Batch 2 - Class 13 - Information and Encryption

• Sherlock Holmes - "The Adventures of the Dancing Men"

• Winding way cipher - Put the message in a grid and define a path through the grid. Write the message in the grid left to right to encrypt. Overlay the path and write ciphertext along it to decrypt.

  Difference between code (like Morse code) and Cipher

  Compression and signaling, versus secrecy

  
  
• Key Sharing - how do we share keys - the problem of secrecy of messages is now translated to secrecy of keys

  Pre-shared lists of keysHow would we share keys between people who have never met?

  Jan and Maria have fallen in love (via the internet) and Jan wishes to mail her a ring. Unfortunately, they live in the world of Kleptopia where anything sent through the mail will be stolen unless it is enclosed in a padlocked box. Jan and Maria each have plenty of padlocks, but none to which the other has a key. How can Jan get the ring safely into Maria's hands?
• Answer: Jan sends Maria a box with the ring in it and one of his padlocks on it. Upon receipt Maria affixes her own padlock to box and mails it back with both padlocks on it. When Jan gets it he removes his padlock and sends the box back to Maria; voila! This solution is not just play; the idea is fundamental in Diffie-Hellman key exchange, a historic breakthrough in cryptography.
• One way functions - easy in one direction, hard in the other
• Color exercise from khan academy https://www.khanacademy.org/computing/computer-science/cryptography/modern-crypt/v/diffie-hellman-key-exchange-part-1 - one way function here is mixing colors
• Diffie Hellman: p^x mod q as a one way function, if p and q are primes, and this function distributes uniformly - p and q are agreed. private keys are x's and result of mods are sent out publicly. substituting p by the shared mod, gives the shared secret.

• Public-Private Key Encryption

  What if each of us has to communicate with millions of other people (like a bank)? Having many pairs of keys is hard. Analogy of a "clip lock". Color example on khan academy (alter the machine as something which can tell you a pair of complementary colors, but there is no way to determine a complementary color of a given color)Other applications - authentication using public keys; secrecy and authentication?

• Take 13 cards of a suite (Ace, 2, 3, 4, ... , King). The game is as follows: you pick two cards, and then the third. If the third card is between the first two cards, you win. What are the chances of your winning?

  If you solve the 13 card puzzle, try the same with all 52 cards