1. Encryption. LEARNING OBJECTIVES: BY THE END OF THE LESSON YOU SHOULD KNOW. What encryption is and why it is important The basics of encryption techniques How to encrypt a message using the Caesar cipher How to encrypt a message using the transposition cipher How to use frequency analysis to help decrypt ciphers

2. Encryption - Starter: Since you were so upset last week. Here is the quiz!! Please enter the link below to play the quiz regarding number systems. http://kahoot.it http://kahoot.it  Learning Objectives: By the end of the lesson you should know. What encryption is and why it is important The basics of encryption techniques How to encrypt a message using the Caesar cipher How to encrypt a message using the transposition cipher How to use frequency analysis to help decrypt ciphers

3. What is Encryption?:  It’s a process of scrambling information so it cannot be understood by anyone intercepting the data.  To decrypt it you need to know the encryption method and key used.Sender Data Receiver

4. Why is it important?:  Data is vulnerable  It could be personal information  It could be sensitive information  Networks and computers can be hacked

5. Encryption Basics:  How does it work?  By changing plaintext into ciphertext  Hello world  Cfzzn anwzx  Hello world  Decrypted by receiver by knowing the cipher method and using a key.

6. Caesar cipher.  Example of a shift cipher ABCDEFGHIJKLMNOPQRSTUVWXYZ ABCDEFGHIJKLMNOPQRSTUVWXYZ

7. Caesar cipher.  Example of a shift cipher ABCDEFGHIJKLMNOPQRSTUVWXYZ VWXYZABCDEFGHIJKLMNOPQRSTU An example of a monoalphabetic cipher. What is the key? Ans: 5

8. Caesar cipher.  Example of a substitution cipher ABCDEFGHIJKLMNOPQRSTUVWXYZ ERNESTBEVIN

9. Caesar cipher.  Example of a substitution cipher ABCDEFGHIJKLMNOPQRSTUVWXYZ ERNSTBVI

10. Caesar cipher.  Example of a substitution cipher ABCDEFGHIJKLMNOPQRSTUVWXYZ ERNSTBVIACDFGHJKLMOPQUWXYZ ERNSTBVI What is the key? Ans:

11. Caesar cipher.  Another example of a substitution cipher ABCDEFGHIJKLMNOPQRSTUVWXYZ ERNSTBVIACDFGHJKLMOPQUWXYZ ABCDEFGHIJKLMNOPQRSTUVWXYZ DQKWROTPJYNUBSIXCLAZGMVEFH An example of a polyalphabetic cipher. What is the key? Ans: a lookup table

12. Frequency analysis  Shift/substitution ciphers are easy to crack.  Done by analysing the frequency of characters.  Who can guess most frequent letter in English alphabet?  Ans: E

13. Transposition Cipher  Please watch this video tutorial.  https://www.youtube.com/watch?v=0um-_4SvPg0

14. Plenary:  How did you get on with the task?  Was it too difficult/too easy?  Are you confident you have learnt the learning objectives?  Homework: Read through section 27 and complete tasks 1 - 11 in AQA A level Computer Science for before next week lesson. And remember UPLOAD TO FRONTER!!!