By Fred Piper, Sean Murphy

This ebook is a transparent and informative advent to cryptography and knowledge 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 may be involved.

Important components are highlighted, akin to move Ciphers, block ciphers, public key algorithms, electronic signatures, and functions equivalent to e-commerce.

This booklet highlights the explosive effect of cryptography on glossy society, with, for instance, the evolution of the net and the creation of extra refined banking equipment.

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

**Example text**

Thus, on the assumption that the interceptor knows the system being used, knowledge of the length of the cryptogram would be sufﬁcient to give the interceptor 100 per cent conﬁdence that he knew the message, even though he could not determine the key used. 54 Another option might be to use the following system in which the two keys, k1 and k2, are equally likely. In order to describe the complete algorithm we use a standard notation. For key k1, the ciphertext for the plaintext message BUY is 0 while the ciphertext for SELL is 1.

Thus, despite the fact that we have perfect secrecy, we have not provided any extra protection to increase the likelihood of the message remaining secret. Nevertheless the weakness is due to the fact that the number of messages is small. It is not the result of poor encryption. 55 Unbreakable ciphers?

As an illustration we might assign numbers to letters as in the diagram. A A B C D E E F G H I J K L M N 01 07 14 21 04 13 17 20 29 31 06 28 12 30 17 00 N O O P Q R R S T T U V W X Y Z 18 26 19 09 10 25 23 02 08 24 22 05 16 15 11 03 34 If we do this then the word TEETH, which has two pairs of repeated letters, might be written as 24 27 13 08 31. To anyone who does not know the key, the ﬁve characters of the cryptogram are different but the genuine receiver is in no danger of confusion. The six letters chosen are likely to be the six most popular letters in the plaintext.