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SPORC: Group Collaboration using Untrusted Cloud Resources OSDI 2010 Presented by Yu Chen

SPORC: Group Collaboration using Untrusted Cloud Resources OSDI 2010 Presented by Yu Chen. Cloud-based Collaborative Services. Pros: -Global accessibility, High availability, -Fault tolerance, -Elastic resource allocation and scaling Cons and Problem: -Sacrifice in security and privacy

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SPORC: Group Collaboration using Untrusted Cloud Resources OSDI 2010 Presented by Yu Chen

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  1. SPORC: Group Collaboration using Untrusted Cloud Resources OSDI 2010 Presented by Yu Chen

  2. Cloud-based Collaborative Services • Pros: -Global accessibility, High availability, -Fault tolerance, -Elastic resource allocation and scaling • Cons and Problem: -Sacrifice in security and privacy • What if the server is malicious?

  3. Solution: SPORC • Agnostic and untrusted server - provides a generic collaboration service - assigns a global order - stores updates in its encrypted history - can be potentially MALICIOUS

  4. Solution: SPORC • Smart Clients -guarantee security by users' cryptographic keys -provides operational transformation -provides fork* consistency -recover from malicious forks -access the documents on behalf of authorized users

  5. Goals • Flexible framework for a broad class of collaborative services • Propagate modifications quickly • Tolerate slow or discounted networks • Keep data confidential • Detect a misbehaving server • Recover from malicious server behavior

  6. Background: Operational Transformation • Problem: Operations might conflict with each other • Example: State: ABCDE Alice: op1='del 4' Bob: op2='del 2' naïve execution: Alice: ACE Bob:ACD • OT enables optimal local updates and eventual consistency

  7. Background:Operational Transformation • Example: State: ABCDE Alice: op1='del 4'; op2' Bob op2='del 2'; op1'

  8. Background: Fork* Consistency • Problem: Divergent views from misbehaving server • Solution: -Clients share information about the history - - Possible partitions into groups, but solvable

  9. Deployment and Threat Model • Deployment -Large number of users and documents -Server: replicating functionality and partitioning state -Client-driven failover and recovery • Threat Model - Server: potentially malicious; unable to corrupt the clients' states - Client: trusts assigned according to the user; genuine code

  10. System Overview

  11. Invariance in SPORC • Local Coherence: Starting from an empty state, applying the operations in commited history and pending queue will result in the current state • Fork* Consistency • Client-Order Preservation

  12. Operations • Labeled with the name of the user • Digitally signed by the user's private key • Includes the client ID • Document Operations - encrypted under a symmetric key • Meta Operations • Why 2 different operations? Solution later.

  13. Sequence Numbers and Hash Chains • Client Sequence Number(clntSeqNo) • Global Sequence Number(seqNo) • Last Commited Operation(opn) • Last Commited Operation Number(prevSeqNo) • Verification: - Client order preservation(Efficiency??) - Fork* consistency

  14. Resolving confliects with OT • Additional Operations from the Server -seqNo>preSeqNo+1 -op'new← T(opnew, <opprevSeqNo+1,...,opseqNo-1>) • Uncommited Operations in the Client's Pending Queue -

  15. Membership Management • Access Control List - reader, editor and administrator - ModifyUserOp • Payloads encrypted by AES + users' public keys • User Removal: new random AES key • Barrier Operation -Continuous Chain of Keys(or Checkpoints)

  16. Extension: Checkpoint • Supported by individual clients • CheckpointOp - Encryption with current document key - contains the hash of encrypted checkpoint data • Verification of CheckpointOp - meta-history

  17. Extension: Checking for ForksOut-of-Band • Fork partition created by the server: -Clients of one fork might never know the history of clients of another fork • Check for Forks Out-of-Band - Message exchanging between clients - <c,d,s,hs> - Request of missing operation from the server

  18. Recovering from a Fork • Recovery via a new server -Both clients will roll back their histories to their last common point before fork -One of them upload the common history to the new server -Both of them will resubmit the operations after the fork

  19. Implementation • generic server • client-libraries -sending, receiving, encryption, OT and consistency checks • Applications: -Key-value store -collaborative text editor

  20. Experimentatal Evaluation • Hardware -2.3GHz AMD Opteron -8GB of RAM -gigabit switched ethernet • Metrics -Latency -Server throughput -Client time-to-join

  21. Latency

  22. Latency

  23. Server Throughput

  24. Client time-to-join

  25. Conclusion • OT enables optimistic updates and reconciles clients' conflicting states • OT and fork* consistency complement each other well • Membership mamangement architecture

  26. Discussion • The extension are not evaluated in this paper • Check for Forks Out-of-Band or Recovering from a Fork: -What if the client is also malicious? -How should we prevent the client-server collusion? • What is the mean time to detect a malicious server with no partition of forks and clients?

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