Protecting Free Expression Online with Freenet Presented by Ho Tsz Kin I. Clarke, T. W. Hong, S. G. Miller, O. Sandberg, and B. Wiley 14/08/2003

Agenda What is Freenet? Freenet Architecture GUID key Routing Network Evolution Managing Storage Performance Analysis Conclusion and Discussion

What is Freenet? Peer-to-peer file storage application Allow publication, and retrieval of information without censorship Each node Contribute storage Cooperate in routing Computer Node Routing Traffic Storing Files

What is Freenet? Five design goals: Anonymity for both readers and authors Deniability for storers Resistance to attempts by third parties to deny access Efficient dynamic storage and routing Decentralization Emphasize on privacy, availability, true freedom of speech Respond adaptively to usage patterns No guarantee on permanent file storage

GUID Keys Each file is assigned with a location- independent globally unique identifier (GUID), i.e. file key Content-hash keys (CHK) Analogous to inodes File key generated by hashing the content Files are encrypted by a randomly-generated key Required in retrieving

GUID Keys Signed-subspace keys (SSK) Analogous to filenames A public-private key pair is randomly-generated A short descriptive string, e.g. “ mcl/research/paper ” Sets up a personal namespace Sign the file to provide integrity check Required in retrieving

Routing Messages travel via node-to-node paths, NOT directly from sender to recipient Each node Knows only about its immediate neighbors Maintains a routing table that lists the addresses of other nodes and the GUID it think they hold Node e ’ s routing table

Retrieval Routing Depth-first Search with backtracking Forward requests according to routing table Request message contains File key Time-to-live (TTL) limit When receive request Check itself Otherwise, forwards to the node with the closest key Expand c first

Retrieval Routing If fail, try using next-closest key If reach a node that is already in the path, bounced back Until TTL expires, or find the file Expand e then Request Bounced back Find the file

Retrieval Routing If success, file together with a note specifying the holder will be passed back To conceal data holder Any node in the reply path can change holder to itself or any arbitrary node Requests will still locate the data, as this node retains the true holder ’ s identity Update routing table File might be cached at all nodes along the reply path Improved availability, fault-tolerance

Insertion Routing Inserting File Route similarly with requesting file Forward message according to the closeness in key if no key collision Fail if key collision occurs Success if TTL expires without collision Insert file along the path Update routing table Any Node along the path can change holder to itself or any arbitrary node

Anonymity in Routing Requesting file Don ’ t know who are requesting Send the file, holder is x Update routing table, but the true holder may or may not be x Inserting file, holder (inserter) is x Store the file, update routing table, but the true holder may or may not be x Retrieval: Insertion:

Training routes Nodes ’ routing tables should specialize in handling clusters of similar keys Node should specialize in storing clusters of files with similar keys The twin effects of clustering should improve the effectiveness of future queries Key clustering GUID keys are derived from hashes Closeness of keys in storing is unrelated to corresponding files ’ content

Adding Nodes New node first generates a public-private key pair Identify the node Use for future trust mechanism Sends an announcement message including the public key and physical address to an existing node, located through some out-of-band means New node Chosen randomly Propagate until TTL expires Assign random GUID in the key-space using shared random number generation

Managing Storage In each node Storage is managed by LRU (Least Recently Used) Cache Frequency of requests per file Routing mechanism Creates more copies in an area of network where a file is requested Files that go un-requested in another part of the network will be deleted Number and location adjust to demand

Degree Distribution Simulation of 10,000 nodes  Degree distribution among Freenet nodes  Close fit to a power-law distribution Max routing table size

Fault-tolerance 10,000 nodes trained network  Remove nodes randomly  TTL = 500  10 trials Median path length below 20 even 30% of nodes fail Robust against quite large failures Power-law gives high degree of fault tolerance

Fault-tolerance 10,000 nodes trained network  Remove randomly nodes  Remove well-connected nodes first Transition to disconnected fragments

Scalability 20 nodes initially connected in ring topology  Insert & request files to random nodes (TTL = 20)  Create a new node after every 5 inserts and requests, announce randomly to a existing node  Measure after every 100 inserts and requests  End till 200,000 nodes  10 trials Median path length in network scales as N 0.28

Conclusion Freenet is P2P application that designed from a different perspective – free flow of information Using Freenet Can ’ t identify who is requesting a document Can ’ t identify who is inserting a document Can ’ t identify where a document resides

Discussion Replicated File is stored as a whole Fragmentation and erasure correction coding No in-build searching function How to search content in Freenet Cost is high, as message need to travel along the whole path How to provide anonymity efficiently Pornographic/offensive content, terrorism humanity should not be deprived of their freedom to communicate Use for good or bad

References I. Clarke, T. W. Hong, S. G. Miller, O. Sandberg, and B. Wiley, “ Protecting Freedom of Information Online with Freenet, ” IEEE Internet Computing, vol6(1), Jan-Feb, 2002, pp I. Clarke, T. W. Hong, O. Sandberg, and B. Wiley, “ Freenet: A distributed anonymous information storage and retrieval system, ” Proc. of the ICSI Workshop on Design Issues in Anonymity and Unobservability, Berkeley, CA,