1. The SecureRing Group Communication System By Kihlstrom, Moser, and Melliar-Smith
ACM Transactions on Information and System Security, November 2001
Presented by Jessica Lunney

2. Motivation
Reliable group communication to create a survivable, distributed system
High throughput
Reasonable latency
Avoid the high overhead of protocols that excessively use digital signatures

3. Features of a SecureRing
Remains correct and reliable despite Byzantine faulty behavior - survivability
Allows one digital signature to cover multiple messages

4. System Model n processors Partially synchronous, distributed
Every processor has unique id
Completely connected network
Processors multicast to everyone, including themselves
Logical ring overlaid upon network
Every processor has private key and access to public keys

5. Assumptions Network will not partition
All processors (servers) operate deterministically
A system of size n always contains: at least ceil((2n + 1)/3) correct processors up to floor((n-1)/3) faulty processors
Faulty processors are unable to forge the signature of correct processors

6. Protocol Hierarchy

7. Message Delivery Protocol - Properties
Non-duplicate Delivery: for any message m, every correct processor p delivers m at most once
Authentication: for any message m that contains id of correct processor p, a correct processor q delivers m only if m was originated by p
Uniqueness of Message ids: if correct processor p delivers m in configuration C, then no correct processor q delivers m’ in C having the same id as m but a different content

8. Message Delivery Protocols - Properties
Reliable Delivery: if p and q are both correct processors in C, and there is no configuration change, if p originates m then q delivers m
Total Order of Messages: if p and q are both correct processors in C that deliver m1 and m2, then p delivers m1 before m2 iff q delivers m1 before m2

9. Message Delivery Protocol – Block

10. Message Delivery Protocol - Token

11. Membership Protocol - Properties
Uniqueness of Configuration ids: if a correct processor p installs C, then no correct processor q installs configuration C’ with the same id as C but different contents
Self-inclusion: if correct processor p installs C, then p is in C
Total order of Configuration: if p and q are both correct and install C1 and C2, then p installs C1 and then C2 iff q installs C1 and then C2

12. Membership Protocol - Properties
Eventual inclusion: if p and q are both correct, there is a time after which p installs a configuration that includes q
Eventual exclusion: if p is correct and q is Byzantine faulty, then there is a time after which p installs a configuration that excludes q, and p never subsequently installs a configuration that includes q
Eventual inclusion + Eventual exclusion = Liveness

13. Membership Protocol - Block

14. Membership Protocol - States

15. Byzantine Fault Detector - Properties
Eventual Strong Byzantine Completeness: there is a time after which every processor that has exhibited a detectable Byzantine fault is permanently suspected by every correct processor
Eventual Strong Accuracy: there is a time after which every correct processor is never suspected by any correct processor
=> ‘Liveness’ of Membership Protocol

16. Byzantine Fault Detector - Block

17. Message Diffusion Protocol - Properties
Self-receipt: if a correct processor D-multicasts a message m, then it eventually D-receives it
Uniform receipt: if a correct processor D-receives a message m, then every correct processor eventually D-receives it

18. Message Diffusion Protocol
Described during faulty operation by Membership and Fault Detection Protocols with complexity O(n2)
Fault free operation could use different protocol to increase overall efficiency

19. Throughput – 300-bit key modulus

20. Throughput – 512-bit key modulus

21. Throughput – 768-bit key modulus

22. Latency – 200 byte messages

23. Membership Change Time