Cryptography (or cryptology; derived from Greek κρυπτός kryptós "hidden," and the verb γράφω gráfo "write" or λεγειν legein "to speak") is the study of message secrecy. One of cryptography's primary purposes is hiding the meaning of messages, but not usually their existence. In modern times, cryptography is considered to be a branch of both mathematics and computer science, and is affiliated closely with information theory, computer security, and engineering.
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Cryptography is used in many applications encountered in everyday life; examples include security of ATM cards, computer passwords, and electronic commerce. It is also valuable for confidential governmental communications, on both domestic and international levels, especially during times of conflict.
Cryptography is about communication in the presence of adversaries
Ron Rivest
Before the modern era, cryptography was concerned solely with message confidentiality (encryption) — conversion of messages 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). In recent decades, the field has expanded beyond confidentiality concerns to include techniques for message integrity checking, sender/receiver identity authentication, digital signatures, interactive proofs, and secure computation, amongst others.
The earliest forms of secret writing required little more than a pen and paper, as most people could not read. An increase in literacy over time required actual cryptography. The main classical cipher types are transposition ciphers, which rearrange the order of letters in a message (e.g. 'help me' becomes 'ehpl em' in a trivially simple rearrangement scheme), and substitution ciphers, 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 alphabet). Simple versions of either offered little confidentiality from enterprising opponents, and still don't. 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. It was named after Julius Caesar who is reported to have used it, with a shift of 3, to communicate with his generals during his military campaigns.
Encryption attempts to ensure secrecy in communications, such as those of spies, military leaders, and diplomats, but it has also had religious applications. For instance, early Christians used cryptography to obfuscate some aspects of their religious writings to avoid the near certain persecution they would have faced had they been less cautious; famously, 666 the Number of the Beast from the Christian New Testament Book of Revelation, is sometimes thought to be a ciphertext referring to the Roman Emperor Nero, one of whose policies was persecution of Christians.^{[1]} There is record of several, even earlier, Hebrew ciphers as well. Steganography (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 shaved man's head  under the regrown hair.^{[2]} More modern examples of steganography include the use of invisible ink, microdots, and digital watermarks to conceal information.
Ciphertexts produced by classical ciphers (and some modern ones) always reveal statistical information about the plaintext, which can often be used to break them. After the Arab discovery of frequency analysis (circa 1000 C.E.), nearly all such ciphers became more or less readily breakable by an informed attacker. Such classical ciphers still enjoy popularity today, though mostly as puzzles. Essentially all ciphers remained vulnerable to cryptanalysis using this technique until the invention of the polyalphabetic cipher, most clearly by Leon Battista Alberti around the year 1467 (there is some indication of early Arab knowledge of them). Alberti's innovation was to use different ciphers (i.e., substitution alphabets) for various parts of a message (often each successive plaintext letter). 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 1800s Babbage showed that polyalphabetic ciphers of this type remained partially vulnerable to frequency analysis techniques.^{[3]}
[[Image:Enigma.jpg240pxthumbnailleftThe Enigma machine, used in several variants by the German military between the late 1920s and the end of World War II, implemented a complex electromechanical polyalphabetic cipher to protect sensitive communications. Breaking the Enigma cipher at the Biuro Szyfrów, and the subsequent largescale decryption of Enigma traffic at Bletchley Park, was an important factor contributing to the Allied victory in WWII.^{[4]}
Although frequency analysis is a powerful and general technique, encryption was still often effective in practice; many a wouldbe cryptanalyst was unaware of the technique. Breaking a message without frequency analysis essentially required knowledge of the cipher used, thus encouraging espionage, bribery, burglary, and defection to discover it. It was finally recognized in the nineteenth century that secrecy of a cipher's algorithm is not a sensible or practical safeguard; in fact, any adequate cryptographic scheme (including ciphers) should remain secure even if the adversary knows the cipher algorithm itself. Secrecy of the key should alone be sufficient for confidentiality when under attack — for good ciphers. This fundamental principle was first explicitly stated in 1883 by Auguste Kerckhoffs and is generally called Kerckhoffs' principle; alternatively and more bluntly, it was restated by Claude Shannon as Shannon's Maxim — 'the enemy knows the system'.
Various 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. In medieval times, other aids were invented such as the cipher grille, also used for a kind of steganography. With the invention of polyalphabetic ciphers came more sophisticated aids such as Alberti's cipher disk, Johannes Trithemius' tabula recta scheme, and Thomas Jefferson's multicylinder (invented independently by Bazeries around 1900). Early in the twentieth century, several mechanical encryption/decryption devices were invented, and many patented, including rotor machines — most famously the Enigma machine used by Germany in World War II. The ciphers implemented by better quality examples of these designs brought about a substantial increase in cryptanalytic difficulty after WWI.^{[5]}
The development of digital computers and electronics after WWII made possible much more complex ciphers. Furthermore, computers allowed for the encryption of any kind of data that is represented by computers in any binary format, unlike classical ciphers which only encrypted written language texts, dissolving the utility of a linguistic approach to cryptanalysis in many cases. 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 (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 usually the case that use of a quality cipher is very efficient (i.e., fast and requiring few resources), while breaking it requires an effort many orders of magnitude larger, making cryptanalysis so inefficient and impractical as to be effectively impossible.
[[Image:Smartcard.JPGthumb250px A credit card with smart card capabilities. The 3 by 5 mm chip embedded in the card is shown enlarged in the insert. Smart cards attempt to combine portability with the power to compute modern cryptographic algorithms.]]
Extensive open academic research into cryptography is relatively recent — it began only in the mid1970s with the public specification of DES (the Data Encryption Standard) by the NBS, the DiffieHellman paper,^{[6]} and the public release of the RSA algorithm. Since then, cryptography has become a widely used tool in communications, computer networks, and computer security generally. The present security level of many modern cryptographic techniques is based on the difficulty of certain computational problems, such as the integer factorization problem or the discrete logarithm problem. In many cases, there are proofs that cryptographic techniques are secure if a certain computational problem cannot be solved efficiently.^{[7]} With one notable exception—the onetime pad—these proofs are contingent, and thus not definitive, but are currently the best available for cryptographic algorithms and protocols.
As well as being aware of cryptographic history, cryptographic algorithm and system designers must also sensibly consider probable future developments in their designs. For instance, the continued improvements in computer processing power have increased the scope of bruteforce attacks when specifying key lengths. The potential effects of quantum computing are already being considered by some cryptographic system designers; the announced imminence of small implementations of these machines is making the need for this preemptive caution fully explicit.^{[8]}
Essentially, prior to the early twentieth century, cryptography was chiefly concerned with linguistic patterns. Since then the emphasis has shifted, and cryptography now makes extensive use of mathematics, including aspects of information theory, computational complexity, statistics, combinatorics, abstract algebra, and number theory. Cryptography is also a branch of engineering, but an unusual one as it deals with active, and intelligent opposition (see cryptographic engineering and security engineering); most other kinds of engineering deal only with natural forces. There is also active research examining the relationship between cryptographic problems and quantum physics (see quantum cryptography and quantum computing).
Until modern times, cryptography referred almost exclusively to encryption, the process of converting ordinary information (plaintext) into unintelligible gibberish (ciphertext). Decryption is the reverse, moving from unintelligible ciphertext to plaintext. A cipher (or cypher) is a pair of algorithms which perform this 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 (known only to the communicants) for a specific message exchange context. Keys are important as ciphers without variable keys are trivially breakable and so rather less than useful 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, apple pie 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 better adapted to computers as well.
Some use the English terms cryptography and cryptology interchangeably, while others use cryptography to refer to the use and practice of cryptographic techniques, and cryptology to refer to the subject as a field of study. In this respect, English usage is more tolerant of overlapping meanings and word origins than are several European languages.
The modern field of cryptography can be divided into several areas of study. The chief ones are as follows:
Symmetrickey 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 1976.^{[9]}
The modern study of symmetrickey ciphers relates mainly to the study of block ciphers and stream ciphers and to their applications. 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.
The Data Encryption Standard (DES) and the Advanced Encryption Standard (AES) are block cipher designs which have been designated cryptography standards by the US government (though DES's designation was finally withdrawn after the AES was adopted). ^{[10]} Despite its deprecation as an official standard, DES (especially its stillapproved and much more secure tripleDES variant) remains quite popular; it is used across a wide range of applications, from ATM encryption ^{[11]} to email privacyand secure remote access. ^{[12]} 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 bitbybit or characterbycharacter, somewhat like the onetime pad. In a stream cipher, the output stream is created based on an internal state which changes as the cipher operates. That state's change is controlled by the key, and, in some stream ciphers, by the plaintext stream as well. RC4 is an example of a wellknown stream cipher.
Cryptographic hash functions (often called message digest functions) do not use keys, but are a related and important class of cryptographic algorithms. They take input data (often an entire message), and output a short, fixed length hash, and do so as a oneway function. For good ones, collisions (two plaintexts which produce the same hash) are extremely difficult to find.
Message authentication codes (MACs) are much like cryptographic hash functions, except that a secret key is used to authenticate the hash value on receipt.
Symmetrickey cryptosystems typically use the same key for encryption and decryption, though this 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 establishing a secret key between two communicating parties, when a secure channel doesn't already exist between them, also presents a chickenandegg problem which is a considerable practical obstacle for cryptography users in the real world.
In a groundbreaking 1976 paper^{[13]}, Whitfield Diffie and Martin Hellman proposed the notion of publickey (also, more generally, called asymmetric key) cryptography in which two different but mathematically related keys are used a public key and a private key. 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. The historian David Kahn described publickey cryptography as "the most revolutionary new concept in the field since polyalphabetic substitution emerged in the Renaissance".^{[14]}
In publickey cryptosystems, the public key may be freely distributed, while its paired private key must remain secret. The public key is typically used for encryption, while the private or secret key is used for decryption. Diffie and Hellman showed that publickey cryptography was possible by presenting the DiffieHellman key exchange protocol.
In 1978, Ronald Rivest, Adi Shamir, and Len Adleman invented RSA, another publickey system.^{[15]}
In 1997, it finally became publicly known that asymmetric key cryptography had been invented by James H. Ellis at GCHQ, a British intelligence organization, in the early 1970s, and that both the DiffieHellman and RSA algorithms had been previously developed (by Malcolm J. Williamson and Clifford Cocks, respectively).^{[16]}
The DiffieHellman and RSA algorithms, in addition to being the first publicly known examples of high quality publickey ciphers, have been among the most widely used. Others include the CramerShoup cryptosystem, ElGamal encryption, and various elliptic curve techniques.
In addition to encryption, publickey 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. Digital signatures can also be permanently tied to the content of the message being signed; they cannot 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 DSA are two of the most popular digital signature schemes. Digital signatures are central to the operation of public key infrastructures and many network security schemes (SSL/TLS, and many virtual private networks (VPNs)).
Publickey algorithms are most often based on the computational complexity of "hard" problems, often from number theory. For example, the hardness of RSA is related to the integer factorization problem, while DiffieHellman and DSA are related to the discrete logarithm problem. More recently, elliptic curve cryptography has developed in which security is based on number theoretic problems involving elliptic curves. Because of the difficulty of the underlying problems, most publickey 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, publickey cryptosystems are commonly "hybrid" systems, in which a fast high quality symmetrickey encryption algorithm is used for the message itself, while the relevant symmetric key is sent with the message, but encrypted using a publickey algorithm. Similarly, hybrid signature schemes are often used, in which a cryptographic hash function is computed, and only the resulting hash is digitally signed.
The goal of cryptanalysis is to find some weakness or insecurity in a cryptographic scheme, thus permitting its subversion or evasion. Cryptanalysis might be undertaken by a malicious attacker, attempting to subvert a system, or by the system's designer (or others) attempting to evaluate whether a system has vulnerabilities, and so it is not inherently a hostile act. In modern practice, however, cryptographic algorithms and protocols must be carefully examined and tested to offer any assurance of the system's security (at least, under clear — and hopefully reasonable — assumptions).
It is a commonly held misconception that every encryption method can be broken. In connection with his WWII work at Bell Labs, Claude Shannon proved that the onetime pad cipher is unbreakable, provided the key material is truly random, never reused, kept secret from all possible attackers, and of equal or greater length than the message.^{[17]} Most ciphers, apart from the onetime pad, can be broken with enough computational effort by brute force attack, but the amount of effort needed may be exponentially dependent on the key size, as compared to the effort needed to use 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 timeconsuming brute force method) can be found to break the cipher. Since no such showing can be made currently, as of today, the onetimepad 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 ciphertextonly attack, the cryptanalyst has access only to the ciphertext (good modern cryptosystems are usually effectively immune to ciphertextonly attacks). In a knownplaintext attack, the cryptanalyst has access to a ciphertext and its corresponding plaintext (or to many such pairs). In a chosenplaintext attack, the cryptanalyst may choose a plaintext and learn its corresponding ciphertext (perhaps many times); an example is 'gardening', used by the British during WWII. Finally, in a chosenciphertext attack, the cryptanalyst may choose ciphertexts and learn their corresponding plaintexts. Also important, often overwhelmingly so, are mistakes (generally in the design or use of one of the protocols involved.
Cryptanalysis of symmetrickey 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 2^{55} decryptions, trying approximately half of the possible keys, to reach a point at which chances are better than even the key sought will have been found. But this may not be enough assurance; a linear cryptanalysis attack against DES requires 2^{43} known plaintexts and approximately 2^{43} DES operations.^{[18]} This is a considerable improvement on brute force attacks.
Publickey algorithms are based on the computational difficulty of various problems. The most famous of these is integer factorization (for example the RSA algorithm is based on a problem related to factoring), but the discrete logarithm problem is also important. Much publickey cryptanalysis concerns numerical algorithms for solving these computational problems, or some of them, efficiently. For instance, the best known algorithms for solving the elliptic curvebased version of discrete logarithm are much more timeconsuming 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, factoringbased encryption techniques must use larger keys than elliptic curve techniques. For this reason, publickey cryptosystems based on elliptic curves have become popular since their invention in the mid1990s.
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 sidechannel attacks. If a cryptanalyst has access to, say, 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 timing attack 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,^{[19]} and can be quite useful to an alert adversary. And, of course, social engineering, and other attacks against the personnel who work with cryptosystems or the messages they handle (such as bribery, extortion, blackmail, espionage) may be the most productive attacks of all.
Much of the theoretical work in cryptography concerns cryptographic primitives — algorithms with basic cryptographic properties — and their relationship to other cryptographic problems. For example, a oneway function is a function intended to be easy to compute but hard to invert. In a very general sense, for any cryptographic application to be secure (if based on such computational feasibility assumptions), oneway functions must exist.
Currently known cryptographic primitives provide only basic functionality. These are usually noted as confidentiality, message integrity, authentication, and nonrepudiation. Any other functionality in a cryptosystem must be built in using combinations of these algorithms and assorted protocols. Such combinations are called cryptosystems and it is they which users will encounter. Examples include PGP and its variants, SSH, SSL/TLS, all PKIs, and digital signatures. Other cryptographic primitives include the encryption algorithms themselves, oneway permutations, trapdoor permutations, etc.
In many cases, cryptographic techniques involve back and forth communication among two or more parties in space (for example between a home office and a branch office). The term cryptographic protocol captures this general idea.
Cryptographic protocols have been developed for a wide range of problems, including relatively simple ones like interactive proofs, ^{[20]} secret sharing,^{[21]} and zeroknowledge, Brands, "Untraceable Offline Cash in Wallets with Observers", In Advances in Cryptology — Proceedings of CRYPTO, SpringerVerlag, 1994.</ref> and secure multiparty computation.^{[22]}
When the security of a good cryptographic system fails, it is rare that the vulnerability leading to the breach will have been in a quality cryptographic primitive. Instead, weaknesses are often mistakes in the protocol design (often due to inadequate design procedures, or less than thoroughly informed designers), in the implementation (for example a software bug), in a failure of the assumptions on which the design was based (for example the proper training of those who will be using the system), or some other human error. Many cryptographic protocols have been designed and analyzed using ad hoc methods, but they rarely have any proof of security. Methods for formally analyzing the security of protocols, based on techniques from mathematical logic (see for example BAN logic), and more recently from concrete security principles, have been the subject of research for the past few decades.^{[23]} Unfortunately, to date these tools have been cumbersome and are not widely used for complex designs.
The study of how best to implement and integrate cryptography in applications is itself a distinct field, see: cryptographic engineering and security engineering.
Cryptography has long been of interest to intelligence gathering agencies and law enforcement agencies. 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 possible widespread access to high quality cryptography.
In some countries, even the domestic use of cryptography is, or has been, restricted. Until 1999, France significantly restricted the use of cryptography domestically. In China, 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, Russia, Singapore, Tunisia, Venezuela, and Vietnam.^{[24]}
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 export of cryptography and cryptographic software and hardware. 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 classified as a munition, like tanks and nuclear weapons.^{[25]} Until the advent of the personal computer and the Internet, this was not especially problematic. Good cryptography is indistinguishable from bad cryptography for nearly all users, and in any case, most of the cryptographic techniques generally available were slow and error prone whether good or bad. However, as the Internet grew and computers became more widely available, high quality encryption techniques became wellknown around the globe. As a result, export controls came to be seen to be an impediment to commerce and to research.
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 of 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.^{[26]} ^{[27]} Also, Daniel Bernstein, then a graduate student at UC Berkeley, brought a lawsuit against the US government challenging some aspects of the restrictions based on free speech grounds. The 1995 case Bernstein v. United States which ultimately resulted in a 1999 decision that printed source code for cryptographic algorithms and systems was protected as free speech by the United States Constitution.^{[28]}
In 1996, 39 countries signed the Wassenaar Arrangement, an arms control treaty that deals with the export of arms and "dualuse" technologies such as cryptography. The treaty stipulated that the use of cryptography with short keylengths (56bit for symmetric encryption, 512bit for RSA) would no longer be exportcontrolled.^{[29]} Cryptography exports from the US are now much less strictly regulated than in the past as a consequence of a major relaxation in 2000; there are no longer very many restrictions on key sizes in USexported massmarket 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 USsourced web browsers such as Mozilla Firefox or Microsoft Internet Explorer, almost every Internet user worldwide has access to quality cryptography (e.g., using long keys at a minimum) in their browser's Transport Layer Security or SSL stack. The Mozilla Thunderbird and Microsoft Outlook Email client 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.
Another contentious issue connected to cryptography in the United States is the influence of the National Security Agency in cipher development and policy. NSA was involved with the design of DES during its development at IBM and its consideration by the National Bureau of Standards as a possible Federal Standard for cryptography.^{[30]} DES was designed to be secure against differential cryptanalysis,^{[31]} a powerful and general cryptanalytic technique known to NSA and IBM, that became publicly known only when it was rediscovered in the late 1980s.^{[32]} According to Steven Levy, IBM rediscovered differential cryptanalysis, but kept the technique secret at NSA's request. The technique became publicly known only when Biham and Shamir rerediscovered it some years later. The entire affair illustrates the difficulty of determining what resources and knowledge an attacker might actually have.
Another instance of NSA's involvement was the 1993 Clipper chip affair, an encryption microchip intended to be part of the Capstone cryptographycontrol initiative. Clipper was widely criticized by cryptographers for two reasons: the cipher algorithm was classified (the cipher, called Skipjack, was declassified in 1998 long after the Clipper initiative lapsed), which caused concerns that 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' principle, as the scheme included a special escrow key held by the government for use by law enforcement, for example in wiretaps.
Cryptography is central to digital rights management, a group of techniques for technologically controlling use of copyrighted material, being widely implemented and deployed at the behest of some copyright holders. In 1998, Bill Clinton 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.^{[33]} This had a very serious potential impact on the cryptography research community since an argument can be made that any cryptanalytic research violated, or might violate, the DMCA. The FBI and the Justice Department have not enforced the DMCA as rigorously as had been feared by some, but the law, nonetheless, remains a controversial one. One wellrespected cryptography researcher, Niels Ferguson, has publicly stated that he will not release some research into an Intel security design for fear of prosecution under the DMCA, and both Alan Cox (longtime number 2 in Linux kernel development) and Professor Edward Felten (and some of his students at Princeton) have encountered problems related to the Act. Dmitry Sklyarov was arrested during a visit to the US from Russia, and jailed for some months for alleged violations of the DMCA which had occurred in Russia, where the work for which he was arrested and charged was then, and when he was arrested, legal. Similar statutes have since been enacted in several countries. See for instance the EU Copyright Directive. In 2007, the cryptographic keys responsible for DVD and HDDVD content scrambling were discovered and released onto the internet. Both times, the MPAA sent out numerous DMCA takedown notices, and there was a massive internet backlash as a result of the implications of such notices on fair use and free speech.
All links retrieved November 25, 2017.



History of cryptography  Cryptanalysis  Cryptography portal  Topics in cryptography  
Symmetrickey algorithm  Block cipher  Stream cipher  Publickey cryptography  Cryptographic hash function  Message authentication code  Random numbers 
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