Unformatted text preview: Shannon defined a cryptosystem whose unicity distance is infinite as one that has ideal secrecy. Note that an ideal cryptosystem is not necessarily a perfect cryptosystem, although a perfect cryptosystem would necessarily be an ideal cryptosystem. If a cryptosystem has ideal secrecy, even successful cryptanalysis will leave some uncertainty about whether the recovered plaintext is the real plaintext. Information Theory in Practice
While these concepts have great theoretical value, actual cryptanalysis seldom proceeds along these lines. Unicity distance guarantees insecurity if it’s too small but does not guarantee security if it’s high. Few practical algorithms are absolutely impervious to analysis; all manner of characteristics might serve as entering wedges to crack some encrypted messages. However, similar information theory considerations are occasionally useful, for example, to determine a recommended key change interval for a particular algorithm. Cryptanalysts also employ a variety of statistical and information theory tests to help guide the analysis in the most promising directions. Unfortunately, most literature on applying information theory to cryptanalysis remains classified, including the seminal 1940 work of Alan Turing. Table 11.1 Unicity Distances of ASCII Text Encrypted with Algorithms with Varying Key Lengths Key Length (in bits) 40 56 64 80 128 256 Unicity Distance (in characters) 5.9 8.2 9.4 11.8 18.8 37.6 Confusion and Diffusion
The two basic techniques for obscuring the redundancies in a plaintext message are, according to Shannon, confusion and diffusion . Confusion obscures the relationship between the plaintext and the ciphertext. This frustrates attempts to study the ciphertext looking for redundancies and statistical patterns. The easiest way to do this is through substitution. A simple substitution cipher, like the Caesar Cipher, is one in which every identical letter of plaintext is substituted for a single letter of ciphertext. Modern substitution ciphers are more complex: A long block of plaintext is substituted for a different block of ciphertext, and the mechanics of the substitution cha...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips