Applied to on-line content the advertising model leads to similar problems like in the traditional media. Dependence on advertising revenue puts pressure on content providers to consider advertising interests. Nevertheless new difficulties caused by the technical structure of online media, missing legal regulation and not yet established ethical rules, appear.
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New opportunities for online advertisers arise with the possibility of one-to-one Web applications. Software agents for example promise to "register, recognize and manage end-user profiles; create personalized communities on-line; deliver personalized content to end-users and serve highly targeted advertisements". The probably ultimate tool for advertisers. Although not yet widely used, companies like
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David Kahn can be considered one of the most important historians on cryptography. His book The Codebreakers. The comprehensive history of secret Communication from Ancient Times to the Internet, written in 1996 is supposed to be the most important work on the history of cryptography.
the word "cipher" comes from the Hebrew word "saphar", meaning "to number". Ciphers are mere substitutions. Each letter of the alphabet gets substituted; maybe by one letter or two or more.
an example:
PLAINTEXT a b c d e f g h i j k l m n o p q r s t u v w x y z
CIPHERTEXT D E F G H I J K L M N O P Q R S T U V W X Y Z A B C