История письма и чтения. Асафова Г.К. - 87 стр.

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A code they could not track

Alberti developed a disc made up of two dials fixed in the centre. Twenty

letters of the alphabet (excluding H,J,K,U,W and Y) and the numbers 1-4 were

written around the edge of the outer plate. A jumbled Latin alphabet was written

around the edge of the inner plate. To create a message, the sender took a letter

from the outer plate, then read off the cipher equivalent from the inner plate.

After a given number of letters, the inner plate was turned an agreed number of

positions so that the alphabet was represented by a different set of ciphers. The

recipient, of course, had to be equipped with the same cipher disc and know when

to turn the plate. Another polyalphabetic system involved writing out the letters of

the alphabet repeatedly in a table. At its simplest, the first line was the normal

alphabet (a-z); the second line began the alphabet with “b” and ended with “a”, the

third ran from “c” to “b” and so on. The sender and the recipient agreed on a secret

“keyword”- usually a term or a phrase.

To stop keywords from falling into the wrong hands, French diplomat Blaise

de Vegenere developed the “autokey”. The message itself contained the keyword,

signalled by a “priming key” – a letter or letters recognised as such by the

recipient. In this way the keyword changed with every message.

Polyalphabetic ciphers were considered uncrackable. However, if either the

sender or the recipient made an error, the text became nonsense. It was until the

age of machine-generated codes that they came into their own. The English

system, used by the Germans during the Second World War, was based on such a

system, as are many computer-generated codes used today – which because of the

speed at which they can be altered, are virtually impossible to break.

VOCABULARY

Abound v.

Изобиловать, кишеть

Break v.

Расшифровывать, разгадывать