Computers in Society

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 0s and
  1s.
• Use a key (the pad) to scramble every bit in a
  message (1 change, 0 dont 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 (0s and 1s)
  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?