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Mixing

links: AC2 TOC - Anonymity - Index

Definition

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Types of mixing

Threshold Mix

A type of mix that waits until it has received messages from a certain number of users (the threshold) before it sends them out in a random order.

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Timed Mix

This mix collects messages for a certain period of time before sending them out in a random order.

  • Advantages: performance guaranty
  • Disadvantages: if only one input \(\rightarrow\) same output, n-1 attack (send all inputs -1)

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Pool Mix

In a pool mix, incoming messages are added to a pool. A certain number of messages are randomly selected from the pool and sent out, while the remaining messages stay in the pool for the next round.

  • Advantages: \(P_{min}\) & \(P_{max}\), n-1 attack not possible
  • Disadvantages: a message may stay forever in the pool

pool-mix.png

Mixminion

  • based on mixmailers: uses mix networks for email communication, effectively anonymizing the sender and recipient of an email (only for email application)
  • possibility to reply: without knowing the sender's actual identity
  • directory servers to evaluate participating remailers: helps users to select reliable and trustworthy nodes for routing their message
  • exit policies: like with Tor, nodes can have policies which allow the operator to determine what type of traffic is allowed to exit their node and to where (legal and ethical considerations)

Key ideas

  • when a message traverses mixminion, each node must decrypt the message using its (ephemeral) private key
  • the key idea behind the replies is splitting the path into two legs:
    • the first half is chosen by the responder to hide the responder identity
    • the second half was communicated by the receiver to hide the receiver identity
    • a crossover-node in the middle is used to switch the headers specifying the path
Alice -------------------------------------------> Bob
Alice <-- second leg -- Crossover <-- first leg -- Bob

Replay attacks

  • servers keep hash of previously processed messages until the server key is rotated
  • bounded amount of state in the server, no possibility for replay attack due to key rotation

Directory Servers

  • inform users about servers
  • probe servers for reliability
  • allow partitioning attack \(\rightarrow\) users are advised to query all directory servers to obtain a complete network view

Nymservers

  • Nyms (short for pseudonyms) allow users to maintain long-term identities without revealing their true identities or their locations
  • A nym server stores nyms and the associated reply blocks
  • Vulnerable to DoS attacks

Problems

  • No benefits / incentives of running a mixer
  • A lot of public key cryptography (slow)
  • Users have to trust directory servers (those also don't have an incentive to act good)
  • Servers have to keep state (resources)
  • Service is limited to E-Mail because of high latency
  • Exit nodes can be liable for legal action
  • No functionality to avoid DoS attacks
  • Statistical correlation of entities communicating over time is still possible if the whole network is observed
  • Bridge between anonymous network and a traditional protocol is difficult

links: AC2 TOC - Anonymity - Index