Cryptography
Short Description
There is a way around this that we’ll discuss in the section on public key cryptography. 2 Simple Substitution Ciphers. In a simple substitution cipher, …
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Cryptography
Tom Davis
tomrdavis@earthlink.net
http://www.geometer.org/mathcircles
February 7, 2000
1 Introduction
The goal of cryptography is to make it possible for two people to exchange a message in such a way that other people
cannot understand the message. There is no end to the number of ways this can be done, but here we will be concerned
with methods of altering the text in such a way that the recipient can undo the alteration and discover the original text.
The original text is usually called “cleartext” and the encoded or altered text is called “ciphertext”. The conversion
from cleartext to ciphertext is called “encoding” or “enciphering”, and the opposite operation is called “decoding” or
“deciphering”. If you are trying to read a secret message that was not intended for you and you initially don’t know
the encoding method, it is called “cracking” the code.
In general, the more ciphertext you have, the easier it is to crack the code. So generally it is a good idea to change
the coding mechanism regularly. For example, if a coding scheme has a keyword (like the Vigen’ere cipher described…
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