Cryptography
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Cryptography (or cryptology) is a discipline of mathematics and, in modern times, of computer science concerned with information security and related issues, particularly encryption and authentication and other applications such as access control.
Cryptography, as an interdisciplinary subject, draws on several fields. Prior to the early 20th 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, and especially number theory. Cryptography is a branch of engineering, but an unusual one as it deals with active, intelligent and malevolent opposition (see cryptographic engineering and security engineering). There is also active research examining the relationship between cryptographic problems and quantum physics (see quantum cryptography and quantum computing).
Cryptography is a central part of the techniques used in computer and network security for such things as access control and information confidentiality. Cryptography is used in many applications that touch everyday life; the security of ATM cards, computer passwords, and electronic commerce all depend on cryptography.
Terminology
The term cryptography ("secret writing", from the Greek kryptós, "hidden," and gráphein, "to write") is often used to refer to the field as a whole, as is cryptology ("the study of secrets"). The study of how to circumvent cryptography is called cryptanalysis or, loosely, "codebreaking."Until modern times, cryptography referred almost exclusively to encryption, the process of converting ordinary information (plaintext) into an unreadable ciphertext. Decryption is the reverse process, recovering a plaintext from an incomprehensible ciphertext version.
A cipher (or cypher) is a pair of algorithms for encryption and decryption (though some ciphers use the same algorithm in both directions). The exact operation of a cipher is controlled by a key, a secret parameter (perhaps with several parts) for the cipher algorithm, at least in practically useful ciphers. Those without variable keys are trivially breakable and less than useful. Historically, ciphers were often used directly for encryption or decryption without additional procedures. Modern work focuses on cryptosystems, algorithms, protocols, and operating procedures for encryption, decryption, key distribution, and key management that use a cipher as one of the cooperating elements. The terms encipher and decipher are used to describe cipher algorithm operations.
In colloquial parlance, the term "code" is often used to mean any method of encryption or meaning concealment. In cryptography, however, code has a specific meaning, referring to a procedure which replaced a unit of plaintext, typically meaningful words or phrases, 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 (eg, Bronco One), since properly chosen ciphers are both more practical and secure than even the best codes, and better adapted to computers as well.
Some use the English terms cryptography and cryptology interchangeably, while others make the following distinction: cryptography refers to the use and practice of cryptographic techniques, while cryptology refers to the subject as a field of study (analogously with biology). Today, cryptography encompasses not only traditional topics like encryption and authentication, but also new ones such as zero-knowledge proofs, secure multiparty computation, etc. The noted cryptographer Ron Rivest has observed that cryptography is about communication in the presence of adversaries.Ronald Rivest, "Cryptography" From the Handbook of Theoretical Computer Science, edited by J. van Leeuwen, Elsevier Science Publishers B.V., 1990
History of cryptography and cryptanalysis
Main article: History of cryptographyHistorically, cryptography was concerned solely with message confidentiality (ie, encryption), the reversible conversion of information from a comprehensible form into an incomprehensible one, rendering it unreadable without secret knowledge (i.e., a key). In recent decades, the field has expanded beyond confidentiality concerns to include techniques for authentication, signatures, interactive proofs, secure computation, steganography, and others.
Cryptography has a long and colorful history. Generally, the earliest forms of secret writing (now often termed classical cryptography) required little more than pen and paper. The main classical cipher classes are transposition ciphers, which rearrange the order of letters in a message (eg, 'help me' becomes 'ehpl em'), and substitution ciphers, which systematically replace letters or groups of letters with other letters or groups of letters (eg, 'fly at once' becomes 'gmz bu podf'). An early, and one of the simplest, substitution ciphers was the Caesar cipher, used by Julius Caesar during his military campaigns. Encryption attempted to ensure secrecy in important communications, such as those of spies, military leaders, and diplomats, but it also had religious applications. For instance, early Christians used cryptography to obfuscate parts of their religious writings to avoid near certain persecution had they been less obscured; famously, 666 (or 616, in a recently discovered early variant text), the Number of the Beast from the Christian New Testament Book of Revelations, is sometimes thought to be a ciphertext referring to the Roman Emperor Nero, one of whose policies was persecution of Christians. Cryptography is also recommended in the Kama Sutra as a way for lovers to communicate without discoveryKama Sutra, Sir Richard F. Burton, translator, Part I, Chapter III, 44th and 45th arts. Steganography was also first developed in the ancient times. Encryption attempts to render a message unreadable if intercepted, but steganography attempts to make a message entirely undetectable. An early example, from Herodotus, concealed a message -- a tattoo on a slave's head -- by regrown hairDavid Kahn, The Codebreakers, 1967, ISBN 0-684-83130--9.. Later examples of steganography include the use of invisible ink, microdots, and digital watermarks to convey hidden information.
The ciphertexts produced by classical ciphers reveal statistical information about the plaintext, which can be used to break them. After the Arab discovery of frequency analysis (circa 1000CE), nearly all such ciphers became more or less readable by an informed attacker. Classical ciphers still enjoy popularity today, though mostly as puzzles (see cryptogram). Essentially all ciphers remained vulnerable to cryptanalysis by this technique until the invention of the polyalphabetic cipher by Leon Battista Alberti, about 1467CE, in which different parts of the message (often each plaintext letter) are enciphered using a different key. In the polyalphabetic Vigenère cipher, for instance, encryption uses a key word, which controls letter enciphering depending on which letter of the key word is used. Despite this improvement, polyalphabetic ciphers, especially of the limited Vignenère type, remain partially vulnerable to frequency analysis techniques.
Although frequency analysis is a powerful and general technique, encryption was still often effective in practice: many a would-be cryptanalyst was unaware of the technique. Breaking a message without frequency analysis essentially required knowledge of the cipher used, thus encouraging espionage, bribery, burglary, defection, et cetera to discover it. It was eventually explicitly recognized in the 19th century that secrecy of a cipher's algorithm is not a sensible, nor practical, safeguard: in fact, any adequate cryptographic scheme (including ciphers) should still be secure even if the adversary knows the cipher itself. Secrecy of the key alone should be sufficient for confidentiality when it is attacked. This fundamental principle was first explicitly stated by Auguste Kerckhoffs and is called Kerckhoffs' principle, or 'Kerchoff's law', or alternatively and more bluntly, as Shannon's Maxim.
Various physical devices and aids have been used to assist cipher operations. 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 own cipher disk, Johannes Trithemius' tabula recta, and Thomas Jefferson's cylinder (reinvented by Bazeries around 1900). Early in the 20th 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 the better designs of these brought about a substantial increase in cryptanalytic difficultyJames Gannon, Stealing Secrets, Telling Lies: How Spies and Codebreakers Helped Shape the Twentieth Century, Washington, D.C., Brassey's, 2001, ISBN 1-57488-367-4..
The development of digital computers and electronics made possible much more complex ciphers. Many computer ciphers can be characterized by their operation on binary bits (sometimes in groups or blocks), unlike classical and mechanical schemes, which generally manipulate traditional characters (ie, letters and digits). However, computers have also assisted cryptanalysis, which 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, while breaking it requires an effort many orders of magnitude larger making cryptanalysis so inefficient and impractical as to be effectively impossible.
Extensive open academic research into cryptography is relatively recent — it began only in the mid-1970s with the public specification of DES (the Data Encryption Standard, the epoch-changing Diffie-Hellman paper Whitfield Diffie and Martin Hellman, "New Directions in Cryptography", IEEE Transactions on Information Theory, vol. IT-22, Nov. 1976, pp: 644-654. ([pdf]), 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 security 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 efficientlyOded Goldreich, ''Foundations of Cryptography, Volume 1: Basic Tools", Cambridge University Press, 2001, ISBN 0-521-79172-3. with one notable exception, the one time pad, these contingent proofs are the best available for cryptographic algorithms and protocols.
As well as being aware of cryptographic history, cryptographic algorithm and system designers must also carefully consider probable future developments in their designs. For instance, the continued effect of Moore's law in increasing the scope of brute-force attacks must be taken into account when specifying key lengths, and the potential effects of quantum computing are already being considered by good cryptographic system designersA. J. Menezes, P. C. van Oorschot, and S. A. Vanstone, [Handbook of Applied Cryptography] ISBN 0-849-38523-7..
Modern cryptography
The modern field of cryptography can be divided into several areas of study. The primary ones are discussed here; see Topics in Cryptography for more.Symmetric-key cryptography
Main article: Symmetric key algorithmSymmetric-key cryptography refers to encryption methods in which both the sender and receiver share the same key (or in which their keys are different, but related in an easily computable way). This was the only kind of encryption publicly known for all of recorded history until 1976.
The modern study of symmetric-key ciphers relates mainly to the study of block ciphers and stream ciphers and to their applications. A block cipher is the 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. Block ciphers are used in a mode of operation to implement a cryptosystem. DES and AES are block ciphers which have been designated cryptography standards by the US government (though DES's designation was eventually withdrawn after the AES was adopted)[FIPS PUB 197: The official Advanced Encryption Standard].. Despite its official deprecation, DES (especially its much more secure triple-DES variant) remains quite popular; it is used across a wide range of applications, from ATM encryption[NCUA letter to credit unions], July 2004 to e-mail privacy[Open PGP Message Format] RFC at the IETF and secure remote access[SSH at windowsecurity.com] by Pawel Golen, July 2004. Many other block ciphers have been designed and released, with considerable variation in quality; see Bruce Schneier, Applied Cryptography, 2nd edition, Wiley, 1996, ISBN 0471117099..
Stream ciphers, in contrast to the 'block' type, operate on a continuous stream of plaintext, and produce an encrypted (ie, enciphered) output stream based on an internal state which changes as the cipher operates. That state's evolution is controlled by the key and, in some stream ciphers, by the plaintext stream as well. RC4 is an example of a well-known stream cipher; see .
Cryptographic hash functions (often called message digest functions) do not use keys, but are a related class of cryptographic algorithms. They take input data (often a entire message) and output a short, fixed length (160 bits is common), hash, and do so as a one-way function. Message authentication codes (MACs) are much like cryptographic hash functions, except that a secret key is used to generate and authenticate the hash value.
Public-key cryptography
Main article: Public-key cryptographySymmetric-key cryptosystems typically use the same key for encryption and decryption. The main drawback of symmetric ciphers is the key management necessary to use them securely. Each distinct pair of communicating parties must share a different secret, randomly chosen key. The number of keys required increases very rapidly and becomes unmanageable very quickly. in fact, it is so generally difficult to establish such a secret key when the parties cannot communicate securely without one as to be a core problem in the use of cryptography 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: typically one for encryption and the other for decryptionWhitfield Diffie and Martin Hellman, "Multi-user cryptographic techniques" [Diffie and Hellman, AFIPS Proceedings 45, pp109-112, June 8, 1976].. A public key system is constructed such that possession of the one key does not permit practical calculation of the other key although they are necessarily related. Instead, both keys are generated, secretly, as an interrelated pair. Ralph Merkle was working on similar ideas at the time, and Hellman has suggested that the term should be Diffie-Hellman-Merkle aysmmetric key cryptography. The cryptography historian David Kahn described public-key cryptography as "the most revolutionary new concept in the field since polyalphabetic substitution emerged in the Renaissance".David Kahn, "Cryptology Goes Public", 58 Foreign Affairs 141, 151 (fall 1979), p. 153.
![Ronald L. Rivest, one of the inventors of RSA, a well-known public-key cryptosystem. RSA is used in a wide variety of encryption and digital signature applications, including PGP and TLS[www.ietf.org/rfc/rfc2246.txt], RFC 2246, Transport Layer Security, 1999.](http://en.wikipedia.org/wiki/Image:Ron_Rivest.jpg "Ronald L. Rivest, one of the inventors of RSA, a well-known public-key cryptosystem. RSA is used in a wide variety of encryption and digital signature applications, including PGP and TLS[www.ietf.org/rfc/rfc2246.txt], RFC 2246, Transport Layer Security, 1999.")
In public-key cryptosystems, the public key may be freely distributed, while its paired private key must remain secret. In public-key encryption, the public key is the encryption key, and the private or secret key is the decryption key. Diffie and Hellman showed that public-key cryptography was possible by presenting the Diffie-Hellman key exchange protocol. In 1978, Ronald Rivest, Adi Shamir, and Len Adleman invented RSA, another public-key systemR. Rivest, A. Shamir, L. Adleman. [A Method for Obtaining Digital Signatures and Public-Key Cryptosystems]. Communications of the ACM, Vol. 21 (2), pp.120–126. 1978. Previously released as an MIT "Technical Memo" in April 1977, and published in Martin Gardner's Scientific American Mathematical Recreations column. In 1997, it finally became publicly known that asymmetric cryptography had been invented by James Ellis at GCHQ, a British intelligence organization, in the early 1970s, and that both the Diffie-Hellman and RSA algorithms had been previously developed (by Malcolm J. Williamson and Clifford Cocks, respectively)[Clifford Cocks. A Note on 'Non-Secret Encryption', CESG Research Report, 20 November 1973]..
Diffie-Hellman and RSA, in addition to being the first two publicly known examples of high quality public-key cryptosystems, have been two of the most popular. Others include the Cramer-Shoup cryptosystem, ElGamal encryption, and various elliptic curve techniques. See
In addition to encryption, public-key cryptography can be used to implement digital signature schemes. A digital signature is somewhat analogous to an ordinary signature; they 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; these variant signatures 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), 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 signtaure schemes. Digital signatures are central to the operation of public key infrastructures and to many network security schemes (e.g., SSL/TLS, many VPNs, etc).
Public-key algorithms are most often based on the computational complexity of 'hard' problems, often from number theory. Because of this, most public-key algorithms involve operations such as modular multiplication and exponentiation, which are much more computationally expensive than the techniques typically used in block ciphers, especially with typical key sizes. As such, public-key cryptosystems are commonly used in a "hybrid" system, in which a fast 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.
Cryptanalysis
Main article: CryptanalysisThe goal of cryptanalysis is to find some weakness or insecurity in a cryptographic scheme. Cryptanalysis might be undertaken by a hostile attacker, attempting to subvert a system, or by the system's designer (or others) attempting to evaluate whether a system has vulnerabilities. In modern practice, however, quality cryptographic algorithms and protocols usually come with proofs that establish practical security of the system (at least, under clear -- and hopefully reasonable -- assumptions).
It is commonly held misconception that every encryption method can be broken. In connection with his WWII work at Bell Labs Claude Shannon proved that the one-time 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"Shannon": Claude Shannon and Warren Weaver, "The Mathematical Theory of Communication", University of Illinois Press, 1963, ISBN 0-252-72548-4. Most ciphers, apart from the one-time pad, can be broken with enough computational effort (by brute force attack if nothing else), but the amount of effort needed to break a cipher may be exponentially dependent on the key size, as compared to the effort needed to use the cipher. In such cases, effective security can still be achieved if some conditions (eg, key size) are such that the effort ('work factor' in Shannon's terminology) is beyond the ability of any adversary.
There are a wide variety of cryptanalytic attacks, and they can be classified in any of several ways. One distinction turns on what an attacker knows and can do. In a ciphertext-only attack, the cryptanalyst has access only to the ciphertext (modern cryptosystems are usually effectively immune to ciphertext-only attacks). In a known-plaintext attack, 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 the gardening used by the British during WWII. Finally, in a chosen-ciphertext 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); one class of these was termed cillies at Bletchley Park during WWII.
Cryptanalysis of symmetric-key techniques 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, before chances are better than even the key will have been found. However, a linear cryptanalysis attack against DES requires 243 known plaintexts and approximately 243 DES operationsPascal Junod, ["On the Complexity of Matsui's Attack"], SAC 2001..
Public-key algorithms are based on the computational difficulty of various problems. The most famous of these is integer factorization (the RSA cryptosystem is based on a problem related to factoring), but the discrete logarithm problem is also important. Much public-key cryptanalysis concerns numerical algorithms for solving these computational problems efficiently. For instance, the best algorithms for solving the elliptic curve-based version of discrete logarithm are much more time-consuming than the best known algorithms for factoring, for problems of equivalent size. Therefore, to achieve an equivalent strength, 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.
While pure cryptanalysis uses weaknesses in the algorithms themselves, other attacks are based upon the implementation, known as side-channel attacks. If a cryptanalyst has access to, say, the amount of time the algorithm 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 analysisDawn Song, David Wagner, and Xuqing Tian, ["Timing Analysis of Keystrokes and Timing Attacks on SSH"], In Tenth USENIX Security Symposium, 2001..
Cryptographic primitives
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 one-way function is a function that is easy to compute but hard to invert. In order for any cryptographic application to be secure (if based on computational assumptions), one-way functions must exist. However, if one-way functions exist, this implies that P ǂ NP. Since the P versus NP problem is unsolved, we don't know if one-way functions exist. If they do, however, we can build other cryptographic tools from them. For instance, if one-way functions exist, then pseudorandom generators and pseudorandom functions existJ. Håstad, R. Impagliazzo, L.A. Levin, and M. Luby, ["A Pseudorandom Generator From Any One-Way Function"], SIAM J. Computing, vol. 28 num. 4, pp 1364–1396, 1999..
Other cryptographic primitives include one-way permutations, trapdoor permutations, and oblivious transfer protocols.
Cryptographic protocols
In many cases, cryptographic techniques involve back and forth communication among two or more parties in space or across time (eg, cryptographically protected backup data). 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 proofsLászló Babai. ["Trading group theory for randomness"]. Proceedings of the Seventeenth Annual Symposium on the Theory of Computing, ACM, 1985., secret sharingG. Blakley. "Safeguarding cryptographic keys." In Proceedings of AFIPS 1979, volume 48, pp. 313-317, June 1979.A. Shamir. "How to share a secret." In Communications of the ACM, volume 22, pp. 612-613, ACM, 1979., and zero-knowledgeS. Goldwasser, S. Micali, and C. Rackoff, "The Knowledge Complexity of Interactive Proof Systems", SIAM J. Computing, vol. 18, num. 1, pp. 186-208, 1989., and much more complex ones like electronic cashS. Brands, ["Untraceable Off-line Cash in Wallets with Observers"], In Advances in Cryptology -- Proceedings of CRYPTO, Springer-Verlag, 1994. and secure multiparty computationR. Canetti, ["Universally composable security: a new paradigm for cryptographic protocols"], In Proceedings of the 42nd annual Symposium on the Foundations of Computer Science (FOCS), pp. 136-154, IEEE, 2001..When a cryptographic system fails, typically breaching security, it is rare that the vulnerabilty 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 (eg, a software bug), in a failure of the assumptions on which the design was based (eg, 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. 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 decadesD. Dolev and A. Yao, ["On the security of public key protocols"], IEEE transactions on information theory, vol. 29 num. 2, pp. 198-208, IEEE, 1983.M. Abadi and P. Rogaway, "Reconciling two views of cryptography (the computational soundness of formal encryption)." In IFIP International Conference on Theoretical Computer Science (IFIP TCS 2000), Springer-Verlag, 2000.D. Song, "Athena, an automatic checker for security protocol analysis", In Proceedings of the 12th IEEE Computer Security Foundations Workshop (CSFW), IEEE, 1999., but the tools available are still essentially inadequate. They are typically cumbersome and not yet 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.
Legal issues involving cryptography
Because of its potential to assist the malicious in their schemes, cryptography has long been of interest to national intelligence-gathering and law enforcement agencies. And because of its facilitation of privacy, and the diminution of privacy attendant on its prohibition, cryptography is also of considerable interest to those committed to civil rights. Accordingly, there has been a history of controversial legal issues surrounding cryptography, especially since the advent of inexpensive computers has enabled wide spread 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, China, Kazakhstan, Mongolia, Pakistan, Russia, Singapore, Tunisia, Venezuela, and Vietnam[RSA Laboratories' Frequently Asked Questions About Today's Cryptography].
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[Cryptography & Speech] from Cyberlaw. Until the advent of the personal computer and the internet, this was not especially problematic as good cryptography was indistinguishable from bad cryptography for most users, and because most of the available cryptography was slow and error prone whether good or bad. However, as the Internet grew, and computers became more widely available, high quality encryption techniques became well-known around the globe. Export controls came to be understood to be an impediment to commerce and to research, particularly in the United States.
In the 1990s, several challenges were launched against US export regulations of cryptography. Philip Zimmermann's Pretty Good Privacy(PGP) encryption program, as well as its source code, was released and found its way onto the Internet in June of 1991. Zimmermann was investigated by the FBI for several years but no charges were filed["Case Closed on Zimmermann PGP Investigation"], press note from the IEEE.Steven Levy, Crypto: How the Code Rebels Beat the Government Saving Privacy in the Digital Age, 2001, ISBN 0-670-85950-8. Also, Daniel Bernstein, then a graduate student at UC Berkeley, brought a lawsuit against the US government challenging aspects of those restrictions on free speech grounds in 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 by the United States Constitution.[Bernstein v USDOJ], 9th Circuit court of appeals decision..
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[The Wassenaar Arrangement on Export Controls for Conventional Arms and Dual-Use Goods and Technologies]. Cryptography exports from the US are much less strictly regulated now than in the past as a consequence of a major relaxation in 2000, and there are no longer many restrictions on key sizes in US-exported mass-market software. See Export of cryptography for more details. In practice, today, due to those US export restrictions being relaxed, and because almost every personal computer connected to the internet, everywhere in the world, includes a US-sourced web browser such as Mozilla Firefox or Microsoft Internet Explorer, almost every internet user worldwide has strong cryptography on their desktop in the form of their browser's Transport Layer Security stack. The Mozilla Thunderbird and Microsoft Outlook E-mail 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 even realize that their basic application software contains such powerful cryptography features. These browsers and email programs are now so ubiquitous that even governments that purport to regulate civilian use of cryptography generally don't find it practical to do much to control their distribution or use, so even when such laws are on the books, enforcement is often lax.
Another contentious issue in cryptography in the United States was the National Security Agency and its involvement in high quality cipher development. NSA was involved with the design of DES during its development at IBM and its consideration as a possible Federal Standard for cryptography["The Data Encryption Standard (DES)"] from Bruce Schneier's CryptoGram newsletter, June 15, 2000. DES was designed to be secure against differential cryptanalysis, a cryptanalytic technique known to NSA and to IBM who (re)discovered it during DES' development (and remained silent at NSA's request), but not publicly known until it was (re)rediscovered in the late 1980s by Biham and ShamirE. Biham and A. Shamir, ["Differential cryptanalysis of DES-like cryptosystems"], Journal of Cryptology, vol. 4 num. 1, pp. 3-72, Springer-Verlag, 1991.. Another instance of NSA's involvement was the 1993 Clipper chip, an encryption microchip intended to be part of the Capstone cryptography-control initiative. Clipper was widely criticized for two cryptographic reasons: the cipher algorithm was classified (the cipher, called Skipjack, was declassified in 1998 after the Clipper initiative lapsed), which led to concerns that the NSA had made the cipher weak on purpose in order to assist its intelligence efforts. The whole initiative was also criticized based its violation of Kerchoff's law, as the scheme included a special escrow key held by the government for use in wiretaps. See Clipper chip for more information.
Cryptography is central to digital rights management (DRM), a technological way of controlling use of copyrighted material at being implemented at the behest of some copyright holders. In 1998, Bill Clinton signed the Digital Millennium Copyright Act (DMCA), which criminalized the production, dissemination, and use of certain cryptanalytic techniques and technology; specifically, those that could be used to circumvent DRM technological schemes[Digital Millennium Copyright Act]. This had a very serious potential impact on the cryptography community since an argument could be made that virtually any cryptanalytic research violated, or might violate, the DMCA. The FBI has not enforced the DMCA as rigorously as had been feared by some, but nonetheless, this law remains a contentious issue in the cryptography community. One well-respected 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 and Professor Edward Felten (and some of his students) have encountered problems related to the Act. Dmitry Sklyarov was arrested, and jailed for some months, for alleged violations of the DMCA which occurred in Russia, where his work was legal.
See also
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- Short and long lists of cryptography topics.
- Short and lists of cryptographers.
- Important books, papers, and open problems in cryptography.
- International Association for Cryptologic Research.
Further reading
See Books on cryptography for a more detailed list.- The Codebreakers by David Kahn, a comprehensive history of classical (pre-WW2) cryptography. The current edition has a brief addendum about WW2 and later.
- The Code Book by Simon Singh, a clearly written anecdotal history of crypto, covering modern methods including public key.
- [[Crypto: How the Code Rebels Beat the Government Saving Privacy in the Digital Age]] by Steven Levy, about the political and legal conflicts in the US about cryptography, such as the Clipper Chip controversy and the Bernstein v. United States lawsuit.
- Applied Cryptography, 2nd edition, by Bruce Schneier. General reference book about crypto algorithms and protocols, aimed at implementers.
- [Handbook of Applied Cryptography] by A. J. Menezes, P. C. van Oorschot, and S. A. Vanstone (PDF download available), somewhat more mathematical than Schneier's book.
- Introduction to Modern Cryptography by Phillip Rogaway and Mihir Bellare, a mathematical introduction to theoretical cryptography including reduction-based security proofs. [PDF download].
- [RSA Laboratories' Frequently Asked Questions About Today's Cryptography].
- Stealing Secrets, Telling Lies: How Spies and Codebreakers Helped Shape the Twentieth Century, by James Gannon.
- [sci.crypt mini-FAQ].
- [NSA's CryptoKids].
- Cryptonomicon by Neal Stephenson (novel, WW2 Enigma cryptanalysis figures into the story, though not always realistically).
- Alvin's Secret Code by Clifford B. Hicks (children's novel that introduces some basic cryptography and cryptanalysis).
References
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