LYCOS RETRIEVER 
Cryptanalysis: Keys
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Cryptanalysis is a tool that all cryptographers use to test and prove their latest and greatest encryption schemes. The only way to determine the strength of a particular method of encryption is to try to break the key and decode the message.
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Even though computation was used to great effect in cryptanalysis in World War II, it ... 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.
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The security of two-key cryptography depends on mathematical questions in a way that single-key cryptography generally did not, and conversely equates cryptanalysis to mathematical research in a new way. 150-digit 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, 150-digit numbers were no longer considered enough for RSA keys. Numbers of several hundred digits are still considered too hard to factor in 2004, though methods will probably continue to improve over time, requiring algorithms to keep pace.
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It is the periodicity of the repeating key which leads to the weaknesses in this method and its vulnerabilities to cryptanalysis. This periodicity of a repeating key can be eliminated by the use of a running-key Vigenère cipher, produced when a non-repeating key is used. However, even though running-key ciphers eliminate periodicity it is still possible to cryptanalyse them by means of several methods... the job of the cryptanalyst is made much harder and a cryptanalyst would require a much larger segment of ciphertext to solve a running-key cipher than one with a repeating key. In fact, the work of Major Joseph Mauborgne of the U.S. Army eventually led to the realisation that the only cryptographic system that is totally secure is that with a one-time completely random key.
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The A.M. Turing Award was named for Alan M. Turing, the British mathematician who articulated the mathematical foundation and limits of computing, and who was a key contributor to the Allied cryptanalysis of the German Enigma cipher during World War II. Since its inception, the Turing Award has honored the computer scientists and engineers who created the systems and underlying theoretical foundations that have propelled the information technology industry. For additional information, click on http://www.acm.org/awards/taward.html
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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 public-key encryption.
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