One bit memory
  D latch
  
  Two inputs:
  D: data
  Control: allow the data to be stored
  
  When control is 0
    D can change, but won't be stored.
    
  When control is 1
    D's value will be stored

Q is the output

D Control   Old_Q   New_Q
0   0         0       0
0   0         1       1
0   1         0       0
0   1         1       0
1   0         0       0
1   0         1       1
1   1         0       1
1   1         1       1

Control=0, no chage
control=1, D is stored

Clock: 0, 1, 0, 1, 0, 1, ...
Synchronize signals in the computer

Main memory is different (dynamic)
  uses capacitors
  cheaper, larger memories
  needs to be refreshed (jog)
  
Our circuit used in:  (static)
  Processor registers
  Video memory
  
----------Chapter 8---------------

Ciphers - encrypt a message

Add a number to each letter in a message
to encipher the message:

Caesar Cipher: C(4)
Plain text message:  HELLO
Cipher text message: LIPPS

Shared secret: 4

Substitution Cipher:
 General map of plain text to cipher text.
 Shared secret: mapping of all letters