1. Cryptography Survey

2. Problem: How would you send a message over an unsecure communication channel?
NECACELTAS

3. Problem: How would you play poker over the internet?

4. Cryptography vocabulary
Plaintext-preencryption text
Ciphertext-text after it has the encryption
Encryption-function from plain text to ciphertext
Decryption-function from ciphertext to plaintext
Key- object which allows encryption/decryption

5. Caesar’s Cipher
NECACELTAS

6. Caesar’s Cipher
RIGEGIPAEX

7. Substitute each letter with the one 4 ahead in the alphabet.
This can augmented by allowing various shifts
How is it cracked?
Try every key

8. Substitution How is it cracked?
a b c d e f g h i j k l m n o p q r s t u v w x y z
t h o m a s j e f r n b c d g i k l p q u v w x y z
Letter          
Frequency           a
8.167% b
1.492% c
2.782% d
4.253% e
12.702% f
2.228% g
2.015% h
6.094% i
6.966% j
0.153% k
0.772% l
4.025% m
2.406% n
6.749% o
7.507% p
1.929% q
0.095% r
5.987% s
6.327% t
9.056% u
2.758% v
0.978% w
2.360% x
0.150% y
1.974% z
0.074%
How is it cracked?

9. Vigenere Cipher theanswerstotheexam facadefacadefacadef
Letter          
Frequency           a
8.167% b
1.492% c
2.782% d
4.253% e
12.702% f
2.228% g
2.015% h
6.094% i
6.966% j
0.153% k
0.772% l
4.025% m
2.406% n
6.749% o
7.507% p
1.929% q
0.095% r
5.987% s
6.327% t
9.056% u
2.758% v
0.978% w
2.360% x
0.150% y
1.974% z
0.074%
facadefacadefacadef
yhgaqwbetswsyhgeaer
How is it cracked?

10. One time pad theanswerstotheexam theveryverysecretke
moivrjuzvjrgxjviqkq
How is it cracked?

11. Claude Shannon
Decided that the security of the system should depend only on the key
Proved that one-time-pad with random key is completely secure

12. Early 20th Century
“Gentlemen don't read each other's mail.”

13. Symmetric Key

14. Asymmetric key

15. RSA
plaintext: t
encryption key: n, e
decryption key: n, d

16. RSA
Choose large primes p and q.
Let n=pq

17. Example of RSA

18. Suppose we want to encrypt 5

19. Signing documents
I’m Spartacus
I’m Spartacus

20. Signing documents I’m Spock (I’m Spock)^d mod n
((I’m Spock)^d mod n)^e mod n
I’m Spock

21. RSA
How is it cracked?

22. AES Symmetric encryption process used in most transactions
Approved by NSA for “Top Secret” documents

23. The Future
Quantum Cryptography

24. Typical Transaction Give me the Amazon key Here it is, signed by us
Lets use this AES key

25. Mental Poker
What do we need?

26. Mental Poker
Agree upon a set of strings to represent the deck

27. Mental Poker Player 1 encrypts every card with key A and shuffles
Player 2 encrypts every card with key B and shuffles

28. Mental Poker
Player 1 decrypts key A from all the cards and encrypts each one with a distinct key (A1,A2 etc)
Player 2 decrypts key B from all the cards and encrypts each one with a distinct key (B1,B2 etc)

29. Mental Poker
When Player 1 agrees that Player 2 should be able to see a card, they hand over the appropriate key