By Fred Piper, Sean Murphy

This e-book is a transparent and informative creation to cryptography and information protection--subjects of substantial social and political significance. It explains what algorithms do, how they're used, the dangers linked to utilizing them, and why governments might be involved.

Important parts are highlighted, resembling circulation Ciphers, block ciphers, public key algorithms, electronic signatures, and purposes akin to e-commerce.

This e-book highlights the explosive impression of cryptography on glossy society, with, for instance, the evolution of the net and the advent of extra subtle banking tools.

**Read Online or Download Cryptography: A Very Short Introduction PDF**

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**Extra info for Cryptography: A Very Short Introduction**

**Sample text**

Once this is done then each row is the result of using the same Simple Substitution Cipher that, for the particular case of a Vigenère Cipher, is an additive cipher. We can now use the statistical arguments of the last section on each individual row. In fact, for a Vigenère Cipher where the length of the cryptogram is long in comparison to the period p, it is probably sufﬁcient to determine the most frequent letter in each row and assume that it represents either E, T, or A. This latter observation exploits the fact that, for each row, the Simple Substitution Cipher being used is a Caesar Cipher.

An example is given here. A D B C D E F G H I J K L M I Q M T B Z S Y K V O F N E O P R J Q R S T A U W P U V W X Y Z X H L C N G The encryption and decryption keys are equal. They are simply the order in which the bold letters are written. The encryption rule is 24 ‘replace each letter by the one beneath it’ while the decryption rule is the opposite procedure. Thus, for example, for the key in this ﬁgure, the cryptogram corresponding to GET is ZTP, while the message corresponding to IYZ is BIG.

Thus 86 is 1010110 in binary. Modular arithmetic Modular arithmetic is only concerned with integers, commonly known as whole numbers. If N is a positive integer, then arithmetic modulo N uses only the integers 0, 1, 2, 3, . . , N-1, that is, integers from 0 to N-1. There are a number of values of N for which arithmetic modulo N is common to most people, although they may not be familiar with the mathematical terminology. For instance when we use a 12-hour clock we use addition modulo 12. If it is now 2 o’clock then everyone ‘knows’ that the time in 3 hours is 5 o’clock, as it will also be in 15 hours.