Credentials Revocation in Vehicular Networks: Design & Evaluation

Credentials Revocation in Vehicular Networks:Design & Evaluation Ghita Mezzour Panos Papadimitratos

Overview • Introduction • Regional CRL • CRL broadcast at low rate • Results • Conclusion

System model – General

Certification authority(CA) Road Side Units (RSUs) Wired communication with the CA Wireless communication with vehicles Each vehicle has A unique identity V A pair of private and public keys {kV , KV} A certificate Cert {V, KV, Lf, attr}CA Each message Signed Accompagnied by the sender’s cert Accepted only within the region of the responsible CA System model – Regional CA

Problem statement • Vehicles can ‘misbehave’ • Attackers : tampered software and hardware • Mulfunctioning devices • Stolen vehicles • Administrative reasons • Once detected, it is necessary to revoke their credentials

Challenges & Constraints • Scalability • Large number of revoked vehicles • Large number of equipped vehicles that need the revocation information • Communication between RSUs and vehicles • Non-pervasive • Short contact times • Bandwidth constrained

Classical credential management schemes (1/2) • Certificates revocation lists (CRLs) • Long lived certificates e.g. 1 year • CRL contains not yet expired certificates that were revoked • CA periodically issues a CRL • CRL can become very large

Classical credential management schemes (2/2) • CRL and D-CRL • CRL issued e.g every month • D-CRL issued e.g every day or week • Problem if some revocation piece is not received • Short lived certificates • Short cert lifetime e.g. 1 day or 1 week • Get a new certificate when certificate expires • Overhead of issuing new Certs

Related work • [RPAJH JSAC 2007] propose two revocation schemes • Revocation of the Trusted Component (RTC)  Reduces the number of Cert in the CRL  Requires to geographically localize vehicles • Revocation using Compressed Certificate Revocation Lists (RC2RL) • CRLs are lossly compressed using Bloom Filters  Scalable  Some legitimate nodes may get revoked as well

CRL based approach  Widely used and tested in many systems  Robust  No false positive  Scalability issues

Agenda • Introduction • Regional CRL • CRL broadcast at low rate • Results • Conclusion

CRL size • Expected CRL size E(NCRL) = Nv * p * r * (Lf /2) Nv Total number of vehicles p Percentage of equipped vehicles r Percentage of revoked vehicles per day Lf Certificate lifetime France Nv = 5.106, 3.105 stolen vehicles per year => 100 – 200 KBytes

B A {a, Ka}A {a, Ka, fr}B {a, Ka}A {a, Ka, fr}B {KB}Root Foreigner Cert (1/2) {a, Ka}A Regular Cert of vehicle a by CAA {a, Ka, fr}B Foreigner Cert of vehicle a by CAB

{a, current time}ka, {a, Ka}A {a, Ka, fr}B, {B, KB}Root If a CRLA {a, ACK, current time}ka Foreigner Cert (2/2) • Delivery protocol • Characteristics • CAs have global revocation information • Need to present a valid regular Cert • Short lifetime • Only valid inside B B a

A Revocation –Misbehavior in the home region B a in CRLA Insert {a} in CRLA {a, Ka}A a

B A C {a, Ka}A {a, Ka, fr}B {a, Ka}A Revocation – Mibehavior in a host region Misbehavior of a a not in CRLA a in CRLA Insert {a} in CRLA Insert {a,fr} in CRLB {a, Ka, fr}B

Foreigner Cert lifetime • Short lifetime • Journeys in host regions are typically short • One week or one month lifetime • Small overhead of issuing foreigner Certs • Foreigner Certs in CRLs • Periodical check of regular Certs that were issued a foreigner Cert • One day lifetime • Overhead of issuing new foreigner Certs if long journey • Implicit revocation: no foreigner Certs in CRLs

Summary • CAs need global revocation information • Vehicles needs regional revocation information • CRL of a region A contains • Certs of region A • Foreigner Certs of foreign vehicles that misbehaved while in A • Small number • Short lifetime => Short CRLs

CA - vehicles communication • Satellites • Wide coverage • Satellite receivers may not be compulsory • Low and expensive bandwidth • Satellite usage loyalties • Cell phones • Expensive • WLAN, buses • City infrastructure • Present in remote areas • RSUs • Non-pervasive • Short contact times • Bandwidth constrained • VANET infrastructure

Background - Erasure codes • Erasure codes for data transmission • The data is cut into M pieces • The blocks are encoded into N >> M encoding pieces • Reception of any slightly larger subset of pieces is enough to recover the original data

Background – Fountain codes • Fountain codes e.g. Raptor code for data transmission • The data is cut into M pieces • The blocks are encoded into a potentially limitless encoded symboly • Reception of any (1 + )M subset of pieces is enough to recover the data

CRL is encoded using an Erasure code / fountain code RSUs broadcast the encoded CRL pieces Vehicles collect CRL pieces as they encounter RSUs Vehicles recover the entire CRL when they receive enough pieces How it works (1/2)

Erasure code: RSUs Shuffles the N pieces pseudorandomly Broadcasts them When the N pieces are over, it starts the broadcast again Fountain code: RSUs Broadcast the encoded pieces How it works (2/2)

Summary • Broadcast based on Erasure/fountain codes • No collaboration between RSUs • No synchronized Broadcast schedule • Requirements • Vehicles complete the CRL reception fast • Small overhead to the system

Number of pieces to be received to complete the reception of the CRL (99.99% confidence) Erasure codes M Number of uncoded CRL pieces N Number of encoded CRL pieces Raptor code M Number of CRL pieces  Code parameter affects the compltexity Number of pieces to receive

CA RSU CRL bcst Bandwidth B v R D R Time to complete the CRL • Total time to complete the CRL Ptot Number of pieces to be received sz Size of a CRL piece + overhead v Speed of the vehicle B Bandwidth of the CRL broadcast R Range of RSUs D Distance between encountering RSUs

Coding schemes comparison Total number of pieces to be received to complete the reception of the CRL (99.99% cofidence) vs. Number of pieces in the CRL

Broadcast bandwidth – RSU range Time duration to complete the reception of the CRL vs. CRL broadcast bandwidth 200 KB CRL, D = 500m, v = 60 km/h

Vehicle speed – Distance between RSUs Time duration to complete the CRL vs. vehicle speed 200KB CRL, B = 3KBytes/s, R = 300m

City vs. Highway scenario Cityscenario V = 40 km/h, dense RSUs Highwayscenario V = 120 km/h, less dense RSUs 200 KB CRL

References • M. Raya, P. Papadimitratos, I. Aad, D. Jungels, and J. –P. Hubaux, Eviction of Misbehaving and Faulty Nodes in VehicularNetworks, IEEE Journal on Selected Areas in Communications (JSAC), Special Issue on Vehicular Network, 4th Quarter, 2007 • Ronald L. Rivest. Can we eliminate certificate revocation lists? In Rafael Hirschfeld, editor, Financial Cryptography, volume 1465, page 178-183, anguilla, British West Indies, February 1998. Springer

Conclusion • Revocation is crucial for VANET • Challenging due to special environmental constraints • CRL approach can be adapted • Regional CRL (Foreigner Certs) • Low rate bandwidth (Erasure/fountain codes)

Credentials Revocation in Vehicular Networks: Design & Evaluation