Friday 16 June 2017

Encryption - Lesson 3 breaking the code

Encryption - Lesson 3 Breaking the code

This lesson is about how we can start to decipher a coded message using knowledge of words and frequency of letters used.

The presentation on this link .......


explains all the tasks you need to do for this lesson.

I think it's fairly self-explanatory and, therefore, am not writing any specific instructions on this post.

Just read each slide carefully and work through the tasks in sequence.

Don't forget to publish your post at the end of the lesson!

Wednesday 14 June 2017

Encryption Lesson 2 - Substitution Cipher

Substitution Cipher

Objectives and Outcomes:
  • be able to explain how a substitution cipher works (band 4)
  • be able to apply your understanding by creating your own substitution cipher and encoding a message (band5)


Many realised that the Caesar Cipher was weak; it was too easy to break.  The Substitution Cipher was developed to overcome this weakness.

The Substitution Cipher involved moving different letters of the alphabet around and put into a random position.

Each letter of the alphabet is replaced by another.



Your tasks
1.  On a new blog post, titled the same as this one, explain how a substitution cipher works.

2.  On a Google doc, create your cipher in the same way as I have shown you, but with different randomly placed letters.

3.  Apply your cipher by encoding the phrase 'Computer science rocks'.  Write the message and the code under your cipher table.

4.  Under this, copy my cipher from my blog.

5.  Using my cipher, decode this message: X  RXSP  OEXTXCK  BPIEPT  VPBBDKPB.

6.  Share your Google doc to 'Anyone with the link' and copy/paste the link to your post, under your first piece of writing.

7.  Explain how this cipher is better than the Caesar Cipher.

8.  Explain what senders and recipients of the coded messages would need to be able to encipher and decipher the messages accurately.

9.  How could the messages be further protected against being deciphered if intercepted regularly by others?  Write your detailed answer on your post.

Publish!




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!