COMPSCI 92L Lecture Notes - Lecture 7: App Inventor For Android, Pseudorandom Number Generator, Modular Arithmetic
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11 May 2016
School
Department
Course
Professor
COMPSCI 92L
Astrachan
Spring 2016
3-2-16
Lecture 7: Abstraction and Encryption
● Whitfield Diffie and Martin Hellman designed an unbreakable encryption code for the
Enigma (WWII machine), but Alan Turing broke it eventually
● Apple’s password encryption algorithm makes it harder to break into someone’s phone
with the wrong password because you have to wait longer with each wrong submission
● Apple v FBI case: Apple, the plaintiff, won a major court victory against the FBI by
refusing to create a backdoor to decrypt the San Bernadino shooter’s iPhone
● HTTPS: secure hypertext transfer protocol
● Caesar cipher: use single number to encrypt/decrypt a number
○ The number is the number of lexographic letters off you transfer each time. One
message only has one key.
●Diffie-Hellman one-way function: easy in one direction, hard in reverse direction
○ Ex. of x1’s private color, x2’s private color, when both colors are mixed, you get
mixed color
○ Easy to mix colors
○ Hard to go back from mixed color to separate private colors
○https://www.youtube.com/watch?v=3QnD2c4Xovk
○ Bob picks secret color, so does Alice
■ Bob #008000, Alice #FF00FF Both mix with secret
○ Bob: #001A57 + #008000= #009A57
○ Alice: #001A57 + #FF00FF= #FF1B56
○ Bob sends to Alice and vice versa
○ Bob computes #FF1B56 + #008000 = #FF9B56
○ Alice computes #009A57+ #FF00FF = #FF9B56
● Pseudo Random Number Generator (PRNG) - uses a seed to choose a random number
from a previously generated list
○ Applications in casino gaming slot machines: computer chip in these slot
machines is set to a designated “tightness” level (ie 93% return on average). All
actions on these slot machines depends on the PRNG on the computer chip.
○ The odds of winning jackpot are constant, as are the tightness levels between
slot machines. Slot machines are rarely adjusted to make the return level
different.
● I Love Candy app:
○ Every texting phone number added to the list gets an Index
○ App Inventor 2 PRNG generates a random index
○ The index is used to pull the corresponding phone number from the list
● Clock arithmetic - used to better approximate randomness
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