1. TrustMe: Anonymous Management of Trust Relationships in Decentralized P2P System Aameek Singh, Ling Liu College of Computing, Georgia Tech International Conference on Peer-to- Peer Computing (P2P’03) Presenter: Jianming Zhou

2. Introduction Open and anonymous nature of P2P invites malicious behavior Sharing harmful content, viruses, etc. Need decentralized mechanism to tack Trust Management Trust based reputation metrics  Measure the trustworthiness of a peer  Transaction-based v.s. user-based rating  Dynamically assign a trust value based on peer reviews Issues Trust Model : What reputation metrics to use Access Protocol : How to access and secure their use

3. Secure Access Protocol Questions: Where should the trust value of a peer be stored? How to securely access other peers’ trust value? Desired Feature Security: protect trust hosting peer from attacks Reliability: Queries get true value Accountability: able to identify malicious peer

4. Problems of existing protocols Poll-based (Cornelli, et al[4]) Every peer before interacting with another peer broadcasts a trust query for that peer All peers that have interacted with that peer send their votes which are combined locally Public Key Cryptography used to secure Problems: No persistence: incomplete review counting due to peer offline; vulnerable to malicious group cooperation No anonymity: identify disclosure in query message; peers giving poor trust value subject to revenge/threat such as DoS attacks. Tedious decision-making: Peer needs to contact all voters to confirm.

5. Problem of existing protocols DHT-based (Eigenrep[8]) Mother peers (Hash from peer ID) hold trust value Peer hash ID and query trust value from mother peers Peer decides trust value by majority rule Shortcomings: Insecure communication: vulnerable to MIB. DHT threats: routing tampering, malicious lookup.. No anonymity: mother peers disclosure Group threats: malicious group hold value for each other

6. TrustMe Protocal Terms: THA peer: peer hold trust value for a particular peer P i : peer i’s private key B i : peer i’s public key SP i : peer i’s special private key SB i : peer i’s special public key TV: trust value TS: time stamp |: concatenate BS: bootstrap server ID: peer identifier BID: special peer identifier assigned by BS K(M): Encryption of message M by key K Offering peer: peer offering resource Querying peer: peer querying for trust value

7. TrustMe: infrastructure Bootstrap Server (BS) Entry point for peers to enter the network Acts as a kind of certification authority Possess a private-public key pair B BS is publicly available to all peers Each Peer Possess two pair of private-public keys and BS assigned ID : BID i =P BS (“Valid Node”|B’ i ) BS maintains a list of active peers

8. TrustMe: Protocal General idea Peer A broadcasts to query trust value of Peer B THA peers for B reply with trust value Peer A decides to interact with B based on trust value Peer A reports new trust value of Peer B THA peers for B update Leverage smart public-key cryptography Stages: Peer Join, Trust Query, Trust Reply, Peer Interaction, Trust Report, Peer leave

9. Peer i: Bootstrap server: Join 1 2 Generate: BID i = P BS (“Valid node”|B i ’ ) B i, B i ’ Assign THA Peer 3 4 Trust Query of peer I : ID i Peer j: Reply with Peer I’s trust value: ID i |B i |SB i |SP i (TV|TS|BID j |P’ j (TS)) 5 Peer x: 6 Collecting Proof-of-interaction P x (TS|B i |ID i ) P i (TS|B x |ID x ) 7 Report Peer X’s trust value for peer I: ID i |SB i (“Report”|V| B x |P x (P i (TS|B x |ID x )))

10. TrustMe: Query/Reply Query: p j query trust value of p i1,p i2,p i3 …,p n Q(j,{i 1,i 2,i 3,…,i n }) = ID i1 |ID i2 |ID i3 |…|ID in Broadcast query message + P2P forwarding mechanism guarantee privacy Reply: THA p x holding trust value of p i R(x,i)=ID i |B i |SB i |SP i (TV|TS|BID x |P ’ x (TS)) ID i : trust value for Pi B i : for future communication with pi SB i : decrypt SPi(M) SP i : Guarantee reply from THA peer BID x : ensure valid replying is from p x (Given B’ x ), + malicious THA peers can be blacklisted by their BID TS/ P’ x (TS): prevent reply attack

11. TrustMe: Anti-attack 1 Manipulating Reply Message: R(x,i)=ID i |B i |SB i |SP i (TV|TS|BID x |P ’ x (TS)) Malicious THA Peer Send wrong value (solution:▼)  multiple THA Peers + Majority rule  Punishment (BID x blacklist)+ random THA peer assignment to reduce possibility of malicious cooperation Send wrong value using other BID (▼)  Use P’ x (TS) Malicious non-THA Peer Replay a genuine message (▼)  TS: old messages are discarded Fake keys (▼)  Multiple THA Peers=> Content Conflict=>Identify

12. TrustMe: Interact/Anti Attack 2 Collecting Proof-of-Interaction (pi  pj) Exchage P i (TS|B j |ID j ) of each other. Prevents replay (TS) Cannot be generated in a fake manner B j and ID j are used for protection against using a message from Peer i’s interaction with some other peer Manipulating Proof-of-Interaction Messages: Replay message (▼) TS : Timestamp Fake (P j, B j ) (▼) Impossible for offering peer because THA Peer send B j to P i in reply message. To prevent query peer fake: offering peer requests its public key from its THA peer

13. TrustMe: Report/Anti-attack 3 Report: update trust value to THA Peers P j files a report for P i ID i |SB i (“Report”|V|B j |P j (P i (TS|B j |ID j ))) Only THA Peer can read (SP i ) THA Peer need P j ’s ID which can be obtained by decrypting with B j and B i Bj and Pj to prevent unlikely scenario that malicious peers get P i (TS|B j |ID j )

14. TrustMe: Peer Join/Leave Peer Join Peer posses two pair of Keys (, ) Why use two pairs and  used only while acting as a THA peer  Prevents mapping of public key to identifier after prolonged monitoring of the network Bootstrap server needs to assign a THA peer (Peer x) Create a new private-public key pair Only the THA peer will have the knowledge of SP i Used for secure transmission of trust values for the reply and the report phase Securely transmits to Peer x Broadcast a message: BID x |P BS (BID x |B’ x (ID i |B i |SP i |SB i )) Only BS can generate and only Peer x can read

15. TrustMe: Peer Leave Peer Leave Create a new THA peer for peers it was responsible for Its trust information is dumped after it is not accessed for some time Not discuss how to handle unexpected leaving!

16. TrustMe: Benefits Persistence: All reviews are counted and stored distributed No Central Trusted Authority BS is just a form of certification authority All trust mechanism within the network Small decision time Only one reply message needed for decision Ease of contribution Easy to contribute its trust value for another peer Just sending one reply message

17. Analysis: Experiments Effect of persistence Non-persistent systems can report highly misleading values Having as little as 10 malicious peers acting together can rate the peer being untrustworthy, even when it is not

18. Analysis: Cost: TrustMe costs more because of more broadcasts Cost varies little with increase in number of THA peers

19. Analysis: Response Time Caching improves response times Increase in number of THA peers also improves response time

20. Conclusion Anonymous trust management possible TrustMe provides secure and reliable access to trust values in a decentralized P2P system Compatible with existing Gnutella style systems