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Copyright (C) 1995 
Tim Philp 
Brantford, Ontario 
Article appeared in The Expositor, Brantford, Ont., Canada 
June 10th, 1995 
		Tim Philp 
	Last week I discussed ways that the computer has 
made our lives less private. Today we take up that thread and 
talk about the stuff of spy novels.Cryptography, in the context 
that we are discussing it, is a means of preventing 
unauthorized people from reading our files or correspondence. 
Cryptography is not new. It has been used throughout history 
and has played its part in shaping events. Mary Queen of Scots 
was beheaded because the cipher that she used to arrange a 
rebellion against Elizabeth I was decoded by the government. 
	Julius Caesar used a primitive cipher, now known as a 
Caesar Cipher, to disguise his messages to Rome during his 
military campaigns. Every letter in his messages was replaced 
by the letter three letters farther in the alphabet. Thus A would 
become D, B would become E, until you reached the end of the 
alphabet. You would then wrap around to make X equal to A, 
Y equal to B and Z equal to C. In this way the word CAESAR 
would become FDHVDU. Simple substitution ciphers like this 
one are easily broken and today are used in puzzle books to 
provide a few moments diversion to puzzle fanciers. 
	The role of computers in cryptography goes back to 
the early days of computers. One of the first computers was 
used to help decode enemy messages in England during the 
second world war. The 'boffins of Bletchley' were the true 
progenitors of the computer, not IBM. 
	Cryptography has progressed far beyond the simple 
Caesar Cipher. Indeed, today's methods of encoding messages 
have progressed to the point where the ciphers are unbreakable 
within any reasonable amount of effort. It does no good to 
break a cipher if it takes 1500 years of computer time to do it! 
	Of interest to computer users is the development of a 
type of cryptography called Public Key Cryptography. With 
this method you have two keys, a public key and a private key. 
Simply speaking, a key is a very long number that is used by 
your program to encode and decode messages. Each person 
that you want to communicate with has a public key that is 
published and is known by everyone.  
	To send a message to your friend you would use his 
public key to encode the message. Your friend receives the 
message and uses his private key to decode the message. 
	The beauty of this technique, is an eavesdropper can 
intercept the encoded message and the public key of the 
recipient, but he cannot use this information to decode the 
message without the secret key of the recipient. 
	In older systems you had to transmit the secret key to 
the recipient. The secret key could be intercepted and used to 
decode the messages. Public key systems require that only the 
public key be transmitted to the recipient. The message cannot 
be decoded using only the public key and the message. 
	The availability to the general public of secure, 
military grade cryptographic methods has generated 
considerable controversy. The US government has placed this 
software under export control and has suggested the use of 
their own system called Clipper. Clipper has a 'backdoor' that 
will allow the government to decode secret messages. They 
claim that they need this power to combat drugs and crime. 
Civil libertarians claim that the government has no business 
intercepting private communications. 
	Cryptography can be used to protect E-Mail messages 
and computer files. The software to do this is freely available 
on the Internet (search for PGP). The US government, it 
seems, is locking the barn door after the horses have escaped. 
This file may not be reproduced by any means without the 
permission of the author 

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