Anonymity and Peer to Peer

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Anonymity and Peer to Peer
PEPITO Workshop Stockholm

• Andrei Serjantov
• University of Cambridge
• Computer Laboratory

July 2, 2002
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Outline

• Can P2P be used to build privacyrelated
  applications?
• Anonymity and P2P
• Traditional Anonymity
• P2P Anonymity
• Weak
• Combining the two is hard
• Censorship Resistance and P2P
• Requires anonymity!

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Traditional Anonymity ---Mix Systems

• A few (trusted) machines, mixes
• Mixmaster
• Email
• Onion Routing
• A variety of protocols
• Not running (yet)
• Resistant to a global passive adversary
• Attacker who can see the whole network

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A (Threshold) Mix
N 4
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How Mixes Work
R - Receiver A - Mix B - Mix
Sender
Receiver
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Mix Network Diagrams
Q
A
R
B
C
S
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Anonymity in P2P

• P2P means anonymity!
• Anonymity by passing messages on
• Not secure against the global passive attacker
• Crowds
• Freenet
• etc
• Too weak for us
• Can we use P2P platforms to build stronger
  anonymity systems?

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DHTs in one slide
A

• Nodes organized into a logical ring
• Each node has a node id
• Know about their neighbours
• And their non-neighbours
• For routing

B

• All thats required to send a message is the node
  id
• If node is down, routes to one of the nearest
  nodes

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P2P Mixes?

• More mixes g Less traffic per mix
• Hosts go up and down
• Unreliable mixes
• Need to learn about the available mixes just
  before sending the message
• Makes a number of strong statistical attacks
  possible
• Tarzan (M. Freedman, MIT)
• Deployment even harder

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Other Privacy Technologies?

• File Sharing!
• Get Crowds anonymity for free
• Steganographic Storage (see IPTPS02)
• Censorship Resistant Systems?
• Have to think about security properties of
  underlying P2P systems
• Other systems???

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Anonymity and Censorship Resistance

• Designing systems which make content hard to
  remove for someone who is more powerful than the
  publisher
• Censorship resistance and anonymity go together
• Publishing
• Retrieval
• Storage
• Censorship resistance is more than just anonymity

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Censorship Resistance

• Availability of documents
• (we want needles, not hay)
• Fault tolerance (attacker taking down servers)
• Denial of service attacks
• Ease of use
• But not
• Searching
• Efficiency
• Can take minutes or hours to retrieve a document

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Assumptions

• Anonymous connection system
• Onion Routing
• P2P layer
• PAST
• Chord
• Anonymous broadcast channel
• Anonymous newsgroup

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Publishing

• Documents
• Split up into shares (using standard algorithms)
• N shares, but need any k to reconstruct document
• Stored in the system (omitted)
• Address of the document
• Gets broadcasted in an anonymous newsgroup

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Architecture

• P2P network, each node takes on the following
  roles
• Storers (storing encrypted shares of a document)
• Chosen randomly
• Forwarders (publicly visible)
• Chosen randomly
• Retrievers (want a document)
• Decrypters (decrypts shares)

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Architecture
Storer
Forwarder
Decrypter
Retriever does not communicate to the storer
directly
Retriever
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Properties

• Storers
• Do not know what they are storing and cannot find
  out (Active Document Anonymity)
• Shares are stored encrypted
• Are resistant to Rubber-Hose Cryptoanalysis
• Forwarders
• Deny forwarding requests for parts of documents
• Forward requests via anonymous pointers
• Retrievers, Publishers
• Deny (almost) everything!
• (Unless caught red-handed with document)

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Architecture
Anonymous Communication via Onions
Storer
Encrypted Share
Request AA
key
Forwarder
Decrypter
Anonymous Address (AA)
Share
Request
Retriever
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How Does P2P Help?

• Built on top of a P2P-like infrastructure
• PAST
• Chord
• Replication of resources
• Discovery
• Routing
• Makes it easier to build and engineer
• Introduces subtle bugs

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Related Work

• Publius
• Few globally known servers
• Free Haven
• Moving document shares around frequently
• Tangler
• Entangles documents together so you cannot censor
  any one

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Future Work

• Prove anonymity properties informally stated here
• Build a prototype
• Address many outstanding issues
• Flooding attack
• Accountability

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Conclusions

• It is unclear how to build P2P anonymity systems
• There are techniques to make censorship resistant
  systems more anonymous
• Separation of duties
• Reply Onions (anonymous pointers)
• P2P helps
• Engineering
• Building