数学日记:

Math and Computer & Web Security - Cryptography


A military commander wants some assurance that the information sent to field commanders does not fall into the hands of opponents. Hence, written communications which can be easily read if intercepted by an an enemy are dangerous. (Asking the messenger to memorize secret messages is not practical, and if one can believe the spy and counter-terrorism thrillers currently on TV, not secure.) Julius Caesar is often credited with one of the earlier attempts at using a cryptological system with a mathematical flavor to disguise messages. It is claimed that he used a system in which each letter of the alphabet in a “plaintext,” the original message, is replaced by the next letter of the alphabet, with the last alphabet letter cycling around to be represented by the first letter of the alphabet. Thus, the phrase Caesar Cipher would be replaced by Dbftbs Djqifs. Coming across a message such as this, one is faced with the tremendous range of possible systems that might have been used to disguise the original message. It might confuse the “enemy” for a while. Within the range of what today have come to be called Caesar Ciphers, one could shift the replacement alphabet by r places, rather than 1 place (r =1) in the example above. When r = 5 the phrase Caesar Cipher becomes hfjxfw hnumjw.


More:
http://mathstopic.blogspot.com/2015/06/math-and-computer-web-security.html


Tumblr:
http://mathpost.tumblr.com/post/121011459280/math-and-computer-web-security-cryptography-a



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