freenet

Company

LOGO

Freenet

By Yogesh Kalyani

OUTLINE

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

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

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

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

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

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)

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

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

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

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

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)

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

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

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

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

Freenet: Routing, Request

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

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)


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


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

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

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

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 will ONLY be removed if space becomes limited

Protocol: Protocol Descriptors

Request.Handshake Reply.Handshake

Initial Contact

Request.Data Send.data

Querying for data

Protocol: Protocol Descriptors

Reply.NotFound Reply.Restart

Failure Message Request.Continue Request.Insert

Request Management Reply.Insert Send.Insert

Inserting Data

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

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

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

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

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.

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

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%

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

Freenet: Design weakness

No file lifetime guarantees

Not an efficient keyword search

Currently, no defense against DoS attacks

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

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

Thank you for your patience

freenet

Documents