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 the secret key. In non-technical language, this is the practice of codebreaking or cracking the code, although these phrases also have a specialised technical meaning.
"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 attacks that do not primarily target weaknesses in the actual cryptography; methods such as bribery, physical coercion, burglary, keystroke logging, and so forth, although these latter types of attack are an important concern in computer security, 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 pen-and-paper methods of the past, through machines like Enigma in World War II, to the computer-based 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 mid-1970s, a new class of cryptography was introduced: assymetric cryptography. Methods for breaking these cryptosystems are typically radically different from before, and usually involve solving carefully-constructed problems in pure mathematics, the best-known being integer factorization.
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 9th century Arabian polymath Abu Yusuf Yaqub ibn Ishaq al-Sabbah Al-Kindi in A Manuscript on Deciphering Cryptographic Messages. This treatise includes a description of the method of frequency analysis (Ibrahim Al-Kadi, 1992- ref-3).
Frequency analysis is the basic tool for breaking 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 given sample of text. Similarly, the digraph "TH" is the most likely pair of letters, 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".
Frequency analysis relies as much on linguistic knowledge as it does on statistics, but as ciphers became more complex, mathematics gradually became the predominant approach to 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 for the first time applied to cryptanalysis with the Bomba device and the Colossus — one of the earliest computers.
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 pen-and-paper 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." (Remarks on the 50th Anniversary of the National Security Agency, 1 November 2002). 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 ciphertext-only attacks in 1998; FEAL-4, 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 real-time using widely-available computing equipment. In 2001, Wired Equivalent Privacy (WEP), a protocol used to secure Wi-Fi wireless networks, was shown to be susceptible to a practical related-key attack.
