MOCA : Mobile Certificate Authority for Wireless Ad Hoc Networks

1. MOCA : Mobile Certificate Authority for Wireless Ad Hoc Networks Seung Yi, Robin Kravets The 2nd Annual PKI Research Workshop (PKI 2003) September. 25, 2003 Presented by Sookhyun, Yang

2. Contents • Introduction • Background • Requirements • MOCA (MObile Certificate Authority) Framework • Evaluation • Conclusion

3. Introduction • Wireless ad hoc networks • Infrastructure-less nature • Inhibit guaranteeing any kind of connectivity • Increased physical vulnerability of the nodes • Key management framework for ad hoc networks • MOCA framework • PKI (Public Key Infrastructure) without infrastrucutre • Threshold cryptography • MOCA nodes • Communication protocol between a client and CAs • MP (MOCA certification protocol)

4. CA CA’s private key : KRauth CA’s public key : KUauth X’s public key : Kux KUb KUa CA CB (1) CA CA = EKRauth[TIme1,IDA,KUa] A B (2) CB threshold Background • PKI • Collection of components and procedure that support the management of cryptographic keys through the use of digital certificates • Public key certificate • Threshold cryptography • Divide up a secret to n pieces • Reconstruct the full secret with any kpieces out of those n MOCAs

5. Requirements • Requirements for MOCA framework • Fault tolerance • Maintain correct operation in the presence of faulty nodes • Tolerant to a fraction of faulty nodes • Security • Act as the trust anchor for the whole network • Operate securely against malicious nodes or adversaries • Availability • Highly dependent on the connectivity of the network • Certification service should be always available to clients

6. MOCA Framework (1/4) • Choosing MOCA nodes • Exploit heterogeneity • More trustworthy • Computationally more powerful • Physically more secure • Threshold cryptography • Distribute the functionality of a CA to the whole network • Secret : CA’s private key • CA : n MOCA nodes • Threshold : k MOCA nodes • Configuration • Total number of nodes in the network (M) • Number of MOCAs (n) • Threshold value for secret reconstruction (1<= k <= n) • Connectivity • MP (MOCA certification protocol) • Communication protocol between client and MOCAs • One-to-many-to-one

7. (1) Partially signed revocation certificate signed with its key share (2) Partial signature signed with key share (2) Full revocation certificate (3) CRL (Certificate revocation list) • At least k • request client MOCA nodes MOCA Framework (2/4) • Revocation • Threshold Cryptography • Certification (3) Collect k partial signature (4) Reconstruct full signature “CA”

8. (1) CREQ (2) CREP MOCA Framework (3/4) • Communication protocol - MP • Success case • k valid CREPs within a fixed period of time • Routing • Reverse path with CREQ • If no CREP within time-out period, reverse path expires

9. MOCA Framework (4/4) • Mechnisms of MP • Flooding • Send and receive packets (CREQ, CREP) to all nodes • Unicast-based optimization • β-unicast • Multiple unicast connections if the client has sufficient routes to MOCAs in its routing cache • Sufficiency • Thresholdk • State of the network • If (routes to MOCAs >= β), then β- unicast else flooding • How to choose among the MOCAs cached in the routing table (>= β) • Random MOCAs • Closest MOCAs • Freshest MOCAs β β (threshold unicast) = k (crypto threshold) + α (safety margin)

10. 600s 1km # of + (mobile nodes) : 150, 300 # of (MOCAs) : 30, 50 # of CREQ : each node 1/1min 1km • Mobility • Node pause time : 0, 10s • Node Max. Speed : 0, 1, 5, 10, 20 ms Evaluation (1/6) • Focus of evaluation • Effectiveness • Success ratio • Flooding-based protocol : (# of total received CREPs) /(# of total CREQs) • Unicast-base optimization : (# of successful certification request)/(# of total CREQs) • Unicast usage in uicast-based optimization • Cost • Packet overhead • Response time : additional communication delay • Simulation set-up

11. Use of unicast CREQs Use of flooding CREQs x : β y : usage of unicast Total # of CREQs = 1000 Evaluation (2/6) • Flooding vs. Unicast • Unicast usage

12. Evaluation (3/6) • Packet overhead • Total number of control packets used for certification services • n = 30 • Setting β as low as possible results in the best improvements in overhead, but endangering security of the whole framework

13. 0.3s Evaluation (4/6) • Certification delay • Arrival time of CREP packets with the closest-unicast approach • Choice between flooding and unicast-based optimizations or choice between different β values does not affect the timing behavior

14. Evaluation (5/6) • Success ratios • αplays an important role in determining the success ratio within a given τ • Helpful when deciding an adequate τfor a given k

15. Evaluation (6/6) • Summary • High success ratio • Flooding-based protocol : almost 99% • Unicast-based optimization : 75%~97% • Reduced overhead • Unicast optimization saves up to about 30% of control packets • Certification delay is acceptable

16. Conclusion • Present a practical key management framework for ad hoc wireless networks using PKI • Clarify the necessity and the problem of providing a PKI framework for ad hoc network • Identify requirements for such a framework • Show effectiveness of paper’s approach through simulation results • Provide some insights into the configuration of such security services in ad hoc networks