Cryptanalysis
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Cryptanalysis (from the Greek kryptós, "hidden", and analýein, "to loosen" or "to untie") is the study of methods for obtaining the meaning of encrypted information, without access to the secret information which is normally required to do so. Typically, this involves finding a secret key. In nontechnical language, this is the practice of codebreaking or cracking the code, although these phrases also have a specialised technical meaning (see code).
"Cryptanalysis" is also used to refer to any attempt to circumvent the security of other types of cryptographic algorithms and protocols in general, and not just encryption. However, cryptanalysis usually excludes methods of attack that do not primarily target weaknesses in the actual cryptography, such as bribery, physical coercion, burglary, keystroke logging, and social engineering, although these types of attack are an important concern and are often more effective than traditional cryptanalysis.
Even though the goal has been the same, the methods and techniques of cryptanalysis have changed drastically through the history of cryptography, adapting to increasing cryptographic complexity, ranging from the penandpaper methods of the past, through machines like Enigma in World War II, to the computerbased schemes of the present. The results of cryptanalysis have also changed — it is no longer possible to have unlimited success in codebreaking, and there is a hierarchical classification of what constitutes a rare practical attack. In the mid1970s, a new class of cryptography was introduced: asymmetric cryptography. Methods for breaking these cryptosystems are typically radically different from before, and usually involve solving carefullyconstructed problems in pure mathematics, the bestknown being integer factorization.
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[edit] History of cryptanalysis
Cryptanalysis has coevolved together with cryptography, and the contest can be traced through the history of cryptography—new ciphers being designed to replace old broken designs, and new cryptanalytic techniques invented to crack the improved schemes . In practice, they are viewed as two sides of the same coin: in order to create secure cryptography, you have to design against possible cryptanalysis.
[edit] Classical cryptanalysis
Although the actual word "cryptanalysis" is relatively recent (it was coined by William Friedman in 1920), methods for breaking codes and ciphers are much older. The first known recorded explanation of cryptanalysis was given by 9thcentury Arabian polymath, AlKindi (also known as "Alkindus" in Europe), in A Manuscript on Deciphering Cryptographic Messages. This treatise includes a description of the method of frequency analysis (Ibrahim AlKadi, 1992 ref3). Italian scholar Giambattista della Porta was author of a seminal work on cryptanalysis "De Furtivis Literarum Notis".^{[1]}
Frequency analysis is the basic tool for breaking most classical ciphers. In natural languages, certain letters of the alphabet appear more frequently than others; in English, "E" is likely to be the most common letter in any sample of plaintext. Similarly, the digraph "TH" is the most likely pair of letters in English, and so on. Frequency analysis relies on a cipher failing to hide these statistics. For example, in a simple substitution cipher (where each letter is simply replaced with another), the most frequent letter in the ciphertext would be a likely candidate for "E".
In practice, frequency analysis relies as much on linguistic knowledge as it does on statistics, but as ciphers became more complex, mathematics became more important in cryptanalysis. This change was particularly evident during World War II, where efforts to crack Axis ciphers required new levels of mathematical sophistication. Moreover, automation was first applied to cryptanalysis in that era with the Polish Bomba device, use of punched card equipment, and in the Colossus — one of the earliest computers (arguably the first programmable electronic digital computer).
[edit] Modern cryptanalysis
Even though computation was used to great effect in cryptanalysis in World War II, it also made possible new methods of cryptography orders of magnitude more complex than ever before. Taken as a whole, modern cryptography has become much more impervious to cryptanalysis than the penandpaper systems of the past, and now seems to have the upper hand against pure cryptanalysis. The historian David Kahn notes, "Many are the cryptosystems offered by the hundreds of commercial vendors today that cannot be broken by any known methods of cryptanalysis. Indeed, in such systems even a chosen plaintext attack, in which a selected plaintext is matched against its ciphertext, cannot yield the key that unlock other messages. In a sense, then, cryptanalysis is dead. But that is not the end of the story. Cryptanalysis may be dead, but there is  to mix my metaphors  more than one way to skin a cat.". ^{[2]} Kahn goes on to mention increased opportunities for interception, bugging, side channel attacks and quantum computers as replacements for the traditional means of cryptanalysis.
Kahn may have been premature in his cryptanalysis postmortem; weak ciphers are not yet extinct, and cryptanalytic methods employed by intelligence agencies remain unpublished. In academia, new designs are regularly presented, and are also frequently broken: the 1984 block cipher Madryga was found to be susceptible to ciphertextonly attacks in 1998; FEAL4, proposed as a replacement for the DES standard encryption algorithm, was demolished by a spate of attacks from the academic community, many of which are entirely practical. In industry, too, ciphers are not free from flaws: for example, the A5/1, A5/2 and CMEA algorithms, used in mobile phone technology, can all be broken in hours, minutes or even in realtime using widelyavailable computing equipment. In 2001, Wired Equivalent Privacy (WEP), a protocol used to secure WiFi wireless networks, was shown to be susceptible to a practical relatedkey attack.
[edit] The results of cryptanalysis
Successful cryptanalysis has undoubtedly influenced history; the ability to read the presumedsecret thoughts and plans of others can be a decisive advantage, and never more so than during wartime. For example, in World War I, the breaking of the Zimmermann Telegram was instrumental in bringing the United States into the war. In World War II, the cryptanalysis of the German ciphers — including the Enigma machine and the Lorenz cipher — has been credited with everything between shortening the end of the European war by a few months to determining the eventual result (see ULTRA). The United States also benefited from the cryptanalysis of the Japanese PURPLE code (see MAGIC).
Governments have long recognised the potential benefits of cryptanalysis for intelligence, both military and diplomatic, and established dedicated organisations devoted to breaking the codes and ciphers of other nations, for example, GCHQ and the NSA, organisations which are still very active today. In 2004, it was reported that the United States had broken Iranian ciphers. (It is unknown, however, whether this was pure cryptanalysis, or whether other factors were involved: [1]).
[edit] Types of cryptanalytic attack
Cryptanalytic attacks vary in potency and how much of a threat they pose to realworld cryptosystems. A certificational weakness is a theoretical attack that is unlikely to be applicable in any realworld situation; the majority of results found in modern cryptanalytic research are of this type. Essentially, the practical importance of an attack is dependent on the answers to the following three questions:
 What knowledge and capabilities are needed as a prerequisite?
 How much additional secret information is deduced?
 How much effort is required? (What is the computational complexity?)
[edit] Prior knowledge: scenarios for cryptanalysis
Cryptanalysis can be performed under a number of assumptions about how much can be observed or found out about the system under attack. As a basic starting point it is normally assumed that, for the purposes of analysis, the general algorithm is known; this is Kerckhoffs' principle of "the enemy knows the system". This is a reasonable assumption in practice — throughout history, there are countless examples of secret algorithms falling into wider knowledge, variously through espionage, betrayal and reverse engineering. (On occasion, ciphers have been reconstructed through pure deduction; for example, the German Lorenz cipher and the Japanese Purple code, and a variety of classical schemes).
Other assumptions include:
 Ciphertextonly: the cryptanalyst has access only to a collection of ciphertexts or codetexts.
 Knownplaintext: the attacker has a set of ciphertexts to which he knows the corresponding plaintext.
 Chosenplaintext (chosenciphertext): the attacker can obtain the ciphertexts (plaintexts) corresponding to an arbitrary set of plaintexts (ciphertexts) of his own choosing.
 Adaptive chosenplaintext: like a chosenplaintext attack, except the attacker can choose subsequent plaintexts based on information learned from previous encryptions. Similarly Adaptive chosen ciphertext attack.
 Relatedkey attack: Like a chosenplaintext attack, except the attacker can obtain ciphertexts encrypted under two different keys. The keys are unknown, but the relationship between them is known; for example, two keys that differ in the one bit.
These types of attack clearly differ in how plausible they would be to mount in practice. Although some are more likely than others, cryptographers will often take a conservative approach to security and assume the worstcase when designing algorithms, reasoning that if a scheme is secure even against unrealistic threats, then it should also resist realworld cryptanalysis as well.
The assumptions are often more realistic than they might seem upon first glance. For a knownplaintext attack, the cryptanalyst might well know or be able to guess at a likely part of the plaintext, such as an encrypted letter beginning with "Dear Sir", or a computer session starting with "LOGIN:". A chosenplaintext attack is less likely, but it is sometimes plausible: for example, you could convince someone to forward a message you have given them, but in encrypted form. Relatedkey attacks are mostly theoretical, although they can be realistic in certain situations, for example, when constructing cryptographic hash functions using a block cipher.
[edit] Classifying success in cryptanalysis
The results of cryptanalysis can also vary in usefulness. For example, cryptographer Lars Knudsen (1998) classified various types of attack on block ciphers according to the amount and quality of secret information that was discovered:
 Total break — the attacker deduces the secret key.
 Global deduction — the attacker discovers a functionally equivalent algorithm for encryption and decryption, but without learning the key.
 Instance (local) deduction — the attacker discovers additional plaintexts (or ciphertexts) not previously known.
 Information deduction — the attacker gains some Shannon information about plaintexts (or ciphertexts) not previously known.
 Distinguishing algorithm — the attacker can distinguish the cipher from a random permutation.
Similar considerations apply to attacks on other types of cryptographic algorithm.
[edit] Complexity
Attacks can also be characterised by the amount of resources they require. This can be in the form of:
 Time — the number of "primitive operations" which must be performed. This is quite loose; primitive operations could be basic computer instructions, such as addition, XOR, shift, and so forth, or entire encryption methods.
 Memory — the amount of storage required to perform the attack.
 Data — the quantity of plaintexts and ciphertexts required.
In academic cryptography, a weakness or a break in a scheme is usually defined quite conservatively. Bruce Schneier sums up this approach: "Breaking a cipher simply means finding a weakness in the cipher that can be exploited with a complexity less than brute force. Never mind that bruteforce might require 2^{128} encryptions; an attack requiring 2^{110} encryptions would be considered a break...simply put, a break can just be a certificational weakness: evidence that the cipher does not perform as advertised." (Schneier, 2000).
[edit] Cryptanalysis of asymmetric cryptography
Asymmetric cryptography (or public key cryptography) is cryptography that relies on using two keys; one private, and one public. Such ciphers invariably rely on "hard" mathematical problems as the basis of their security, so an obvious point of attack is to develop methods for solving the problem. The security of twokey cryptography depends on mathematical questions in a way that singlekey cryptography generally does not, and conversely links cryptanalysis to wider mathematical research in a new way.
Asymmetric schemes are designed around the (conjectured) difficulty of solving various mathematical problems. If an improved algorithm can be found to solve the problem, then the system is weakened. For example, the security of the DiffieHellman key exchange scheme depends on the difficulty of calculating the discrete logarithm. In 1983, Don Coppersmith found a faster way to find discrete logarithms (in certain groups), and thereby requiring cryptographers to use larger groups (or different types of groups). RSA's security depends (in part) upon the difficulty of integer factorization — a breakthrough in factoring would impact the security of RSA.
In 1980, one could factor a difficult 50digit number at an expense of 10^{12} elementary computer operations. By 1984 the state of the art in factoring algorithms had advanced to a point where a 75digit number could be factored in 10^{12} operations. Advances in computing technology also meant that the operations could be performed much faster, too. Moore's law predicts that computer speeds will continue to increase. Factoring techniques may continue do so as well, but will most likely depend on mathematical insight and creativity, neither of which has ever been successfully predictable. 150digit numbers of the kind once used in RSA have been factored. The effort was greater than above, but was not unreasonable on fast modern computers. By the start of the 21st century, 150digit numbers were no longer considered a large enough key size for RSA. Numbers with several hundred digits are still considered too hard to factor in 2005, though methods will probably continue to improve over time, requiring key size to keep pace or new algorithms to be used.
Another distinguishing feature of asymmetric schemes is that, unlike attacks on symmetric cryptosystems, any cryptanalysis has the opportunity to make use of knowledge gained from the public key.
[edit] Quantum computing applications for cryptanalysis
Quantum computers, which are still in the early phases of development, have potential use in cryptanalysis. For example, Shor's Algorithm could factor large numbers in polynomial time, in effect breaking some commonly used forms of publickey encryption.
By using Grover's algorithm on a quantum computer, bruteforce key search can be made quadratically faster. However, this could be countered by increasing the key length.
[edit] Methods of cryptanalysis
Classical cryptanalysis:
Symmetric algorithms:
 Boomerang attack
 Brute force attack
 Davies' attack
 Differential cryptanalysis
 Impossible differential cryptanalysis
 Integral cryptanalysis
 Linear cryptanalysis
 Meetinthemiddle attack
 Modn cryptanalysis
 Relatedkey attack
 Slide attack
 XSL attack
Hash functions:
Network attacks:
External attacks:
[edit] See also
[edit] General
[edit] Historic cryptanalysts
 Conel Hugh O'Donel Alexander
 Lambros D. Callimahos
 Alastair Denniston
 Agnes Meyer Driscoll
 Elizebeth Friedman
 William F. Friedman, the father of modern cryptology
 Meredith Gardner
 Dilly Knox
 Solomon Kullback
 Marian Rejewski
 Frank Rowlett
 Abraham Sinkov
 Giovanni Soro, the Renaissance's first outstanding cryptanalyst
 Brigadier John Tiltman
 Alan Turing
 Herbert Yardley
[edit] National
[edit] External links
 A lot of real encrypted messages on newsgroups
 Basic Cryptanalysis (files contain 5 line header, that has to be removed first)
 Distributed Computing Projects
 Simon Singh's crypto corner
 The National Museum of Computing
 UltraAnvil tool for attacking simple substitution ciphers
[edit] References
 ^ Crypto History
 ^ David Kahn, Remarks on the 50th Anniversary of the National Security Agency, November 1, 2002.
 Ibrahim A. AlKadi ,"The origins of cryptology: The Arab contributions”, Cryptologia, 16(2) (April 1992) pp. 97–126.
 Friedrich L. Bauer: "Decrypted Secrets". Springer 2002. ISBN 3540426744
 Helen Fouché Gaines, "Cryptanalysis", 1939, Dover. ISBN 0486200973
 David Kahn, "The Codebreakers  The Story of Secret Writing", 1967. ISBN 0684831309
 Lars R. Knudsen: Contemporary Block Ciphers. Lectures on Data Security 1998: 105126
 Bruce Schneier, "SelfStudy Course in Block Cipher Cryptanalysis", Cryptologia, 24(1) (January 2000), pp. 18–34.
 Abraham Sinkov, Elementary Cryptanalysis: A Mathematical Approach, Mathematical Association of America, 1966. ISBN 0883856220
 Christopher Swenson, Modern Cryptanalysis: Techniques for Advanced Code Breaking, ISBN 9780470135938
 Friedman, William F., Military Cryptanalysis, Part I, ISBN 0894120441
 Friedman, William F., Military Cryptanalysis, Part II, ISBN 0894120646
 Friedman, William F., Military Cryptanalysis, Part III, Simpler Varieties of Aperiodic Substitution Systems, ISBN 0894121960
 Friedman, William F., Military Cryptanalysis, Part IV, Transposition and Fractionating Systems, ISBN 0894121987
 Friedman, William F. and Lambros D. Callimahos, Military Cryptanalytics, Part I, Volume 1, ISBN 0894120735
 Friedman, William F. and Lambros D. Callimahos, Military Cryptanalytics, Part I, Volume 2, ISBN 0894120743
 Friedman, William F. and Lambros D. Callimahos, Military Cryptanalytics, Part II, Volume 1, ISBN 0894120751
 Friedman, William F. and Lambros D. Callimahos, Military Cryptanalytics, Part II, Volume 2, ISBN 089412076X
