11 - Chapter 11: Cipher Techniques Some Problems Types of...

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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #11-1 Chapter 11: Cipher Techniques Some Problems Types of Ciphers Networks Examples
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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #11-2 Overview Problems What can go wrong if you naively use ciphers Cipher types Stream or block ciphers? Networks Link vs end-to-end use Examples Privacy-Enhanced Electronic Mail (PEM) Secure Socket Layer (SSL) Security at the Network Layer (IPsec)
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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #11-3 Problems Using cipher requires knowledge of environment, and threats in the environment, in which cipher will be used Is the set of possible messages small? Do the messages exhibit regularities that remain after encipherment? Can an active wiretapper rearrange or change parts of the message?
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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #11-4 Attack #1: Precomputation Set of possible messages M small Public key cipher f used Idea: precompute set of possible ciphertexts f ( M ), build table ( m , f ( m )) When ciphertext f ( m ) appears, use table to find m Also called forward searches
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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #11-5 Example Cathy knows Alice will send Bob one of two messages: enciphered BUY, or enciphered SELL • Using public key e Bob , Cathy precomputes m 1 = { BUY } e Bob , m 2 = { SELL } e Bob • Cathy sees Alice send Bob m 2 Cathy knows Alice sent SELL
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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #11-6 May Not Be Obvious Digitized sound Seems like far too many possible plaintexts Initial calculations suggest 2 32 such plaintexts Analysis of redundancy in human speech reduced this to about 100,000 ( 2 17 ) This is small enough to worry about precomputation attacks
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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #11-7 Misordered Blocks Alice sends Bob message n Bob = 77, e Bob = 17, d Bob = 53 Message is LIVE (11 08 21 04) Enciphered message is 44 57 21 16 Eve intercepts it, rearranges blocks Now enciphered message is 16 21 57 44 Bob gets enciphered message, deciphers it He sees EVIL
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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #11-8 Notes Digitally signing each block won’t stop this attack Two approaches: Cryptographically hash the entire message and sign it Place sequence numbers in each block of message, so recipient can tell intended order Then you sign each block
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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #11-9 Statistical Regularities If plaintext repeats, ciphertext may too Example using DES: input (in hex): 3231 3433 3635 3837 3231 3433 3635 3837 corresponding output (in hex): ef7c 4bb2 b4ce 6f3b ef7c 4bb2 b4ce 6f3b Fix: cascade blocks together (chaining) More details later
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June 1, 2004 Computer Security: Art and Science ©2002-2004 Matt Bishop Slide #11-10 What These Mean
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This note was uploaded on 05/04/2008 for the course CS 526 taught by Professor Wagstaff during the Fall '07 term at Purdue University-West Lafayette.

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11 - Chapter 11: Cipher Techniques Some Problems Types of...

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