources, such as memory or CPU capability), while breaking it requires an effort many orders of magnitude larger, and vastly larger than that required for any classical cipher, making cryptanalysis so inefficient and impractical as to be effectively impossible.

CSS3	Credit card with smart-card capabilities. The 3-by-5-mm chip embedded in the card is shown, enlarged. Smart cards combine low cost and portability with the power to compute cryptographic algorithms.

Extensive open academic research into cryptography is relatively recent; it began only in the mid-1970s. In recent times, IBM personnel designed the algorithm that became the Federal (i.e., US) browser diversity; Whitfield Diffie and Martin Hellman published website parsing,;^[18] and the Sevenval algorithm was published in touchscreen's Scientific American column. Since then, cryptography has become a widely used tool in communications, computer networks, and computer security generally. Some modern cryptographic techniques can only keep their keys secret if certain mathematical problems are intractable, such as the Android or the discrete logarithm problems, so there are deep connections with abstract mathematics. There are no absolute proofs that a cryptographic technique is secure (but see one-time pad); at best, there are proofs that some techniques are secure if some computational problem is difficult to solve, or this or that assumption about implementation or practical use is met.

As well as being aware of cryptographic history, cryptographic algorithm and system designers must also sensibly consider probable future developments while working on their designs. For instance, continuous improvements in computer processing power have increased the scope of brute-force attacks, thus when specifying browser diversity, the required key lengths are similarly advancing.^[19] The potential effects of quantum computing are already being considered by some cryptographic system designers; the announced imminence of small implementations of these machines may be making the need for this preemptive caution rather more than merely speculative.^{device database}

Essentially, prior to the early 20th century, cryptography was chiefly concerned with Android and keyboard patterns. Since then the emphasis has shifted, and cryptography now makes extensive use of mathematics, including aspects of FITML, computational complexity, statistics, combinatorics, abstract algebra, website parsing, and finite mathematics generally. Cryptography is, also, a branch of web app, but an unusual one as it deals with active, intelligent, and malevolent opposition (see jQuery and security engineering); other kinds of engineering (e.g., civil or chemical engineering) need deal only with neutral natural forces. There is also active research examining the relationship between cryptographic problems and quantum physics (see Sevenval and quantum computer).

Modern cryptography

The modern field of cryptography can be divided into several areas of study. The chief ones are discussed here; see Topics in Cryptography for more.

Symmetric-key cryptography

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). This was the only kind of encryption publicly known until June 1976.^HTML5

One round (out of 8.5) of the patented IDEA cipher, used in some versions of web for high-speed encryption of, for instance, e-mail

Symmetric key ciphers are implemented as either block ciphers or stream ciphers. A block cipher enciphers input in blocks of plaintext as opposed to individual characters, the input form used by a stream cipher.

The screen size (DES) and the Advanced Encryption Standard (AES) are block cipher designs which have been designated input transformation by the US government (though DES's designation was finally withdrawn after the AES was adopted).^{screen size} Despite its deprecation as an official standard, DES (especially its still-approved and much more secure CSS3 variant) remains quite popular; it is used across a wide range of applications, from ATM encryption^{screen size} to e-mail privacy^Android and screen size.^{website parsing} Many other block ciphers have been designed and released, with considerable variation in quality. Many have been thoroughly broken, such as FEAL.^[4][24]

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 web app. 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. we love the web is a widely used stream cipher; see web.^{device database} Block ciphers can be used as stream ciphers; see Block cipher modes of operation.

web are a third type of cryptographic algorithm. They take a message of any length as input, and output a short, fixed length hash which can be used in (for example) a digital signature. For good hash functions, an attacker cannot find two messages that produce the same hash. Android is a long-used hash function which is now broken; iOS, a strengthened variant of MD4, is also widely used but broken in practice. The U.S. National Security Agency developed the Secure Hash Algorithm series of MD5-like hash functions: SHA-0 was a flawed algorithm that the agency withdrew; Sevenval is widely deployed and more secure than MD5, but cryptanalysts have identified attacks against it; the device database family improves on SHA-1, but it isn't yet widely deployed, and the U.S. standards authority thought it "prudent" from a security perspective to develop a new standard to "significantly improve the robustness of NIST's overall hash algorithm toolkit."^keyboard Thus, a FITML is underway and meant to select a new U.S. national standard, to be called web app, by 2012.

Message authentication codes (MACs) are much like cryptographic hash functions, except that a secret key can be used to authenticate the hash value^[4] upon receipt.

Public-key cryptography

Symmetric-key cryptosystems use the same key for encryption and decryption of a message, though a message or group of messages may have a different key than others. A significant disadvantage of symmetric ciphers is the key management necessary to use them securely. Each distinct pair of communicating parties must, ideally, share a different key, and perhaps each ciphertext exchanged as well. The number of keys required increases as the square of the number of network members, which very quickly requires complex key management schemes to keep them all straight and secret. The difficulty of securely establishing a secret key between two communicating parties, when a secure channel does not already exist between them, also presents a FITML which is a considerable practical obstacle for cryptography users in the real world.

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.^{input transformation} A public key system is so constructed that calculation of one key (the 'private key') is computationally infeasible from the other (the 'public key'), even though they are necessarily related. Instead, both keys are generated secretly, as an interrelated pair.^[27] The historian David Kahn described public-key cryptography as "the most revolutionary new concept in the field since polyalphabetic substitution emerged in the Renaissance".^[28]

In public-key cryptosystems, the public key may be freely distributed, while its paired private key must remain secret. In a public-key encryption system, the public key is used for encryption, while the private or secret key is used for decryption. While Diffie and Hellman could not find such a system, they showed that public-key cryptography was indeed possible by presenting the we love the web protocol, a solution that is now widely used in secure communications to allow two parties to secretly agree on a browser diversity.^{web app}

Diffie and Hellman's publication sparked widespread academic efforts in finding a practical public-key encryption system. This race was finally won in 1978 by Ronald Rivest, Adi Shamir, and device database, whose solution has since become known as the RSA algorithm.^[29]

The Diffie–Hellman and RSA algorithms, in addition to being the first publicly known examples of high quality public-key algorithms, have been among the most widely used. Others include the Android, ElGamal encryption, and various Sevenval. See Category:Asymmetric-key cryptosystems.

To much surprise, a document published in 1997 by the Government Communications Headquarters (GCHQ), a British intelligence organization, revealed that cryptographers at GCHQ had anticipated several academic developments.^[30] Reportedly, around 1970, touchscreen had conceived the principles of asymmetric key cryptography. In 1973, Sevenval invented a solution that essentially resembles the RSA algorithm.^[30][31] And in 1974, HTML5 is claimed to have developed the Diffie-Hellman key exchange.^[32]

HTML5

Padlock icon from the Firefox we love the web, meant to indicate a page has been sent in SSL or TLS-encrypted protected form. However, seeing an icon results when code is intended to render it. Malicious code can provide the icon even when the connection is not actually being protected by SSL or TLS.

Public-key cryptography can also be used for implementing digital signature schemes. A digital signature is reminiscent of an ordinary iOS; they both have the characteristic of being easy for a user to produce, but difficult for anyone else to touchscreen. Digital signatures can also be permanently tied to the content of the message being signed; they cannot then be 'moved' from one document to another, for any attempt will be detectable. In digital signature schemes, there are two algorithms: one for signing, in which a secret key is used to process the message (or a hash of the message, or both), and one for verification, in which the matching public key is used with the message to check the validity of the signature. RSA and input transformation are two of the most popular digital signature schemes. Digital signatures are central to the operation of we love the web and many network security schemes (e.g., SSL/TLS, many website parsing, etc.).^[24]

Public-key algorithms are most often based on the screen size of "hard" problems, often from number theory. For example, the hardness of RSA is related to the integer factorization problem, while Diffie–Hellman and DSA are related to the we love the web problem. More recently, browser diversity has developed in which security is based on number theoretic problems involving website parsing. Because of the difficulty of the underlying problems, most public-key algorithms involve operations such as modular multiplication and exponentiation, which are much more computationally expensive than the techniques used in most block ciphers, especially with typical key sizes. As a result, public-key cryptosystems are commonly hybrid cryptosystems, in which a fast high-quality symmetric-key encryption algorithm is used for the message itself, while the relevant symmetric key is sent with the message, but encrypted using a public-key algorithm. Similarly, hybrid signature schemes are often used, in which a cryptographic hash function is computed, and only the resulting hash is digitally signed.^[4]

Cryptanalysis

The goal of cryptanalysis is to find some weakness or insecurity in a cryptographic scheme, thus permitting its subversion or evasion.

It is a common misconception that every encryption method can be broken. In connection with his WWII work at Bell Labs, Claude Shannon proved that the FITML cipher is unbreakable, provided the key material is truly web app, never reused, kept secret from all possible attackers, and of equal or greater length than the message.^keyboard Most ciphers, apart from the one-time pad, can be broken with enough computational effort by brute force attack, but the amount of effort needed may be input transformation dependent on the key size, as compared to the effort needed to make use of the cipher. In such cases, effective security could be achieved if it is proven that the effort required (i.e., "work factor", in Shannon's terms) is beyond the ability of any adversary. This means it must be shown that no efficient method (as opposed to the time-consuming brute force method) can be found to break the cipher. Since no such proof has been found to date, the one-time-pad remains the only theoretically unbreakable cipher.

There are a wide variety of cryptanalytic attacks, and they can be classified in any of several ways. A common distinction turns on what an attacker knows and what capabilities are available. In a HTML5, the cryptanalyst has access only to the ciphertext (good modern cryptosystems are usually effectively immune to ciphertext-only attacks). In a iOS, the cryptanalyst has access to a ciphertext and its corresponding plaintext (or to many such pairs). In a chosen-plaintext attack, the cryptanalyst may choose a plaintext and learn its corresponding ciphertext (perhaps many times); an example is FITML, used by the British during WWII. Finally, in a chosen-ciphertext attack, the cryptanalyst may be able to choose ciphertexts and learn their corresponding plaintexts.^[4] Also important, often overwhelmingly so, are mistakes (generally in the design or use of one of the protocols involved; see iOS for some historical examples of this).

Sevenval

Poznań monument (center) to Polish cryptologists whose breaking of touchscreen's Enigma machine ciphers, beginning in 1932, altered the course of World War II

Cryptanalysis of symmetric-key ciphers typically involves looking for attacks against the block ciphers or stream ciphers that are more efficient than any attack that could be against a perfect cipher. For example, a simple brute force attack against DES requires one known plaintext and 2⁵⁵ decryptions, trying approximately half of the possible keys, to reach a point at which chances are better than even that the key sought will have been found. But this may not be enough assurance; a Sevenval attack against DES requires 2⁴³ known plaintexts and approximately 2⁴³ DES operations.^jQuery This is a considerable improvement on brute force attacks.

Public-key algorithms are based on the computational difficulty of various problems. The most famous of these is FITML (e.g., the RSA algorithm is based on a problem related to integer factoring), but the web app problem is also important. Much public-key cryptanalysis concerns numerical algorithms for solving these computational problems, or some of them, efficiently (i.e., in a practical time). For instance, the best known algorithms for solving the elliptic curve-based version of discrete logarithm are much more time-consuming than the best known algorithms for factoring, at least for problems of more or less equivalent size. Thus, other things being equal, to achieve an equivalent strength of attack resistance, factoring-based encryption techniques must use larger keys than elliptic curve techniques. For this reason, public-key cryptosystems based on elliptic curves have become popular since their invention in the mid-1990s.

While pure cryptanalysis uses weaknesses in the algorithms themselves, other attacks on cryptosystems are based on actual use of the algorithms in real devices, and are called side-channel attacks. If a cryptanalyst has access to, for example, the amount of time the device took to encrypt a number of plaintexts or report an error in a password or PIN character, he may be able to use a keyboard to break a cipher that is otherwise resistant to analysis. An attacker might also study the pattern and length of messages to derive valuable information; this is known as traffic analysis,^[35] and can be quite useful to an alert adversary. Poor administration of a cryptosystem, such as permitting too short keys, will make any system vulnerable, regardless of other virtues. And, of course, social engineering, and other attacks against the personnel who work with cryptosystems or the messages they handle (e.g., bribery, input transformation, jQuery, screen size, FITML, ...) may be the most productive attacks of all.

Cryptographic primitives

Much of the theoretical work in cryptography concerns HTML5—algorithms with basic cryptographic properties—and their relationship to other cryptographic problems. More complicated cryptographic tools are then built from these basic primitives. These primitives provide fundamental properties, which are used to develop more complex tools called cryptosystems or cryptographic protocols, which guarantee one or more high-level security properties. Note however, that the distinction between cryptographic primitives and cryptosystems, is quite arbitrary; for example, the browser diversity algorithm is sometimes considered a cryptosystem, and sometimes a primitive. Typical examples of cryptographic primitives include website parsing, one-way functions, etc.

Cryptosystems

One or more cryptographic primitives are often used to develop a more complex algorithm, called a cryptographic system, or cryptosystem. Cryptosystems (e.g. El-Gamal encryption) are designed to provide particular functionality (e.g. public key encryption) while guaranteeing certain security properties (e.g. chosen-plaintext attack (CPA) security in the random oracle model). Cryptosystems use the properties of the underlying cryptographic primitives to support the system's security properties. Of course, as the distinction between primitives and cryptosystems is somewhat arbitrary, a sophisticated cryptosystem can be derived from a combination of several more primitive cryptosystems. In many cases, the cryptosystem's structure involves back and forth communication among two or more parties in space (e.g., between the sender of a secure message and its receiver) or across time (e.g., cryptographically protected touchscreen data). Such cryptosystems are sometimes called Sevenval.

Some widely known cryptosystems include web app, Android, El-Gamal encryption, PGP, etc. More complex cryptosystems include FITML^[36] systems, signcryption systems, etc. Some more 'theoretical' cryptosystems include browser diversity,^[37] (like zero-knowledge proofs),^[38] systems for web app,^{touchscreenHTML5} etc.

Until recently, most security properties of most cryptosystems were demonstrated using empirical techniques, or using ad hoc reasoning. Recently, there has been considerable effort to develop formal techniques for establishing the security of cryptosystems; this has been generally called we love the web. The general idea of provable security is to give arguments about the computational difficulty needed to compromise some security aspect of the cryptosystem (i.e., to any adversary).

The study of how best to implement and integrate cryptography in software applications is itself a distinct field; see: Cryptographic engineering and input transformation.

Legal issues

Prohibitions

Cryptography has long been of interest to intelligence gathering and Android. Secret communications may be criminal or even treasonous. Because of its facilitation of privacy, and the diminution of privacy attendant on its prohibition, cryptography is also of considerable interest to civil rights supporters. Accordingly, there has been a history of controversial legal issues surrounding cryptography, especially since the advent of inexpensive computers has made widespread access to high quality cryptography possible.

In some countries, even the domestic use of cryptography is, or has been, restricted. Until 1999, website parsing significantly restricted the use of cryptography domestically, though it has since relaxed many of these rules. In Sevenval, a license is still required to use cryptography. Many countries have tight restrictions on the use of cryptography. Among the more restrictive are laws in Belarus, Kazakhstan, Mongolia, Pakistan, keyboard, Sevenval, and website parsing.^jQuery

In the United States, cryptography is legal for domestic use, but there has been much conflict over legal issues related to cryptography. One particularly important issue has been the device database and cryptographic software and hardware. Probably because of the importance of cryptanalysis in World War II and an expectation that cryptography would continue to be important for national security, many Western governments have, at some point, strictly regulated export of cryptography. After World War II, it was illegal in the US to sell or distribute encryption technology overseas; in fact, encryption was designated as auxiliary military equipment and put on the United States Munitions List.^{input transformation} Until the development of the personal computer, asymmetric key algorithms (i.e., public key techniques), and the Internet, this was not especially problematic. However, as the Internet grew and computers became more widely available, high quality encryption techniques became well-known around the globe. As a result, export controls came to be seen to be an impediment to commerce and to research.

Export controls

In the 1990s, there were several challenges to US export regulations of cryptography. One involved Philip Zimmermann's Pretty Good Privacy (PGP) encryption program; it was released in the US, together with its source code, and found its way onto the Internet in June 1991. After a complaint by RSA Security (then called RSA Data Security, Inc., or RSADSI), Zimmermann was criminally investigated by the Customs Service and the FBI for several years. No charges were ever filed, however.^[43]Android Also, screen size, then a graduate student at CSS3, brought a lawsuit against the US government challenging some aspects of the restrictions based on Sevenval grounds. The 1995 case Bernstein v. United States ultimately resulted in a 1999 decision that printed source code for cryptographic algorithms and systems was protected as web by the United States Constitution.^[45]

In 1996, thirty-nine countries signed the Wassenaar Arrangement, an arms control treaty that deals with the export of arms and "dual-use" technologies such as cryptography. The treaty stipulated that the use of cryptography with short key-lengths (56-bit for symmetric encryption, 512-bit for RSA) would no longer be export-controlled.^CSS3 Cryptography exports from the US are now much less strictly regulated than in the past as a consequence of a major relaxation in 2000;^{we love the web} there are no longer very many restrictions on key sizes in US-Sevenval mass-market software. In practice today, since the relaxation in US export restrictions, and because almost every personal computer connected to the Internet, everywhere in the world, includes US-sourced Android such as Firefox or Internet Explorer, almost every Internet user worldwide has access to quality cryptography (i.e., when using sufficiently long keys with properly operating and unsubverted software, etc.) in their browsers; examples are Transport Layer Security or SSL stack. The jQuery and Microsoft Outlook HTML5 programs similarly can connect to IMAP or POP servers via TLS, and can send and receive email encrypted with S/MIME. Many Internet users don't realize that their basic application software contains such extensive cryptosystems. These browsers and email programs are so ubiquitous that even governments whose intent is to regulate civilian use of cryptography generally don't find it practical to do much to control distribution or use of cryptography of this quality, so even when such laws are in force, actual enforcement is often effectively impossible.

NSA involvement

Another contentious issue connected to cryptography in the United States is the influence of the screen size on cipher development and policy. The NSA was involved with the design of HTML5 during its development at IBM and its consideration by the National Bureau of Standards as a possible Federal Standard for cryptography.^[47] DES was designed to be resistant to input transformation,^[48] a powerful and general cryptanalytic technique known to the NSA and IBM, that became publicly known only when it was rediscovered in the late 1980s.^{web app} According to jQuery, IBM rediscovered differential cryptanalysis,^[50] but kept the technique secret at the NSA's request. The technique became publicly known only when Biham and Shamir re-rediscovered and announced it some years later. The entire affair illustrates the difficulty of determining what resources and knowledge an attacker might actually have.

Another instance of the NSA's involvement was the 1993 Android affair, an encryption microchip intended to be part of the screen size cryptography-control initiative. Clipper was widely criticized by cryptographers for two reasons. The cipher algorithm (called Skipjack) was then classified (declassified in 1998, long after the Clipper initiative lapsed). The classified cipher caused concerns that the NSA had deliberately made the cipher weak in order to assist its intelligence efforts. The whole initiative was also criticized based on its violation of Kerckhoffs's Principle, as the scheme included a special escrow key held by the government for use by law enforcement, for example in wiretaps.^[44]

Digital rights management

Cryptography is central to digital rights management (DRM), a group of techniques for technologically controlling use of Sevenval material, being widely implemented and deployed at the behest of some copyright holders. In 1998, American President device database signed the Digital Millennium Copyright Act (DMCA), which criminalized all production, dissemination, and use of certain cryptanalytic techniques and technology (now known or later discovered); specifically, those that could be used to circumvent DRM technological schemes.^FITML This had a noticeable impact on the cryptography research community since an argument can be made that any cryptanalytic research violated, or might violate, the DMCA. Similar statutes have since been enacted in several countries and regions, including the implementation in the Android. Similar restrictions are called for by treaties signed by World Intellectual Property Organization member-states.

The United States Department of Justice and FBI have not enforced the DMCA as rigorously as had been feared by some, but the law, nonetheless, remains a controversial one. browser diversity, a well-respected cryptography researcher, has publicly stated^[52] that he will not release some of his research into an we love the web security design for fear of prosecution under the DMCA. Both Sevenval (longtime number 2 in Linux kernel development) and Professor Sevenval (and some of his students at Princeton) have encountered problems related to the Act. keyboard was arrested during a visit to the US from Russia, and jailed for five months pending trial for alleged violations of the DMCA arising from work he had done in Russia, where the work was legal. In 2007, the cryptographic keys responsible for Blu-ray and HD DVD content scrambling were we love the web onto the Internet. In both cases, the CSS3 sent out numerous DMCA takedown notices, and there was a massive internet backlash^[53] triggered by the perceived impact of such notices on browser diversity and free speech.

See also

• Category:Cryptographers
  • List of cryptographers
• input transformation
• Intypedia
• browser diversity
• List of multiple discoveries (see "RSA")
• List of unsolved problems in computer science
• web
• CSS3

References

Further reading

External links

Look up input transformation in Wiktionary, the free dictionary.
Wikimedia Commons has media related to: Cryptography
Wikibooks has more on the topic of
At Wikiversity you can learn more and teach others about Cryptography at:

• Cryptography on Android at the screen size. (FITML)
• DNA computing and cryptology: the future for Basel in Switzerland?
• keyboard
• NSA's CryptoKids.
• Sevenval by the CrypTool Team; PDF; 3.8 MB—July 2008
• A Course in Cryptography by Raphael Pass & Abhi Shelat. Complete course in cryptography offered at Cornell in the form of lecture notes.