Cryptography - Mount Holyoke Collegeblerner/cs101/Slides/Lecture25Cryptogr… · Caesar Cipher...
Transcript of Cryptography - Mount Holyoke Collegeblerner/cs101/Slides/Lecture25Cryptogr… · Caesar Cipher...
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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
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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
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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
Caesar Cipher Example
Key = 3
Plaintext: Mount Holyoke
Ciphertext: P
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
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Caesar Cipher Example
Key = 3
Plaintext: Mount Holyoke
Ciphertext: PR
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
Caesar Cipher Example
Key = 3
Plaintext: Mount Holyoke
Ciphertext: PRX
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
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Caesar Cipher Example
Key = 3
Plaintext: Mount Holyoke
Ciphertext: PRXQ
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
Caesar Cipher Example
Key = 3
Plaintext: Mount Holyoke
Ciphertext: PRXQW
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
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Caesar Cipher Example
Key = 3
Plaintext: Mount Holyoke
Ciphertext: PRXQW KROBRNH
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
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
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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
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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
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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
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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
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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
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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
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