Freenet

1. Freenet By Yogesh Kalyani

2. OUTLINE • Introducing P2P • Overview of Freenet • Key’s • Protocol • Security Analysis • Improvement • Strength and Weakness • Conclusion

3. What is Peer-to-Peer (P2P)? • “…an entity with capabilities similar to other entities in the system.” • Every end host known as peers, has equal roles and capabilities • They can act as both client and server, and provide for direct communication between them

4. Client Server Model Vs P2P Model Client Server Model: • Contact a server and download a web page • Server has all the resources and capabilities P2P Model: • Direct communication between the peers • Peer acts as both client and server

5. Freenet • What is Freenet ? • A decentralized distributed file storage system • How does it work? • Files stored and replicated across a distributed network environment, with a peer-to-peer query and data access system. • No centralized system management

6. Motivation • What does it provide ? • Anonymity for both producers and consumers of information • Deniability for storers of information • Efficient dynamic storage and routing of information • Decentralization of all network functions

7. Key’s • Files in Freenet are identified by binary file keys, which are obtained by applying a hash function • Freenet use SHA-1 function for hashing • Freenet Uses three types of file keys 1) Keyword-signed key (KSK) 2) Signed-subspace key (SSK) 3) Content Hash Key (CHK)

8. Structure of Keys • Each Freenet key has the following structure • “freenet:” is the standard prefix • First three chars state key type: KSK, SSK, CHK • “@” symbol separates the key type from the rest of the message • Then a long set of characters used to identify the file Example:freenet:KSK@papers/p2p/freenet/keys

9. Keyword-signed key (KSK) • Easiest to use of all the key types • KSK is derived from a short descriptive text string Ex: text/philosophy/sun-tzu/art-of-war • This string is then used as input to generate a public/private key pair Public key is then hashed to yield the file key Private key is used to sign the file

10. Keyword-signed key • Advantages 1) Only the file description needs to be published 2) Easy to remember • Disadvantages 1)Global Namespace 2) Nothing prevents two users from independently choosing the same descriptive string for different files

11. Keyword-signed key • Disadvantages(2) 3) Users can abuse the names of popular files by inserting their file with the same name 4) Users can abuse the names of popular files by inserting their file with the same name • Possible because the file description is published 5) Attacker can use dictionary attack against this sign by compiling the list of descriptive string

12. Signed-subspace key (SSK) • Addresses the problems suffered by KSK by allowing user to make personal name space • User creates a namespace by randomly generating a public/private key pair, • Which will used to identify his namespace • Private Key • Only the person who posses the private key can insert files to the namespace in the network • Allows others to ensure a file was posted by a certain person • Public Key • Allows users to retrieve the file from the network (with Descriptive String)

13. Signed-subspace key • To get the key for the subspace: • First he chooses a short descriptive text string like text/philosophy/art-of-war. 1) Public key is hashed 2) The descriptive string is hashed 3) (1) XOR (2) 4) (3) is hashed 5) (4) is encrypted using the file description

14. Content Hash Key (CHK) • A content-hash key is derived by directly hashing the contents of the corresponding file. • It gives every file a pseudo-unique file key • Files are also encrypted by a randomly-generated encryption key. • Content-hash keys are most useful in conjunction with signed-subspace keys using an indirection mechanism • For Retrieval user has to publishes the content-hash key with the decryption key

15. Clustering of Keys • When a node successfully receives a file from another node. It associates that node in its routing table with the hash key of the file • All future requests from this node will send the request to the node listed in the routing table associated with the key closest to the key of the file being requested

16. Freenet: Routing Request • Hash key for data (160-bit SHA-1) • Find node with closest match • Forward query to this node by specifying key and #HTL • Return data, replicating along the way

17. Freenet: Routing, Request

18. Freenet: Routing Insert: • Hash key for data (160-bit SHA-1) • Initiate a query with key, #HTL • Node will check into its data store whether they have the data • If then will response with the preexisting file assuming request has been made • Else will forward the query to next node

19. Freenet: Routing, Insert • Process goes on till #HTL Expire • If no collision i.e. or request is successful return “AllClear” Message • Push data onto all the nodes (#HTL)

20. Freenet: Routing, Insert • Quality of routing improves over time • Each time a requestor successfully receives a file from another node • It adds that node to it’s routing table • Associates the file key with that node in the routing table • All future requests for files with similar keys will be sent to nodes associated with these keys

21. Freenet: Routing, Insert • Eventually a node will become specialize in serving the request for a families of keys which are close together and will store more files Reasons: • Other node will send request for files that have similar keys • If it does not have the file it forwards the request to another node based on it’s routing table • When the file is located, the response gets passed back • Each node on the responses path gets a copy of the file stored • This includes the node the request was initially sent to • Over time this node will start to store more and more files with this key type

22. Freenet: Routing Managing Data • Node storage is managed as an LRU (Least Recently Used) cache in which data items are kept sorted in decreasing order by time of most recent request or time of insert, if an item has never been requested

23. Freenet: Routing, Managing Data • Each node has limited storage capacity • The user has no knowledge of what files their node stores • As well No user knows the identity of a node that provides a file they have requested • Or knows the identity of a node that has requested a file from them

24. Freenet: Routing, Managing Data • NO file lifetime guarantees • Popular files will spread to many nodes • Each requested file located, will be copied to every node it passes through on the path from the source node to the requestor node • Rarely accessed files will slowly be removed from the network as room is required for new files i.e. removing least-recently used (LRU) files • As a node runs out of space, files will be deleted in order of least recently requested to make room • Rarely requested files willONLYbe removed if space becomes limited

25. Protocol: Protocol Descriptors • Request.Handshake • Reply.Handshake • Initial Contact • Request.Data • Send.data • Querying for data

26. Protocol: Protocol Descriptors • Reply.NotFound • Reply.Restart • Failure Message • Request.Continue • Request.Insert • Request Management • Reply.Insert • Send.Insert • Inserting Data

27. Security Analysis • Eavesdropping • No protection against eavesdropping between the user and the first node contacted in basic Freenet • Users are recommended to first connect to a node running on their own machine in basic Freenet • Pre-Routing to protect from eavesdropping in new version of freenet

28. Security Analysis: Eavesdropping • As routing depends on knowledge of the search key • Key anonymity is not possible. • Use of hashes as keys provides some obscurity against casual eavesdropping • But still vulnerable to dictionary attacks • Can be easily done by setting up a node and "eavesdropping" any messages that are routed through her

29. Denial of Service Attacks • An attacker can insert a large number of junk files into the network • Counter attack: • By dividing the datastore into two sections, one for new inserts and one for established files • New inserts would only replace new inserts; therefore a flood of them could not displace existing files

30. Denial of Service Attacks Problem with dividing Datastore: • Difficult for genuine new inserts to survive long enough to be requested by others and become established • An attacker may be able to legitimize his bogus files, by requesting them from strategic locations where it will be cached on as many nodes as possible

31. Traffic Analysis • Traffic analysis is possible by setting up nodes in strategic locations and by monitoring encrypted traffic between nodes • Achieved by observing Search Key Closeness, Hope-to-live, Depth. • Simplest case, if a node receives a request for a search key that it has never served anything close to, he can guess that the request originated from a directly connected node or that he is very early in the chain. 

32. Improvement Original Algorithm in Freenet: • The assessment of whether a peer X has common interests with a peer Y is based on historical data, i.e., on whether peer X was able to respond to previous queries sent by peer Y

33. Improvement *New algorithm[3] • Principle: The "current" content stored at a peer reflects its current interest. Thus, common interest between two peers is assessed directly from the current content locally stored at both peers and is periodically re-evaluated • In this way each peer maintains a community of peers, which share similar interests i.e. number of files • study shows: It reduces the average Freenet request and response path lengths by up to 39%

34. Freenet: Design strength • Anonymity for both producers and consumers of information • Completely Decentralized: No single point of failure • Scales well • Dynamic routing adapts well to changing network topology • High resilience to attacks

35. Freenet: Design weakness • No file lifetime guarantees • Not an efficient keyword search • Currently, no defense against DoS attacks

36. Conclusion • Freenet is a loosely unstructured decentralized network, in which every node has equal roles and do the same task thereby increasing the fault tolerance • Freenet uses depth first search with backtracking which makes it scalable when the size of network is increased or network is overloaded • Overall, Freenet is good P2P network for file sharing system; deploying efficient routing for the location and the storage of the data

37. References • Freenet, http://freenet.sourceforge.net. • Freenet documentation, http://freenet.sourceforge.net/doc/book.html. • Marcelo Werneck Barbosa, “Using locality of reference to improve performance of peer-to-peer applications” Proceedings of the fourth international workshop on Software and performance, ACM Press, Year of Publication: 2004, Pages: 216 - 227 • Clarke, O. Sandberg, B. Wiley, and T. W.Hong, “Freenet: A distributed anonymous information storage and retrieval system”, in ICSI Workshop of Design Issues in Anonymity and Unobservability, July 2000. • SM Lui, SH Kwok, “Interoperability of Peer-To-Peer File Sharing Protocols” ACM SIGecom Exchanges, Volume 3, Issue 3   Summer, 2002, ACM Press, Year of Publication: 2002 Pages: 25 - 33

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