cryptography - mount holyoke collegeblerner/cs101/slides/lecture25cryptogr… · caesar cipher...

CryptographyDecember 9, 2008

Intercepting Information Scenario 1

http://www.geeksquad.com/uploadedImages/

wwwgeeksquadcom/Tools/Tech_Articles/

going_wireless_1.jpg

Wireless broadcasts information using radio signals

Any computer on a wireless network CAN listen to any network traffic

A computer SHOULD only listen to traffic addressed to it

It is impossible to know if anyone is eavesdropping

1

2

Tuesday, December 9, 2008

Intercepting Information Scenario 2

Spam filter reads a mail message to decide if it is spam

What could an unscrupulous person do when he/she writes a spam filter?

Mail serverInternet

Your laptop

Cryptography

3

4


Caesar Cipher Example

Key = 3

Plaintext: Mount Holyoke

Ciphertext:

PLAINTEXT LETTER M N O P Q R S T U V W X YCIPHERTEXT LETTER P Q R S T U V W X Y Z A B


Key = 3


Ciphertext: P


5

6



Key = 3


Ciphertext: PR



Key = 3


Ciphertext: PRX


7

8



Key = 3


Ciphertext: PRXQ



Key = 3


Ciphertext: PRXQW


9

10



Key = 3


Ciphertext: PRXQW KROBRNH


General Letter Substitution

Algorithm: Substitute 1 letter for another

Key

Ciphertext: LUGAU

PLAINTEXT LETTER A B C D E F G H I J K L MCIPHERTEXT LETTER G J A O U N E Z Y P H S T

PLAINTEXT LETTER N O P Q R S T U V W X Y ZCIPHERTEXT LETTER V W L M I B D C R K F Q X

11

12


WWII Cryptography

http://upload.wikimedia.org/wikipedia/commons/a/ae/Enigma.jpg

Enigma machine

Used by Germans in World War II

Led to great success for Uboats - sunk 50 ships every month!

Broken by British, led by Alan Turing

Enigma Machine• Top shows A being

typed and encoding to G

• Typing a letter also moves the rotors.

• Next time, shown on the bottom, A encodes to C.

http://upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Enigma-action.svg/527px-Enigma-action.svg.png

13

14


An Enigma Codebook

http://upload.wikimedia.org/wikipedia/commons/thumb/8/82/Kenngruppenheft.jpg/800px-Kenngruppenheft.jpg

Alan Turing

http://encyclozine.com/History/Biography/Turing/turing.jpg

Group at Bletchley Park in England figured out the algorithm

Claim they shortened the war by 2 years!

Father of computer scienceTuring machinesTuring test

15

16


Symmetric Key Cryptography

Problem: Sender and receiver must agree on the algorithm and key

Public Key Cryptography

Keys come in pairs:

Public key - anyone can see

Private key - must be kept secret

Anything encrypted with public key can be decrypted with the private key and vice versa

17

18


Secret vs. Public Key

Text

Secret key

Public key

Shared key

Bob’s public keyBob’s private key

Internet

Amazon’s computer

Your computer

Ecommerce Application

Your credit card number

Ciphertext

Encrypt with Amazon’s public key

Your credit card number

Decrypt with Amazon’s private key

19

20


Public key challenges

What algorithm would use separate keys for encoding and decoding?

Why isn’t the decoding key guessable if you know what the encoding key is?

RSA algorithm depends on the fact that it is very difficult to factor large numbers

Secure Socket Layer (SSL)

Client

CONNECT

Server public key

Symmetric key

Real communication

Server

No encryption

Encrypted with server public key

Encrypted with symmetric key

21

22


Digital SignatureDigital signature uses the sender’s keys

Alice’s private key Alice’s public key

What’s next?

Quantum computing

Computers based on quantum physics

Expect that quantum computers will ultimately break RSA

Encryption with quantum computers might also be able to detect eavesdropping

23

24


cryptography - mount holyoke collegeblerner/cs101/slides/lecture25cryptogr… · caesar cipher...

Documents