CMSC 414 Computer and Network Security Lecture 20

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CMSC 414Computer and Network SecurityLecture 20

• Jonathan Katz

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Administrative items

• Midterm next week
• Based on everything from last midterm through
  today

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Zero knowledge (ZK)

• ZK proofs can offer deniability and secrecy
• A zero-knowledge protocol for graph
  3-colorability
• Warning the aim here is simplicity, and many
  subtleties and details are purposely being ignored

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Applications of zero-knowledge

• (Deniable) authentication
• Generalization of the protocol we saw last time
• Again, many subtleties and details omitted!
• Anonymous credentials
• Group signatures/trusted computing

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Anonymity vs. pseudonymity

• Anonymity
• No one can identify the source of any messages
• Unlinkability cannot even tell that messages
  originated from the same person
• Pseudonymity
• No one can identify the source of a set of
  messages
• but they can tell that they all came from the
  same person, with a known pseudonym
• There is a broad scale of achievable anonymity
• Best you can hope for is limited by the network
  size!

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Traffic analysis

• May be possible to learn who is communicating
  with whom using traffic analysis
• Typically, even if communication is encrypted the
  headers are not
• Need unencrypted headers for routing
• How is it possible to communicate anonymously?

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Anonymous communication

• You are sitting around a table with n people
• How do you send an anonymous message to another
  person?
• How do you broadcast a message to everyone
  without revealing your identity?
• Linear-round protocol?
• Constant-round protocol (DC-nets)
• Is this secure only for honest-but-curious
  behavior, or also for malicious behavior?

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Anonymizers

• Single anonymizer proxy
• How to achieve bidirectional communication
• Note one side need not know the other
• Anonymizers already exist!
• Email
• http

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Anonymizers

• Issues/drawbacks?
• Robustness
• Useful for hiding the source from the
  destination less useful for preventing
  full-fledged traffic analysis
• Unless encryption is used, which it typically
  would not be
• Possible attacks
• Latency vs. timing correlation
• 0-latency solution using spurious messages?
• One user sending multiple messages to the same
  server
• Message sizes
• Replay attacks