A Reputation-Based Approach for Choosing Reliable Resources in

Peer-to-Peer Networks

E. Damiani S. De Capitani di Vimercati E. Damiani S. De Capitani di Vimercati

S. Paraboschi P. Samarati F. ViolanteS. Paraboschi P. Samarati F. Violante

http://seclab.dti.unimi.it/p2prep/

Outline

1. P2P Networks & Gnutella protocol [1]

2. XRep protocol [2]

3. Security considerations on XRep

4. Conclusions

5. References

P2P Networks

All the nodes offer the same services and follow the same behavior.

Nodes act both as servers and as clients. There are no nodes with a special responsibility to

monitor or supervise the network behavior.

P2P networks for file sharing involves two phases:1 Search of the peers where the requested file resides.2 Download from the exporting peers the requested information.

Gnutella Gnutella is a protocol for distributed search. Nodes are called servents.(server + client) Each servent has a servent_id Servents communicate by exchanging descriptors. Ping, Pong – are used to discover servents Query, QueryHit – Searching files in the P2P network Push – allows a firewalled servent to contribute files

to the P2P network

The Gnutella Protocol Specification v0.4 www9.limewire.com/developer/gnutella_protocol_0.4.pdf

Descriptor routing 1

A servent P requiring a resource broadcasts a Query out.

A servent S will respond with a QueryHit if a match is found against its local database. And S will forward incoming Query descriptors to all of its directly connected servents, except the one that delivered the incoming Query.

QueryHit descriptors may only be sent along the same path that carried the incoming Query descriptor.

Descriptor routing 2 A servent will decrement a descriptor header’s TTL A servent will decrement a descriptor header’s TTL

field, and increment its Hops field, before it forwards field, and increment its Hops field, before it forwards the descriptor to any directly connected servent.the descriptor to any directly connected servent.

If, after decrementing the header’s TTL field, the TTL If, after decrementing the header’s TTL field, the TTL field is found to be zero, the descriptor is not field is found to be zero, the descriptor is not forwarded along any connection.forwarded along any connection.

A servent receiving a descriptor with the same Payload Descriptor and Descriptor ID as one it has received before, should attempt to avoid forwarding the descriptor to any connected servent.

Gnutella

11

22

33

6655

77

88

1: initiator

2&7: responders

Query

QueryHit

Not a descriptor

44Match Match

A servent requiring a file broadcasts a Query out.

Servents will forward incoming Query descriptors to all of its directly connected servents, except the one that delivered the incoming Query.

A servent receiving a descriptor which has received before will not forward the descriptor

A servent will respond with a QueryHit if a match is found against its local database.

QueryHit descriptors may only be sent along the same path that carried the incoming Query descriptor

99

If the TTL field is found to If the TTL field is found to be zero, the descriptor is be zero, the descriptor is not forwarded along any not forwarded along any connection.connection.

Structure of Gnutella descriptor

Every descriptor has two parts:

1. Header

Descriptor ID Payload Descriptor TTL Hops Payload Length

Minimum Speed Search criteria

Number of Hits

Port IP Speed Result Set Trailer Servent IDFile

IndexFile Size

FileNamefile1

file2

file3

Query: sent by initiator

QueryHit: sent by responder

2. Payload

+

Motivation of Reputation systems Most P2P systems protect peers’ anonymity.

Anonymity opens the door to possible misuses and abuses. No way to verify the source or content of files -- Bad service, low quality files -- The content of a file is different than the title -- Trojan horses and viruses

e.g. Mandragore – a Gnutella worm -- Act as a servent and answer all Queries. -- Provides a renamed copy of itself for downloading.

Peer review process: the peers’ opinions is used to

establish a reputation for peers and files.

XRep: Basic Assumptions

Each servent maintains information on its own experience on files and other servents and share such experience with others upon request

Each servent has a servent_id which is a digest of a public key obtained using a secure hash function

Servent reputations are associated with the servent_id

Each file has a resource_id which is a digest computed by applying a secure hash function to the file content

File reputations are coupled to resource_id

Reputations Storage & votes calculation 1

Each servent maintains a resource_repository & a servent_repository that store its opinions about files and servents it had experiences

A servent votes on files and servents with which it had experiences.Votes are its opinion on files and servents

Votes are expressed on the basis of information available in the resource_repository & servent_repository.

resource_repository: set of pairs

(resource_id, value) value=0 or 1 servent_repository: set of triples

(servent_id, num_plus, num_minus)

num_plus, num_minus are positive integers VoteVote = 0 or 1 = 0 or 1 VoteVote of servent =1, if of servent =1, if num_plus>>num_minus VoteVote of file = of file = value

Reputations Storage & votes calculation 2

XRep: Polling Protocol

Phase 1: Resource searching.

p sends a Query message for searching files, and servents matching the request respond with a QueryHit

Query (Min_Speed, Search_criteria)

QueryHit(num_hit,IP,port,speed,Result_set,trailer,servent_id)

Trailer: resource_ids of files in result set

Initiator p Servent s

Phase 2: Vote polling P selects a file r that best seems to satisfy its request. Such

selection may be guided by the user’s preference p polls its peers about the reputation of a file and the set of

servents that offer it. Servents wishing to respond vote on the resource_id and

servent_ids and send back a PollReply

Initiator p Servent s

Poll (resource_id, {servent_id1… servent_idn}, PKpoll)

PollReply ({(IP,port,Votes)}PKpoll)

Phase 3: Vote evaluation & reliability check

1. p decrypts PollReply, discards tampered ones.

2. p clusters Voter’s IP and weight the votes within a cluster --Reducing the effect of a clique

3. p selects a set of voters in each cluster, contacts them directly, and expects back confirmation messages. If not enough responses, then p repeats step 3.

Initiator p Servent s

TrueVote ( Vote )

TrueVoteReply ( responses )

Phase 4: Best servent check p cannot always download file from best servent p contacts the best servent S to check the fact that

it exports file which p wants to download Preventing ID stealth attack.

Initiator p Servent s

AreYou (servent_ids, resource_id)

AreYouRepley({response}SKs, PKs)

Phase 5: Resource download p selects a servent s, downloads the file r

and checks it against its digest Update its resource_repository &

servent_repository

Initiator p Servent s

download ( r )

resource ( r )

Query (Min_Speed, Search_criteria)

QueryHit(num_hit,IP,port,speed,Result,trailer,servent_id)

Initiator p Servent s

Poll (resource_id, {servent_id1… servent_idn}, PKpoll)

PollRepley ({(IP,port,Votes)}PKpoll)

TrueVote ( Vote )

TrueVoteRepley ( responses )

AreYou (servent_ids, resource_id)

AreYouRepley({response}SKs, PKs)

download ( r )

resource ( r ){

{

{

{

{

Vote polling

Resource Searching

Vote evaluation

download

Best servent check

XRep: Security Considerations

XRep allows to protect P2P against following attacks Self replication Man in the middle ID Stealth Pseudospoofing Shilling

Self replication: A malicious servent could answer all Queries

and provide doctored content. Even honest peers, unaware of the malicious

content, could share it and contributing to its diffusion.

e.g. Mandragore – a Gnutella worm Bad reputations of file

-- Worms slightly modifying themselves Cannot collect positive recommendations Check reputation of the servent

Man in the middle: A broadcasts a Query. B responds a QueryHit. E replaces IP and Port of the QueryHit with E’s IP and

Port, sends it back to A. A may download from E. The fake content provided by E will not match the digest

of the legitimate file, then be discarded. (Phase 5)

A

BEQuery

Query

QueryHit

Modified QueryHit

Download

ID Stealth:

A malicious peer answers with two QueryHits, carrying the digest of a doctored file and one of them carrying the ID of a reputable servent

Xrep checks whether the best servent is offering that file (Phase 4).

Psedospoofing & Shilling: 1 Attackers create and control multiple servents.

They give positive votes to the attacker. Four cases:

Multiple servents have same IP address IP cluster (phase 3)

Servents have different but faked IP address TrueVote/TrueVoteRepley (phase 3)

These two cases are called Psedospoofing

Psedospoofing & Shilling: 2 Servents have different real IP address. And

those IP addresses have same net_id. IP cluster may reduce the effect (phase 3)

Servents have different real IP address. And those IP addresses have different net_id.

To ensure a high number of voters.

These two cases are call Shilling.

Distribution of Servent & Resource

An important aspect for the applicabilty of this approach

frequent files are more frequently searched

=> the number of votes will be high few servents offering many files

=> these servents will probably well know Cold-start problem

Conclusions

XRep is a reputation management protocol for anonymous P2P environments

It prevents malicious behaviors in P2P network

Future work:-- reputation mechanism with supernodes

-- performance optimization

References

[1] The Gnutella Protocol Specification v0.4

www9.limewire.com/developer/gnutella_protocol_0.4.pdf

[2] “ A Reputation-based Approach for Choosing Reliable Resources in Peer-to-Peer Networks," E. Damiani, etc.

[3] http://www.limewire.com/