Diplom Thesis Faculty of Engineering and Computer Science Ulm University Conditional Pseudonymity in Vehicular Ad Hoc Networks Florian Marcus Schaub presented on November 13, 2008 Supervisors Prof. Dr.-Ing. Michael Weber Dr. Frank Kargl Advisor Zhendong Ma Faculty of Engineering and Computer Science, Ulm University James-Franck-Ring, 89081 Ulm, Germany This thesis has been prepared in the summer semester 2008 as a requirement for the completion of the Diplom course Medieninformatik at Ulm University. Cover photo: Night blur by Mando Gomez http://www.mandolux.com Printed on November 9, 2008. Some rights reserved. This work is licensed under the Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 Germany License. To view a copy of this license, visit http://creativecommons.org/ licenses/by-nc-nd/3.0/de/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA. Computer Science is no more about computers than astronomy is about telescopes. Edsger W. Dijkstra Contents 1 Introduction 1 2 Problem Analysis 5 2.1 Characteristics of Vehicular Ad Hoc Networks . 5 2.2 Security and Privacy in Vehicular Ad Hoc Networks . 9 2.3 Conditional Pseudonymity . 14 2.4 Summary . 18 3 Requirements for Privacy in VANETs 19 3.1 Assumptions . 20 3.2 General VANET Requirements . 21 3.3 Pseudonym Requirements . 23 3.4 Authentication Requirements . 28 3.5 Summary . 30 4 Privacy in other Domains 33 4.1 Health Care . 34 4.2 E-Government . 39 4.3 E-Commerce . 43 4.4 Internet . 51 4.5 Ad Hoc Networks . 59 4.6 Summary . 65 5 Research on Privacy in VANETs 67 5.1 Public Key Cryptography . 67 5.2 Symmetric Cryptography . 73 5.3 Group Signatures . 75 5.4 Identity-Based Cryptography . 77 5.5 Other Approaches . 79 5.6 Summary . 81 6 Framework for Conditional Pseudonymity 83 6.1 Pseudonyms with Embedded Identities . 84 6.2 Privacy-Preserving Pseudonym Issuance . 88 6.3 Distribution of Resolution Authority . 94 6.4 Summary . 99 7 Analysis and Discussion 101 7.1 Attack Analysis . 101 7.2 Anonymity Analysis . 106 7.3 Performance Analysis . 108 7.4 Comparison with Requirements . 114 Contents 7.5 Comparison with Other Approaches . 116 7.6 Summary . 117 8 Conclusion 119 Bibliography 123 vi List of Figures 2.1 Communication Technologies in Vehicular Communication Scenarios . 6 2.2 Communication Patterns in Vehicular Ad Hoc Networks . 7 2.3 Structure of a Digitally Signed Message . 12 2.4 The Process of Pseudonym Resolution . 17 4.1 Three Ballot Voting . 43 4.2 Carbon Paper-lined Envelope Analogy for Blind Signatures . 45 4.3 Layer Structure of an Onion for Onion Routing . 54 6.1 V-token Structure . 87 6.2 WAVE OBU Certificate Structure . 88 6.3 Shamir's Secret Sharing Scheme . 95 6.4 Blakley's Secret Sharing Scheme . 96 7.1 Anonymity Analysis of the Pseudonym Issuance Protocol . 107 7.2 Anonymity Analysis of the Identity Resolution Protocol . 108 7.3 Bandwidth Requirements for Messages with V-tokens . 113 List of Tables 2.1 Dimensions of an Attacker . 10 3.1 Requirements for Privacy in VANETs . 31 6.1 Notation Overview . 85 7.1 Potential Attacks and Prevention Mechanisms . 105 7.2 Comparison of Different Encryption Schemes . 109 7.3 Comparison of Certificate Storage Requirements . 111 7.4 Total Message Sizes for Different Payloads . 112 List of Tables viii List of Acronyms 3G 3rd Generation of mobile phone standards ABS Anti-lock Braking System ACL Access Control List ANODR ANonymous On-Demand Routing ASR Anonymous Secure Routing ARCPO Anonymous Receiver-Contention POsitioning routing BAN Burrows-Abadi-Needham logic C2C-CC Car 2 Car Communication Consortium CA Certificate Authority CALM Communications, Air-interface, Long and Medium range COR Core Onion Router CRL Certificate Revocation List DHT Distributed Hash Table DLP Discrete Logarithm Problem DMV Department of Motor Vehicles DRM Digital Rights Management DoS Denial of Service DSRC Dedicated Short Range Communications ECC Elliptic Curve Cryptography ECDSA Elliptic Curve Digital Signature Algorithm EDR Electronic Data Recorder EFF Electronic Frontier Foundation EHR Electronic Health Record ELP Electronic License Plate EML Election Markup Language EPIC Electronic Privacy Information Center ESP Electronic Stability Programme ETP Electronic Transfer of Prescriptions EU European Union FCC Federal Communications Commission GPS Global Positioning System GSM Global System for Mobile communications HIPAA Health Insurance Portability and Accountability Act HMAC keyed-Hash Message Authentication Code HTTP HyperText Transfer Protocol IBC Identity-Based Cryptography IEEE Institute of Electrical and Electronics Engineers IP Internet Protocol ISO International Standards Organization IR Identity Repository ISP Internet Service Provider ITS Intelligent Transportation Systems IVC Inter-Vehicle Communication List of Acronyms LTP Long-Term Pseudonym MAC Message Authentication Code MANET Mobile Ad hoc NETwork MPC Multi-Party Computation NoW Network on Wheels OASIS Organization for the Advancement of Structured Information Standards OBU On-Board Unit P2P Peer-to-Peer P3P Platform for Privacy Preferences PKI Public Key Infrastructure PRECIOSA PRivacy Enabled Capability In co-Operative Systems and safety Applications PRIME Privacy and Identity Management for Europe RA Resolution Authority RFID Radio-Frequency IDentification RSU Road-Side Unit SeVeCom Secure Vehicular Communication project SK Signature of Knowledge STP Short-Term Pseudonym SPARTA Secure Pseudonym Architecture with Real Time Accounting TCP Transmission Control Protocol TESLA Time Efficient Stream Loss-tolerant Authentication TLS Transport Layer Security Tor The Onion Router TPD Tamper-Proof Device TPM Trusted Platform Module TTP Trusted Third Party V2I Vehicle-to-Infrastructure communication V2V Vehicle-to-Vehicle communication VANET Vehicular Ad hoc NETwork VLR Verifier-Local Revocation VSC Vehicle Safety Communications project W3C World Wide Web Consortium WAVE Wireless Access in the Vehicular Environment WSN Wireless Sensor Network ZKPK Zero-Knowledge Proof of Knowledge x 1 Introduction The traffic volume on roads worldwide has been steadily increasing in recent years. According to the annual road statistics report of the European Union Road Federation [60], road transport has been growing with an annual rate of 2.4% in the European Union (EU 27) since 1995|corresponding to about 15.53 million new car registrations every year. Despite the increasing traffic volume, the number of car accidents has been slightly decreasing from 1.43 million accidents involving injury, in 2000, down to 1.28 million, in 2006. The number of fatal accidents even dropped by 23.0% in the same period. This positive development can be partly attributed to the improvement of passive safety systems, like airbags and seat-belt tighteners, which lessen the impact of an accident, but also to the advancement of active safety systems. Active safety systems aim to prevent accidents by warning, supporting, and actively assisting drivers in critical situations. If a collision is inevitable, active safety technology can proactively prepare the vehicle for the impact to reduce injury. Typical active safety systems are traction control, ESP, ABS, and four-wheel drive. In the course of improving active safety, more advanced technologies find their way into vehicles. Sensors of all kinds are employed to measure and assess a vehicle's condition and environment, enabling the issuance of early warnings to drivers. Radar technology, laser systems, and camera sensors detect obstacles on the road and warn drivers, but can also provide parking assistance. Camera sensors can actively scan road sides and alert the driver when the vehicle is unintentionally crossing lines or moving into another lane|seat or steering wheel vibrations can provide corresponding haptic feedback to the driver. All these assistive technologies aim to alert the driver as early as possible to potentially dangerous situations before they occur. The next steps in that direction are cooperative safety systems that leverage wireless communication between vehicles to extend the telematic horizon. Laser sensors can only detect visible obstacles ahead, but vehicles that communicate wirelessly can alert and warn each other, and therefore the drivers, about situations out of sight, e.g. the upcoming end of a traffic jam or a stranded vehicle around the next bend. Inter-vehicle communication (IVC) enables vehicles to continuously exchange messages with other vehicles in their proximity, and establish detailed information about their surrounding environment. Based on this context information, potential dangers can be detected in early stages and appropriate counter-measures can be initiated. With ever increasing traffic density, vehicle-to-vehicle communication can improve road safety in the future. The potential of the technology has been recognized by car manufacturers, governments, and research institutions alike. Many projects of national and international scale are under way worldwide which represent the joint efforts of these stakeholders to develop reliable vehicular communication solutions for commercial use. Simultaneously, the definition of corresponding standards is pursued to ensure interoperability. Although safety is one of the strongest selling points of vehicular communications, the technology has further advantages. Intelligent transportation systems (ITS) [82], consisting of interconnected smart vehicles, can achieve better traffic efficiency and enhance driver convenience, for example by enabling