Batch 2 - Class 12 - Information and Encryption

• Shift Cipher Device

• Telephone numbers, and STD and ISD codes - what are the assumptions in this design with respect to frequency of calling? How does the system know whether we are dialing local or STD? (notion of non-repeating prefix again)
• Error Detection: Go back to ascii codes, and ask kids how they would design a system so that if there was one bit error in a character, they could detect it. Show that in standard ascii, one bit error may lead to wrong interpretation - A and E differ by one bit, so BAT could become BET.
• Hint: Introduce notion of repetition or redundancy to detect errors.
• Parity bit
• However, with parity, one can't correct the error - Error Correction - 4 bits need 3 parity bits to correct one error. 
• Let kids learn about it at home if they would like to

• Form teams of three for the following exercises:
• Post the problem of secret communication to kids and let them work in pairs to solve it. Note that to be able to communicate secretly, and not let a person-in-the-middle break in, they need to have something (key) in advance which is a secret
• Most groups came up with some form of substitution cipher
• Puzzle:  Justin Case and Auntie Bellum are fellow con artists who deliver coded messages to each other to communicate. Recently Auntie Bellum was put in jail for stealing a rare and expensive diamond. Only a few days after this, Justin Case sent her a friendly letter asking her how she was. On the inside of the envelope of the letter, he hid a code. Yesterday, Auntie Bellum escaped and left the envelope and the letter inside the jail cell. The police did some research and found the code on the inside of the envelope, but they haven't been able to crack it. Could you help the police find out what the message is?

  This is the code: llwatchawtfeclocklnisksundialcirbetimersool
• Hint: It was a TIMED exit
• Answer: "Loose Bricks In Left Wall"

• Shift Cipher/ Caeser Cipher - Use the device kids have made at home, and let kids communicate using it
• Rhea made a shift cipher device - nice job!

• What is the "key" here? Note that this is not a very strong cipher - either because the person-in-the-middle knows its a shift, or due to frequency of letters in english language we used earlier
• Substitution cipher - what is the key? The frequency problem is still there!
• Explore frequency fingerprint - https://www.khanacademy.org/computing/computer-science/cryptography/crypt/p/frequency-fingerprint-exploration 

• Polyalphabetic cipher - How would you avoid the fingerprint problem? What would happen if all letters were roughly the same frequency? What is the "key" in polyalphabetic shift?

• How would you break the polyalphabetic cipher?
• How would you use this for maximum secrecy? One time pad

• Homework Puzzle: Read "The Adventures of the Dancing Men", a Sherlock Homes story. Or solve it here www.aamt.edu.au/content/download/18716/.../nmd12_dancing_men.pdf