1. Computer Security: Problem
The Problem: Billions in Losses
Outright theft
Online scams
Viruses / Worms
Actual damage
Actions to avoid damage
Denial of Service
Etc.
Possible Traps (Public Systems ! ! ! )
Trojan Horse
Onlooker
Cameras

2. Computer Security: Defenses
Passwords
Using Secure Passwords
Keeping them Secure
Encryption
Simple (Classical Encryption – Pre-Computer)
Strong (Modern – Computer Based)
Good Practices
Like all fields, doing something stupid …
Tradeoffs
Is the cure worse than the disease?
Long Live Common Sense!

3. Good Passwords and Cracking
Briefcase (style) Combination Locks
Brute force methods: Try all combinations
Number of wheels
Number of position per wheel
Time per trial
How long does it take?
Contrast to BRUTE brute force method (Always Consider!)
Password on a computer
+ More possibilities per “wheel”
+ More “wheels” (often up to user)
- Computer based cracking faster!
- Dictionary attacks
Picking a good UNIX password

4. Classical Encryption When passwords fail, encryption can be fallback
Also provides extra level of difficulty
Security vs. Privacy
Many levels of encryption sophistication:
Go through some of them
Single Alphabetic Substitution
Caesar: L FDPH, L VDZ, L FRQTXHUHG
Magic decoder ring?
Cryptoquote
Cracking single alphabetic substitution
Character frequency -- ETAONIRSH
(Length of text)

5. Classical Encryption: Single Alphabet
I F O J L K F J N D C E L N P N C X N D J L D V F
F O J I D M R N J L U J F O V R M I F J M R F C
M R S L K F C M S C N C M, D C N Q C D M S F C,
K F C K N S P N E S C B S U N J M X, D C E
E N E S K D M N E M F M R N G J F G F L S M S F C
M R D M D B B A N C D J N K J N D M N E N H O D B.
-- D B S C K F B C
A 1 H 1 O 4 V 2
B 6 I 3 P 2 W 0
C 16 J 11 Q 1 X 2
D 14 K 7 R 6 Y 0
E 7 L 6 S 10 Z 0
F 15 M 15 T 0
G 2 N 18 U 2

6. Classical Encryption Polyalphabetic Substitution Cypher Reuse !
The Vignere Cypher
The Babbit Solution
How many alphabets used?
digraph frequency “th”
Several Single-Alphabet problems
Cypher Reuse !
Bigger pool of data
Patterns become obvious
One Time Pads
Can be Absolutely Secure
Computers and Random Number Generators ?!

7. Classical Encryption The Key Exchange Problem Threats
Using your “secure” channel
A padlock analogy
Diffie, Hellman, and Merkle solution