Freenet

“…an adaptive peer-to-peer network application that permits the publication, replication, and retrieval of data while protecting the anonymity of both authors and readers.” Clarke, et.al.

Goals & Properties

• Anonymity for producers and consumers

• Deniability for storers

• Resistance to denial of service attacks

• Efficient storage and routing

• Decentralization

• Whole file is the unit of storage • No guarantee of permanent storage• Operates at application layer• Files named by location-independent keys• Transparent lazy replication

Goa

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Assigning a Key

• keyword-signed key : key is derived from a short descriptive string chosen by the user when the file is inserted into the system

• signed-subspace key : key is derived from the public key generated by the user; creates a personal namespace

• content-hash key : key is derived from hashing the file’s contents

Note: content-hash key and signed-subspace key can be used with an indirection mechanism to handle updatable files.

Keyword-signed key

key generationstring

private key

public key hash

file key

file

signature

encrypt

stored file

Note: retrieval requires only “string”

Problems resulting from “flat” name space:• Collisions (different users selecting same string for different files)• Key-squatting (junk files in popularly named positions)

Signed-subspace key

string

public key hash

file key

encrypt

stored file

private key

file

signature

hash

hash

xor

Notes: •retrieval requires (string, public key)•directory simulation through a file containing descriptive strings for other files

Content-hash key

random key

file key

encrypt

stored file

file

hash

Note: retrieval requires (file key, random key)

Updateable Files

Notes:• A combination of content-hash keys and signed-subspace keys• To update:

1. Insert new file using CHK method using new (file key, random key)2. Insert new indirect file using original (public key, string)

• Key collisions on indirect file causes older version to be discarded• Old version of file still available using its (file key, random key) index• Technique can also be used to split large files into parts

file

(1) insert using CHK

(file key, random key)

indirect file

(2) insert using SSK

(3) publish (public key, string)

Message Structure

•Incremented at each hop•Used to set hops-to-live in response•Initialized to small random value•When =1, not automatically incremented with probability p

transaction ID hops-to-live depth payload

•Set by sender to limit propogation•Decremented on each hop•When = 1, forwarded again with probability p

•64 bit randomly generated•Used to prevent routing cycles request/reply

Retrieval (key, hops-to-live)

local node

localstore

(1) request

(2) check local store

(3) send request using routing table

(5) cache

(4) file & source

(6) Update route table

(7) deliver file

Notes:

• Request contains (key, hops-to-live)• Any node on reply path can change source to

be itself or any other node• File cached at all nodes along return path

1. Improved subsequent access2. Redundancy improve fault tolerance

key1 node1

key2 node2

… …

Notes:• Each node maintains routing table• Route request to node which has the closest lexicographic matching key• Use depth-first-search with backtracing• Table entries added as new files acquired by the node

routing table

Routing

Insertion

local node

localstore

(1) insert

(2) check local store

(3) send insert using routing table

(5a) if file, cache

(4) file or OK

(5a) if file, update route table

file key

(5b) If OK, send file

Notes:• Inserted file stored at all nodes along search path• Any node on path can change the source to itself or any other node• Inserts announce the existence of the node• Attempts to insert junk files are rejected and the real file is further propagated

Scalability

Fault Tolerance

Free Haven

“…the Free Haven Project aims to design, implement, and deploy a

functioning distributed anonymous storage service.” Dingledine et.al.

Forms of Anonymity

• Anonymity of agents (authors, publishers, readers, and servers) – no link between the agent and a given document

• Document anonymity – servers do not know what documents they store

• Query anonymity – servers do not know the identity of documents which satisfies a users request

Structure

• Documents (file)– Each document is divided into shares– Each document is assigned an expiration date

• Servers– Community of servers – servnet– Each server has a persistent identification – pseudonym– Each server exposes a public key and a (set of) remailer reply

blocks– Each server has a database of the public keys and the remailer

reply blocks of all other servers– Servers form contracts to store shares for a specified interval of

time– Fulfilling a contract increases that server’s reputation

Insertion

<share><PKdoc> ce41f889d7569704e89edbdddf243662d8c784</PKdoc><sharenum> 1 </sharenum><totalshares> 100 </totalshares><sufficientshares> 60 </sufficientshares><expiration> 2000-06-11-22:25:24</expiration><data> … </data><signature> …digital signature of above… </signature></share>

• A file F is broken into shares f1, …, fn where k (<n) shares are needed to reconstruct the file• A key pair (PKdoc, SKdoc) is generated for F• Each share is signed• All shares of a given file are indexed by Hash(PKdoc)

Possible share representation:

Retrieval

• Reader– Generates (PKclient, SKclient) key pair for the transaction– Generates a one-time remailer reply block (rrb)– Broadcast request (H(PKdoc), PKclient, rrb) to all servers

• Server– Checks for availability of any shares with index of

H(PKdoc)– Encrypts each found share with PKclient

– Sends each encrypted share using rrb

Remailer Reply Blocks

Goal: a sender is provided by a receiver (nym) with the means to transmit a message to that receiver such that the sender has no knowledge of the actual receiver

rem@isp usr@a rem@b server

Constructing a reply block

Anon-To: usr@aEncrypt-Key: key1

Encrypt with public key of rem@b Anon-To: rem@b

Encrypt-Key: key2

replyblock-1

Anon-To: rem@ispEncrypt-Key: key3

replyblock-2

Encrypt with public key of rem@isp

Encryptions in transit

message

cyphertext-A

sign; encrypt with nym public key

encrypt with key3

cyphertext-B

cyphertext-A

encrypt with key2

cyphertext-C

cyphertext-B

encrypt with key1

Flow of data

replyblock-2

cyphertext-A

server usr@arem@isp rem@b

cyphertext-Creplyblock-1

cyphertext-B