Puzzles in P2P Systems

1
Puzzles in P2P Systems

• Andrei Serjantov and Stephen Lewis
• University of Cambridge Computer Laboratory

2
Introduction

• Here we look at P2P stuff-sharing systems
• Kazaa, Gnutella, etc, etc, etc
• Bit Torrent
• Abstract from the politics
• RIAA
• ISPs

3
Motivation

• The Free Riding Problem
• Downloading without sharing
• The Slashdot Effect
• MojoNation
• Performing Community Tasks
• Quality Rating
• Indexing

4
Sketch Solution

• Protocol
• When user asks for content, hand them a puzzle
• They solve the puzzle, send you the solution
• You send them the content
• Both are happy
• What is the puzzle?
• Who generates it? How?
• Different answers help provide different
  properties
• Serious thought needed

5
An Example Protocol
B
Yep, Solve The Halting Problem!
Puzzle Server
LOTR?
Halting problem?
A
Here it is!
6
Solving the Halting Problem
A
Hmmmmmmm Hmmmmmmm Hmmmmmmm Hmmmmmmm Halts!
7
Exmaple Protocol (cont)
B
Halts!
LOTR III
A
8
Encouraging Content Sharing

• Assume a central puzzle repository
• Much more stuff than puzzles
• When a piece of stuff is requested, the to be
  solved puzzle is specified (as above)
• A user who shares lots of stuff has many puzzle
  solutions, hence does not have to solve often
  when downloading
• Periodically, a fraction of the puzzles in the
  repository is changed

9
What is a Puzzle?

• Computational puzzle
• Processor-based computation
• Hashcash
• Memory-bound
• Community puzzle
• Rating content quality
• Indexing

• Turing puzzle
• Humans can do and computers cannot
• See below

10
Computational Puzzles

• Processor puzzles
• Finding a hash function collision
• Easy to check
• Memory-bound computational puzzles
• Used for anti-spam schemes
• How fast you solve the puzzle depends on the
  amount of memory, not (so much) on processor speed

11
Community Puzzles

• Essentially as above, but the user has to perform
  some task which benefits the entire community at
  the cost of his convenience
• Quality rating
• Indexing
• Verifiable
• Majority voting will do
• Subset of Turing puzzles

12
Money Money Money

• Micropayments as puzzles ??
• Popularity tends to follow the power law
  distribution
• Hence, collecting a small payment from many will
  enable a good payoff to the popular few.
• (Do you care if your subscription charge is /- 1
  Euro?)
• Micropayments are pure speculation
• More likely to succeed in the context of a single
  system

13
Research

• Investigate security protocols with entirely new
  properties
• non-cheating
• equal benefit
• Rather than secrecy or authenticity
• Some properties will be ensured not by
  cryptographic means, but by rationality of the
  users
• Information Security Economics
• Which puzzle are best?

14
What I really do(!)

• Censorship Resistance
• Privacy
• Mostly politics(!)
• Information Security Economics

• Anonymity
• Web browsing
• Email
• P2P?
• P2P

See http//www.cl.cam.ac.uk/aas23/