Tuesday, 6 June 2017

Encryption lesson 1 - Caesar Cipher

Encryption lesson 1 - Caesar Cipher

Objectives and Outcomes

  • be able to describe what a cipher is (band 4)
  • be able to create your own cipher algorithm (band 5)

You have been given 2 circles of a Caesar Cipher wheel.  The smaller circle sits on top of the larger one,  if you put a pen-point on the dot in the middle, you should be able to turn the top circle slowly over the bottom one which should stay still.

This cipher wheel was a very early machine used to help enciphering and deciphering secret messages.

It works by matching the "A" on the inner wheel to the appropriate shift letter on the outer wheel (turning clockwise - to the right); so for a shift of 3, "A" would be lined up with "D" on the outer wheel and all the other letters can then be read off.  You can use negative number shifts by going backwards (anti-clockwise - to the left), eg -1 "A" would line up with "Z".

Tasks
1.  On a new blog post, titled the same as this one, explain what the Caesar Cipher is, how it works and how a cipher wheel can be used to help (do not just copy/paste what I've written!).

2.  Decode this message on a shift 3:  ZKHQ BRX KDYH GHFRGHG WKLV ZRUN RXW WZHQWB VHYHQ WLPHV QLQH DQG WHOO BRXU WHDFKHU. 

3.  Write the code and your deciphered message on your blog, explaining how you worked it out.

4.  Write a message of your own and encipher it using the Wheel.  Write this on your blog.  Get a partner to try and decipher it.

5.  On your post, explain how easy it might be for someone who intercepts a secret message using this cipher to work out the original.

6.  On your post, write ideas of how this system could be adapted to make it harder to work out the coded message.

Publish!

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