Cryptanalysis: Encyclopedia II - Cryptanalysis - History of cryptanalysis

Cryptanalysis - 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.

Cryptanalysis - 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 Arabic 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 (Ibraham, 1992).

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.

Cryptanalysis - 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 [1].

Kahn may have been premature in his cryptanalysis postmortem; weak ciphers are not yet extinct. 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.

Cryptanalysis - The results of cryptanalysis

Successful cryptanalysis has undoubtedly influenced history; the ability to read the presumed-secret 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. Even as of 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: [2]).

Adapted from the Wikipedia article "History of cryptanalysis", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki