Abstract. It is usually the case that before a transaction can take place, some mutual trust must be established between the participants. On-line, doing so requires the exchange of some certified information about the participants. The easy solution is to disclose one’s identity and reveal all of one’s certificates to establish such a trust relationship. However, it is clear that such an approach is unsatisfactory from a privacy point of view. In fact, often revealing any information that uniquely corresponds to a given individual is a bad idea from the privacy point of view. In this survey paper we describe a framework where for each transaction there is a precise specification of what pieces of certified data is revealed to each participant. We show how to specify transactions in this framework, give examples of transactions that use it, and describe the cryptographic building blocks that this framework is built upon. We conclude with bibliographic notes on the state-of-the-art in this area. 1

Self-checking software tamper resistance mechanisms employing checksums, including advanced systems as recently proposed by Chang and Atallah (2002) and Horne et al. (2002), have been promoted as an alternative to other software integrity verification techniques. Appealing aspects include the promise of being able to verify the integrity of software independent of the external support environment, as well as the ability to automatically integrate checksumming code during program compilation or linking. In this paper, we show that the rich functionality of many modern processors, including UltraSparc and x86-compatible processors, facilitates automated attacks which defeat such checksumming by self-checking programs.

Interactions in electronic media require mutual trust to be established, preferably through the release of certified information. Disclosing certificates for provisioning the required information often leads to the disclosure of additional information not required for the purpose of the interaction. For instance ordinary certificates unnecessarily reveal their binary representation. We propose a certificate-based framework comprising protocol definitions and API specifications for controlled, i.e., well-specified, release of data. This includes controlled release during the certification of data and controlled release of certified data. The protocols are based on proofs of knowledge of certificates and relations over the attributes, ensuring that no side information but only the specified data is revealed. Furthermore, the protocols allow for releasing certified data in plain or encrypted form and allow one to prove general expressions over the data items. Our framework can be seen as a generalization of anonymous credential systems, group signature, traceable signature, and e-cash schemes. The framework encompasses a specification language that allows one to precisely specify what data to release and how to release them in the protocols. We show how our framework can be implemented cryptographically and how a privacy-enhanced PKI that integrates into today’s PKI on the Internet can be built using the framework. We consider our framework a central building block to achieve privacy on the Internet. 1

Trusting a computer for a security-sensitive task (such as checking email or banking online) requires the user to know something about the computer’s state. We examine research on securely capturing a computer’s state, and consider the utility of this information both for improving security on the local computer (e.g., to convince the user that her computer is not infected with malware) and for communicating a remote computer’s state (e.g., to enable the user to check that a web server will adequately protect her data). Although the recent “Trusted Computing ” initiative has drawn both positive and negative attention to this area, we consider the older and broader topic of bootstrapping trust in a computer. We cover issues ranging from the wide collection of secure hardware that can serve as a foundation for trust, to the usability issues that arise when trying to convey computer state information to humans. This approach unifies disparate research efforts and highlights opportunities for additional work that can guide real-world improvements in computer security. 1

Abstract. Trusted Computing Platforms provide the functionality of remote attestation, i.e. attesting the configuration and status of a system to a remote entity. Remote attestation hereby proves integrity and authenticity of system environments. This is crucial for policy enforcement, which in turn is needed in many usage scenarios, e.g., DRM. However, applying remote attestation solely allows masquerading attacks. These attacks are possible since the concept of remote attestation does not provide any means for establishing secured communication channels. In this paper we describe this kind of attacks against protocols for remote attestation and present a protocol for preventing masquerading attacks. 1

Self-hashing has been proposed as a technique for verifying software integrity. Appealing aspects of this approach to software tamper resistance include the promise of being able to verify the integrity of software independent of the external support environment, as well as the ability to integrate code protection mechanisms automatically. In this paper, we show that the rich functionality of most modern general-purpose processors (including UltraSparc, x86, PowerPC, AMD64, Alpha, and ARM) facilitate an automated, generic attack which defeats such self-hashing. We present a general description of the attack strategy and multiple attack implementations that exploit different processor features. Each of these implementations is generic in that it can defeat self-hashing employed by any user-space program on a single platform. Together, these implementations defeat self-hashing on most modern general-purpose processors. The generality and efficiency of our attack suggests that self-hashing is not a viable strategy for high-security tamper resistance on modern computer systems.

Based on the notion of accumulators, we propose a new cryptographic scheme called universal accumulators. This scheme enables one to commit to a set of values using a short accumulator and to efficiently compute a membership witness of any value that has been accumulated. Unlike traditional accumulators, this scheme also enables one to efficiently compute a nonmembership witness of any value that has not been accumulated. We give a construction for universal accumulators and prove its security based on the strong RSA assumption. We further present a construction for dynamic universal accumulators; this construction allows one to dynamically add and delete inputs with constant computational cost. Our construction directly builds upon Camenisch and Lysyanskaya’s dynamic accumulator scheme. Universal accumulators can be seen as an extension to dynamic accumulators with support of nonmembership witness. We also give an efficient zero-knowledge proof protocol for proving that a committed value is not in the accumulator. Our dynamic universal accumulator construction enables efficient membership revocation in an anonymous fashion.

A large fraction of email spam, distributed denial-ofservice (DDoS) attacks, and click-fraud on web advertisements are caused by traffic sent from compromised machines that form botnets. This paper posits that by identifying human-generated traffic as such, one can service it with improved reliability or higher priority, mitigating the effects of botnet attacks. The key challenge is to identify human-generated traffic in the absence of strong unique identities. We develop NAB (“Not-A-Bot”), a system to approximately identify and certify human-generated activity. NAB uses a small trusted software component called an attester, which runs on the client machine with an untrusted OS and applications. The attester tags each request with an attestation if the request is made within a small amount of time of legitimate keyboard or mouse activity. The remote entity serving the request sends the request and attestation to a verifier, which checks the attestation and implements an application-specific policy for attested requests. Our implementation of the attester is within the Xen hypervisor. By analyzing traces of keyboard and mouse activity from 328 users at Intel, together with adversarial traces of spam, DDoS, and click-fraud activity, we estimate that NAB reduces the amount of spam that currently passes through a tuned spam filter by more than 92%, while not flagging any legitimate email as spam. NAB delivers similar benefits to legitimate requests under DDoS and click-fraud attacks. 1

Abstract: Despite the popularity of adding sensors to mobile devices, the readings provided by these sensors cannot be trusted. Users can fabricate sensor readings with relatively little effort. This lack of trust discourages the emergence of applications where users have an incentive to lie about their sensor readings, such as falsifying a location or altering a photo taken by the camera. This paper presents a broad range of applications that would benefit from the deployment of trusted sensors, from participatory sensing to monitoring energy consumption. We present two design alternatives for making sensor readings trustworthy. Although both designs rely on the presence of a trusted platform module (TPM), they trade-off security guarantees for hardware requirements. While our first design is less secure, it requires no additional hardware beyond a TPM, unlike our second design. Finally, we present the privacy issues arising from the deployment of trusted sensors and we discuss protocols that can overcome them. 1.

Abstract. Opportunistic sensing allows applications to “task ” mobile devices to measure context in a target region. For example, one could leverage sensorequipped vehicles to measure traffic or pollution levels on a particular street, or users ’ mobile phones to locate (Bluetooth-enabled) objects in their neighborhood. In most proposed applications, context reports include the time and location of the event, putting the privacy of users at increased risk—even if a report has been anonymized, the accompanying time and location can reveal sufficient information to deanonymize the user whose device sent the report. We propose AnonySense, a general-purpose architecture for leveraging users’ mobile devices for measuring context, while maintaining the privacy of the users. AnonySense features multiple layers of privacy protection—a framework for nodes to receive tasks anonymously, a novel blurring mechanism based on tessellation and clustering to protect users ’ privacy against the system while reporting context, and k-anonymous report aggregation to improve the users ’ privacy against applications receiving the context. We outline the architecture and security properties of AnonySense, and focus on evaluating our tessellation and clustering algorithm against real mobility traces. 1