1. Computers in Society Encryption

2. Representing Sensory Experience Some objects correspond to human sensory experience – these representations are created to all these experiences to be replicated. This ALWAYS involves the conversion from analog (continuous time and value) to digital (discrete time and value)

3. From Analog to Digital Media is recorded by turning an analog signal in the real world into a digital signal in a computer. The sampled signal is on a grid: the X direction determines the sampling interval (rate), the Y direction the sampling values. The precision of the sampling determines how closely the digitized signal matches the original.

4. Approximating Sensation When representing pictures, sounds, or movies there is always a trade-off between size and precision. For example, sound can be represented as.wav or.mp3; pictures as.bmp or.jpg Media formats such as mp3 and jpg degrade the media quality, usually in ways that are not detectable to human senses.

5. The Next Level So far, we've ignored "meaning". That is, a picture is just a bunch of pixels A sound is just a waveform A book is just a string of characters The hard problem is assigning a deeper meaning to these objects. Go to the seminar of Friday for a good look at this problem!

6. Ontology Why is ontology important? How do Ontologies relate to object-oriented programming? What is a partition? What are some common relationships? What is the difference between a domain ontology and upper ontology? What is OWL? Why is it important?

7. Privacy and Security Encryption Anonymity Trust

8. Encryption The problem: private communication on public channels Is the Internet a public or private channel? Why?

9. History of Encryption This is a classic example of a problem that has been around for thousands of years that can be addressed directly by computation! Two basic ideas: a computational mechanism to perform encryption and a shared secret between the parties

10. Encryption Our basic encryption scenario: Alice and Bob are trying to communicate A third party, Trudy, is trying to understand what Bob and Alice are saying. Any message from Alice to Bob is also seen by Trudy

11. Shared Secret Encryption Until recently, all encryption has been based on a “shared secret”. The classic example is the “One Time Pad” – a sequence of random 0’s and 1’s. Use a key (the pad) to scramble every bit in a message (1 = change, 0 = don’t change) Receiver must have same pad Pad has to be “truly random” Can only use pad once! Code is mathematically unbreakable Pad must have the same length as the message

12. Using “Keys” The problem with a one time pad is that it gets used up. Instead of a pad, we would rather use a “key”. A key is a piece of information (0’s and 1’s) that is reused continuously to encrypt an arbitrary amount of data Keys are measured by their length (128 bit encryption, for example) Note that “cracking” the key opens up lots of potential information. How hard is it to guess an N-bit key?

13. Encryption Standards There are many “standard” encryption algorithms (DES, AES, …). We generally rely on public standards rather than private ones – why??? Shared key (symmetric) encryption is computationally efficient – encrypting a long message is no problem at all.

14. Public Key Encryption This is somewhat like the “invention of the wheel” in the crypto world. A truly world-changing feat that is not much recognized by the public. This was first published publicly by Whitfield Diffie and Martin Hellman in 1976 Malcolm J. Williamson actually developed this stuff first but it was not published

15. Public Key Encryption Basic idea: you need two keys, a public key that everyone knows and a private key only you know. Sender uses public key of receiver to encode message Only receiver has private key. No need to trust the sender with your secret!

16. The Lockbox I want to be able to receive something from a friend without worrying about anybody peeking in. My solution: use an unbreakable lockbox with a lock that can't be picked. I'll give my friend the lockbox and one key, I'll keep the other key. He can mail me the locked box and only I can open it. What sort of encryption is this?

17. The Key Problem I don't want to meet my friend in private to hand him the key but I can't mail him the key either (why?). So what if instead I put a diagram of the key on my website so he can build it himself? Will that work?

18. Locks Since anyone can build a key, anyone can pick locks on my private message. Instead of keys, let's talk about locks. Think of a combination lock – if it's open, you can lock something with it even if you don't know the combination. You only need the combination to unlock! Now instead of sharing keys, I give an unlocked lock to my friend.

19. Building Locks Instead of telling everyone in the world how to build my key, I'll tell everyone how to build an open lock than only I can unlock. Wouldn't seeing the plans for this lock make it possible for others to deduce the combination?