Cryptography: A Very Short Introduction by Fred Piper, Sean Murphy

By Fred Piper, Sean Murphy

This publication is a transparent and informative creation to cryptography and knowledge protection--subjects of substantial social and political value. It explains what algorithms do, how they're used, the hazards linked to utilizing them, and why governments can be involved. vital parts are highlighted, akin to circulate Ciphers, block ciphers, public key algorithms, electronic signatures, and functions equivalent to e-commerce. This booklet highlights the explosive impression of cryptography on sleek society, with, for instance, the evolution of the net and the advent of extra refined banking methods.

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The classic example of a perfectly secure cipher system is the one-time pad. If the message is a passage of English text containing n letters with all punctuation and spaces removed, then the key, which is only used once to protect a single message, is a randomly generated sequence of n letters from the alphabet. The encryption rule is precisely that used for the Vigenère Cipher with the key replacing the keyword. , kn, the ith component of the cryptogram is given by: ci = (mi + ki) mod 26 Note that the fact that the key is the same length as the message guarantees that there is no need to start repeating the key during the encryption process.

Clearly, as we have already seen, the key size provides an upper bound of an algorithm's cryptographic strength. However, as we saw with the Simple Substitution Ciphers, having a large number of keys is no guarantee of strength. A symmetric algorithm is said to be well designed if an exhaustive key search is the simplest form of attack. Of course, an algorithm can be well designed but, if the number of keys is too small, also be easy to break. Designing strong encryption algorithms is a specialized skill.

For a hardware attack, an attacker might use a purpose-built device. Any underestimation of this time is likely to lead to insecurity, whilst overestimating may make security a larger overhead than is needed. A lucky attacker who attempts an exhaustive key search might find the key with the first guess. One result of having a large number of keys is to make the probability of this happening very small. At the other extreme, a very unlucky attacker might not find the key until the last guess. In practice, it is very unlikely that the attacker would need to perform a complete search before finding the key.

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