1. Group Spreading: A Protocol for Provably Secure Distributed Name Service
Christian Scheideler
Dept. of Computer Science
Johns Hopkins University
Joint work with Baruch Awerbuch

2. Robust Distributed Name Service
Peer: entity with a unique identity, i.e. identified by (Name(p), IP(p))
Goal: provide name service, i.e. each execution of Lookup(name) returns IP(p) of the peer p with Name(p)=name, or NULL if there is no such peer
Robust Distributed Name Service

3. Robust Distributed Name Service
Static set of peers.
Easiest solution: every peers knows every other peer.
Problem: not scalable!
Better solution: peers organize in a searchable overlay network of low degree.
Robust Distributed Name Service

4. Robust Distributed Name Service
Chord
[Stoica, Morris, Karger, Kaashoek, and Balakrishnan ’01]
Idea: organize peers in cycle according to
hash function h:Names [0,1).
0.28
0.43
0.58
Robust Distributed Name Service

5. Robust Distributed Name Service
Hypercubic Shortcuts
+1/2
+1/4
fingers
+1/8
+1/16 x
Robust Distributed Name Service

6. Hypercubic Pointer Structure
Robust Distributed Name Service

7. Lookup Operation Lookup(name): ~log n number of hops
Pure Chord cannot handle adversarial peers!!
Can also handle dynamic set of peers.
Robust Distributed Name Service

8. Distributed Name Service
Dynamic set of peers, some adversarial.
Operations:
Join(p): peer p wants to join the system
Leave(p): peer p wants to leave the system
Lookup(name): returns IP(p) of the peer p with Name(p)=name, or NULL if there is no such peer
Goal: Provide provably survivable distributed name service
Robust Distributed Name Service

9. Robust Distributed Name Service
Prerequisites
Certification authority:
certifies (Name,IP) pairs so that collisions are avoided
prevents adversarial peers from taking over identity of honest peers
cannot prevent adversarial peers from registering (potentially many) own pairs
Allow adversary to have infinitely many identities, but resources finite!
Fighting at this front is fighting the wrong battle!
Robust Distributed Name Service

10. Robust Distributed Name Service
Correctness
Once Join(p) or Leave(p) terminates in p, it is called completed.
p is called a member once Join(p) has completed.
p ceases to be a member once it initiates Leave(p).
Lookup(name) request executed correctly:
If an honest peer p with Name(p)= name is currently a member of the system, returns IP(p).
Otherwise, if such a peer has never been in the system or completed Leave(p) , returns NULL.
Otherwise, returns IP(p) or NULL.
Robust Distributed Name Service

11. Robust Distributed Name Service
Efficiency
Time unit: max time a message needs to travel from one honest peer to another
An overlay network operation is called
work-efficient if it is completed using a comm volume of at most polylog(n) bits
time-efficient if it is completed in at most polylog(n) time units
Overlay network maintainance should only need polylog(n) bits of comm per time unit per peer
Robust Distributed Name Service

12. Peer-to-Peer Name Service
Problem: adversarial peers a priori indistinguishable from honest peers
Idea: randomly distribute peers in overlay network [CAN,Chord,Pastry,…]
Strategy: use pseudo-random hash function h:Names [0,1)
Robust Distributed Name Service

13. Peer-to-Peer Name Service
Place peer p at h(name(p)) in [0,1):
randomly distributes honest peers
may not randomly distribute adversarial peers
Robust Distributed Name Service

14. Peer-to-Peer Name Service
Problem: adversarial peers can isolate honest peers
Chord: connect to
direct neighbors
closest successor of h(Name(p))+1/2 i
Robust Distributed Name Service

15. Peer-to-Peer Name Service
Alternative:
Assign each peer p to a random ID(p) in [0,1)
Robust Distributed Name Service

16. Peer-to-Peer Name Service
New attempt:
Assign each peer p to a random ID(p)
Limit lifetime of a peer
Robust Distributed Name Service

17. Peer-to-Peer Name Service
So good approach:
Assign random IDs
Enforce limited lifetime
But:
How to interconnect the peers?
How to generate random IDs?
How to enforce limited lifetime?
How to implement operations?
Robust Distributed Name Service

18. Peer-to-Peer Name Service
How to interconnect the peers?
Region approach: each peer has several nodes, each node v maintains rv=log n - loglog n + Q(1) 1
Region: interval of size 1/2r starting at an integral multiple of 1/2r.
1/2rv = Q(log n / n)
+/- 1/2
-1/4
+1/4
-1/8
+1/8 x
Robust Distributed Name Service

19. Peer-to-Peer Name Service
How to generate random IDs?
Group approach: each peer maintains Q(log n) nodes, ID of new node generated by region a) b) c) R R R
Robust Distributed Name Service

20. Peer-to-Peer Name Service
How to enforce limited lifetime?
Lifetime approach:
Join operation implemented s.t. it either terminates in Q(log n) steps or does not succeed
Honest nodes take median of age views of a node reported by nodes in region
Honest nodes cut connections to nodes that are more than L=Q(log n) steps old
Robust Distributed Name Service

21. Peer-to-Peer Name Service
How to implement operations?
Join(p): p contacts a peer q in the system, q includes Q(log n) nodes for p into the system, from there p inserts entry (Name(p),IP(p)) into relevant region and takes over maintenance of its nodes
Leave(p): p deletes entry (Name(p),IP(p)) and leaves the system
Lookup(name): region routing, majority decision
Robust Distributed Name Service

22. Robust Distributed Name Service
Assumptions
adversary e/log n - bounded
join/leave rate of peers is O(1/log2 N)
honest peers have infinite bandwidth (no DOS)
assumptions about messages…
A peer is honest if
it is not under the control of the adversary at any time,
it is reliable, and
it contacted an honest peer in order to join the network.
Robust Distributed Name Service

23. Robust Distributed Name Service
Related Work
Classical distributed computing: Byzantine agreement, two-phase commit, linear overhead
Proactive security: uses coding to protect data in dynamic environment, linear overhead
Fixed-topology networks: only fail-stop faults, no Byzantine behavior
Hash-based P2P networks: hinge on assumption that IDs are random
Random or unpredictable placement (Freenet, Gnutella): hard to attack, but hard to find data
Robust Distributed Name Service

24. Robust Distributed Name Service
Related Work
[Sit and Morris 2001]: investigate various attacks and defenses including routing attacks, storage and retrieval attacks, DoS, and join/leave attacks
[Castro, 2001]: replication algorithm tolerating Byzantine faults as long as 1/3 fraction of replicas faulty
[Douceur, 2002]: Sybil attacks (adversaries forge multiple identities)
[Castro, Druschel, Ganesh,… 2002]: strategies for secure nodeID assignment, routing table maintenance, and message forwarding
BUT: no provably robust overlay construction
Robust Distributed Name Service

25. Robust Distributed Name Service
Comparison
Group Spreading: e/log N-bounded adversary
Enforced Spreading: e-bounded adversary N
Trust-but-Verify
work
overhead
Group Spreading
polylog(N)
Enforced Spreading
fraction of adv nodes
e/log N e
1/2
Robust Distributed Name Service

26. Robust Distributed Name Service
Conclusions
It is possible to come up with provably robust overlay network constructions!
Analysis hard…
Model still unrealistic, but Rome was also not built in one day 
Questions????
Robust Distributed Name Service

27. Robust Distributed Name Service
STOC 2005
When: May 22-24, 2005
Where: Baltimore, MD, USA
Submission deadline: Nov 4, 5:59 pm EST
Web Site:
Robust Distributed Name Service