UW EE 299 - Coding for Security (14 pages)

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Coding for Security



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Coding for Security

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Pages:
14
School:
University of Washington
Course:
Ee 299 - Introductory Topics in Electrical Engineering
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Lecture 19 Coding for Security The Digital World of Multimedia Prof Mari Ostendorf Announcements Lab 5 due this week No homework this week writing HW next week Exams Grading is almost done will be handed out on Weds Solutions posted tomorrow Grade records will be available with exams 25 February 2008 EE299 Lecture 19 Plan for Today Coding for Security Issues in information security Focus on cryptography Simple systems Encoding binary strings Public keys 25 February 2008 EE299 Lecture 19 Coding for Security Privacy is important to us as individuals but is a challenge in our digital world Bank account info medical records academic records are all online now often accessible via the internet Wireless phone calls and email are easy to eavesdrop laptops can be easily stolen Secure information technology is needed for Military operations Commercial development and sales Humanitarian aid and human rights workers 25 February 2008 EE299 Lecture 19 Threats to Information Security Data disclosure Someone can read and comprehend the information Fraud Misrepresenting identity Data insertion removal and modification Changing stored data or data in transmission Denial of service Preventing access by overloading computers or network equipment 25 February 2008 EE299 Lecture 19 Tradeoff of Convenience Security Convenient to be able to access information remotely e g from home while traveling Convenient to have wireless access Convenience for users makes information less secure Solutions Limit access firewalls passwords PINs selfdestructive devices Ensure integrity of the data alteration detection ensure originator recipient cannot deny access Encrypt the data if you get access you can t read it unless you have the key 25 February 2008 EE299 Lecture 19 Cryptography Like compression and error coding change the format of the data Difference don t publish the recipe for changing it back user needs a special data key to reconstruct the signal Terminology Encoding Encryption or encipherment Decoding Decryption or decipherment Codebreaking figuring out the key 25 February 2008 EE299 Lecture 19 Simple Example Rotational Code Replace the k th letter in the alphabet with the k N th letter wrapping around Key N N 1 26 so easy to break N 4 Example meet me at the qfc at qiixdqidexdxlidujgdex 25 February 2008 EE299 Lecture 19 Slightly Better Permutation Code Any letter can map to any other letter Represent in a 26x26 permutation table 1 for each letter transformation 0 everywhere else Key The position of all 1 s in the table K 1 6 2 10 3 17 4 1 5 24 6 11 or equivalently K 6 10 17 1 24 11 A F B J C 26 4x1025 roughly possibilities Much harder to break but easier than you might think if you know the original language 25 February 2008 EE299 Lecture 19 What makes a code good Size of the key bigger key has more possibilities so harder to break Often the coding mechanism is not secret only the key 25 February 2008 EE299 Lecture 19 Encrypting Binary Sequences Change the bit sequence by combining the message and key with mod 2 addition exclusive OR 0 0 0 0 1 1 1 0 1 1 1 0 Example S signal K key E encrypted signal S 101100010100100001010010 K 001001101001110100110101 E 100101111101010101100111 Security is lost if the key becomes available Key must be the same length as the signal 25 February 2008 EE299 Lecture 19 Generating Keys Pseudo random number generator X n 1 AX n B mod N Example A 533 B 227 N 64 X 0 125 X 1 533 125 227 mod 64 66852 mod 64 36 X 2 533 36 227 mod 64 23 X 3 6 X 4 33 Binary form 100100 010111 000110 100001 N needs to be large enough to produce a long enough key without repetition Cryptography system Many people can know A B N X 0 usually kept secret 128 bit keys 2128 or approx 1038 key seeds 25 February 2008 EE299 Lecture 19 Public Key Cryptography Communicate encrypted information without sharing the key itself Each user has a public key that is generated from a private key but difficult to find the private key from the public key X Private key gives public key G X Two people work together to make a seed Seed F G X Y seed built by Y F X G Y seed built by X Mathematical challenge designing F G 25 February 2008 EE299 Lecture 19 Cryptography in History Rotation code used around 50BC by Julius Caesar Cryptography played a major role in World War II mathematics and intuition are important in codebreaking Widespread use of digital computers makes codebreaking easier Idea of public keys first introduced in 1976 25 February 2008 EE299 Lecture 19


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