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Class 13 Introduction to Anonymity CIS 755: Advanced Computer Security Spring 2014 Eugene Vasserman http://www.cis.ksu.edu/~eyv/CIS755_S14/
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Administrative stuff Quizzes and Exam II – Scheduling – Format
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Outline Anonymity concepts and background The Dining Cryptographers problem Anonymous email – Chaum mixes – Mixminion Anonymous web browsing – Tor Problems with Tor
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Anonymity Concepts Privacy – Confidentiality Anonymity/Pseudonymity – Unobservability – Unlinkability
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Dining Cryptographers Three people toss coins: heads=1, tails=0 Menus hide right-hand coin XOR your coin flip result and left neighbor’s result Report value to everyone Report opposite value to send a single bit If the sum is odd, someone sent a message
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Dining Cryptographers II Slow Error-prone Needs tamper detection Does not scale Provides unobservability
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Unobservability k-anonymity (scalable dining cryptographers) – Must be implemented very carefully Link padding – Inefficient – Cover traffic knowledge
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Unlinkability Sender can’t identify receiver Receiver can’t identify sender Neither knows who the other is – How do we handle authentication? Unobservability implies unlinkability (?)
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For Bob from Alice For Carol from Alice For David from Alice Onion Encryption
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Message for Bob Wrapping for Carol Wrapping for Doug Onion Encryption II Bob Alice Wrapping for Edward Edward Doug Carol
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Chaum Mixes Bob Alice Output in lexographic order
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Global Adversary Bob Alice
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Chaum Mix Cascade Bob Alice
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Anonymous Reply Address for replies: Reply: Mix0 decrypts N,A; sends: Mix decrypting reply does not know destination Mix encrypting reply does not know source
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Mixminion AB C D E Bob A,B,C,D,E Alice Bob
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Problems with MixMinon Centralized entities required – Availability failure – Anonymity failure (how?) Malicious nodes: – Control entry and exit – Unlikely
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Anonymous Email High-latency Low-throughput Provides unlinkability – Have to be careful about authentication No default end-to-end confidentiality (PGP) – Actually, there is for replies Secure against global adversary
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Anonymous Web Browsing Low-latency Medium-throughput Server does not know client Provides sender unlinkability – Have to be careful about authentication No default end-to-end confidentiality (SSL) NOT secure against global adversary
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Tor ABC TCP over TCP (UGH!)
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Problems Tor Does NOT Address Global adversary – Functional vs. actual? Packet counting Packet sampling
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Anonymous Web Services Web service does not know client Client does not know web service Provides sender and receiver unlinkability Rendezvous
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Tor Hidden Services ABCDEF
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Questions? Reading discussion
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