Transcript Freenet: A Distributed Anonymous Information Storage and

FREENET
UDAYA S PISIPATI
02/26/08
CONTENTS
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Introduction
Design goals
Architecture
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Protocol details
Performance Analysis
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Basic model
Keys in searching
Retrieving data
Storing data
Managing data
Adding nodes
Small world model
Security
Conclusion
INTRODUCTION
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Drawbacks of current networked systems:
Privacy
Availability
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Systems offering greater security and reliability are needed
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Freenet -distributed information storage and retrieval system designed to
address these concerns
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Preliminary implementation can be downloaded from
http://www.freenetproject.org/
DESIGN GOALS
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Anonymity for both producers and consumers of information
Deniability for stores of information
Resistance to attempts by third parties to deny access to
information
Efficient dynamic storage and routing of information
Decentralization of all network functions
ARCHITECTURE
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Data files are named by location independent keys
Nodes in the network query one another to store and retrieve these
files
Each node maintains its own local datastore, available to the network
for reading and writing
Dynamic routing table contains addresses of other nodes and the keys
that they are thought to hold
Users are provided hard drive extensions
Requests for keys are passed from node to node
Local decisions are made by each node
Nodes only have knowledge of their immediate upstream and
downstream neighbors in the chain
Hops-to-live limit and pseudo-unique random identifiers are assigned
to each node
No node is privileged over any other node
KEY BASED SEARCHING
•Keyword signed key(KSK)
•Easy for retrieval – only need ‘Desc’
•Minimal protection against tampering- Dictionary attacks, same
Descriptive string for two files
Desc
FILE
‘Desc’– key generation Pb + Pr ; SHA(Pb)
+ Pr
E(FILE, Desc)
Encrypted FILE
Signature
KSK
KEYS AND SEARCHING…..
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Problems with KSK – flat namespace (collisions), key squatting, dictionary
attacks
Signed Subspace Key (SSK)
 Randomly generated key pair  namespace ID
 SSK = SHA( SHA(‘D’) XOR SHA(Pb) )
 Pr used to sign the file, file encrypted by using D
 (-)Publish – subspace Pb + ‘D’
 (+)Owner can construct hierarchical space of arbitrary depth - using
indirect files
KEYS AND SEARCHING…
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Content Hash Keys (CHK) = SHA (file contents)
 Files encrypted by a random encryption key
 Publish CHK + decryption key
 CHK + SSK  easily updateable files
 2 step process – publish file, publish pointer
 Results in pointers to newer version
 Older versions accessed thru CHK
 Can be used for splitting files
KEYS AND SEARCHING: INSERT, UPDATE AND SPLIT
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Insert a file under its CHK
An indirect file (whose contents are the CHK) is inserted under the SSK
Retrieval possible in two steps when SSK is known
For updating the file, insert a new version under its CHK (different from the
CHK of the old version)
Insert a new indirect file under the original SSK pointing to the updated
version
Key collision occurs when the insert reaches the node which possesses the
old version
Check the signature on the new version and replace the old version if the
signature is valid and most recent
Split the file into a number of parts and insert each part under a CHK, also
create an indirect file to point to the individual parts
RETRIEVING DATA
STORING DATA
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User calculates the binary file key for the file and sends a message to the
his/her own node specifying the proposed key and a hops to live value
The node checks to see if the key is already taken, if the key is found, the
node returns the file like the result of a search, the user now chooses a
different key and repeats the procedure
If key is not found, the user node looks up the nearest key in its routing
table and forwards the insert to the corresponding node
If the hops-to-live limit has been reached without a key collision being
detected, an “all clear” result will be propagated to the original inserter
MANAGING DATA
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Node storage is managed as an LRU cache
When a new file arrives which would cause the datastore to exceed the
designated size, LRU files are evicted in order until there is room
Advantage: This method allows outdated documents to fade away naturally
ADDING NODES
PROTOCOL DETAILS
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Packet oriented protocol
Request.Handshake
Reply.Handshake
Request.Data
Reply.Restart
Send.Data
Reply.NotFound
Request.Insert
RING TOPOLOGY
•1000 nodes in ring topology
•Datastore = 50 items
•RT = 250 items
•Keys associated with links are hash of
destn IPs
PERFORMANCE ANALYSIS: SMALL WORLD MODEL
Identification of a small-world
network: Existence of a scale-free
power-law distribution of links within
the network
The tail of the distribution provides
the highly connected nodes needed to
create short paths
Result: The distribution closely
approximates a power law except for
the anomalous point representing
nodes with filled 250-entry routing
tables
Loss of poorly connected nodes will
not greatly affect routing in the
network
SECURITY: ANONYMITY PROPERTIES OF FREENET
System
Attacker
Sender
Anonymity
Key anonymity
Basic Freenet
Local
Eavesdropper
Exposed
Exposed
Basic Freenet
Collaborating
nodes
Beyond suspicion Exposed
Freenet +
Pre-routing
Local
Eavesdropper
Exposed
Collaborating
nodes
Beyond suspicion Exposed
Beyond suspicion
CONCLUSION
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Freenet provides an effective means of anonymous information storage
and retrieval
It keeps information anonymous and available while remaining highly
scalable
WIP: Implementing of a simulation and visualization suite which will enable
more rigorous tests of the protocol and routing algorithm
WIP: Implementation of a public-key infrastructure to authenticate nodes
and create a searching mechanism
REFERENCES
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"Freenet: A Distributed Anonymous Information Storage and Retrieval
System”
http://www.doc.ic.ac.uk/~twh1/academic/papers/icsi-revised.pdf