{"id":1810,"date":"2011-05-25T20:16:57","date_gmt":"2011-05-25T20:16:57","guid":{"rendered":"http:\/\/cemclinux1.math.uwaterloo.ca\/~cscircles\/wordpress\/?page_id=1810"},"modified":"2011-05-25T20:16:57","modified_gmt":"2011-05-25T20:16:57","slug":"15c","status":"publish","type":"page","link":"https:\/\/olescs.hkmu.edu.hk\/python\/15c\/","title":{"rendered":"15C: Caesar's JVTIVK JRCRU IVTZGV"},"content":{"rendered":"\n

Exercises 15A, 15B, and 15C can be completed in any order.<\/em><\/p>\n

Cryptography is the art and science of hiding the meaning of information, in a way that only some people can see it. In this lesson we will introduce one of the simplest cryptographic methods, the Caesar cipher<\/a> <\/strong>(also known as a\u00a0shift cipher<\/strong>), and you will to write a program to break it. You will design every aspect of your solution (unlike lesson 15A, which we broke into sub-parts).<\/p>\n

The Caesar cipher works by replacing each letter of the alphabet by another letter. To be precise, whenever you want to encode some text you need to pick a shift value<\/strong> S<\/em>, which is a number between 0 and 25. Then, you replace each letter in the text with the letter which is S<\/em> positions later in the alphabet, circling around to the start after you reach Z at the end of the alphabet.<\/p>\n

Example<\/h2>\n

Suppose we want to encode the secret message<\/p>\n

JOIN ME AT EIGHT BY THE ZOO<\/code><\/p>\n

using the shift value S<\/em>=2. The encryption rule says each letter is replaced by the one which occurs 2 positions later in the alphabet. For example, since the alphabet is ABCDEFGHIJ<\/strong>KL<\/strong>..., the first letter J<\/code> will be replaced by the letter L<\/code>. Continuing, the O<\/code> is replaced by Q<\/code>, the I<\/code> is replaced by K<\/code>, et cetera. To encode the letter Y, we have to circle back to the start: after Y comes Z, then A, so Y<\/code> is replaced by A<\/code>. Likewise Z<\/code> is replaced by B<\/code>. So the encoded version of our secret message is:<\/p>\n

LQKP OG CV GKIJV DA VJG BQQ<\/code><\/p>\n

If some spy saw this message, it would not be obvious to them what your message was about.<\/p>\n

\n
Short Answer Exercise: <\/span>Spy Coder<\/span><\/div>What is the result of encrypting SPY CODER<\/code> with the shift value S<\/em>=5? (Use uppercase.) You can write a program in the console<\/a> to compute this, if you like \u2014 it might be useful later.