Introducing Accountability to Anonymity Networks

Many anonymous communication (AC) networks rely on routing traffic through proxy nodes to obfuscate the originator of the traffic. Without an accountability mechanism, exit proxy nodes risk sanctions by law enforcement if users commit illegal actions through the AC network. We present BackRef, a generic mechanism for AC networks that provides practical repudiation for the proxy nodes by tracing back the selected outbound traffic to the predecessor node (but not in the forward direction) through a cryptographically verifiable chain. It also provides an option for full (or partial) traceability back to the entry node or even to the corresponding user when all intermediate nodes are cooperating. Moreover, to maintain a good balance between anonymity and accountability, the protocol incorporates whitelist directories at exit proxy nodes. BackRef offers improved deployability over the related work, and introduces a novel concept of pseudonymous signatures that may be of independent interest. We exemplify the utility of BackRef by integrating it into the onion routing (OR) protocol, and examine its deployability by considering several system-level aspects. We also present the security definitions for the BackRef system (namely, anonymity, backward traceability, no forward traceability, and no false accusation) and conduct a formal security analysis of the OR protocol with BackRef using ProVerif, an automated cryptographic protocol verifier, establishing the aforementioned security properties against a strong adversarial model

Leak-Free Mediated Group Signatures

Group signatures are a useful cryptographic construct for privacy-preserving non-repudiable authentication, and there have been many group signature schemes. In this paper, we introduce a variant of group signatures that offers two new security properties called leak-freedom and immediate-revocation. Intuitively, the former ensures that an insider (i.e., an authorized but malicious signer) be unable to convince an outsider (e.g., signature receiver) that she indeed signed a certain message; whereas the latter ensures that the authorization for a user to issue group signatures can be immediately revoked whenever the need arises (temporarily or permanently). These properties are not offered in existing group signature schemes, nor captured by their security definitions. However, these properties might be crucial to a large class of enterprise-centric applications because they are desirable from the perspective of the enterprises who adopt group signatures or are the group signatures liability-holders (i.e., will be hold accountable for the consequences of group signatures). In addition to introducing these new securit

Can an Industry Voluntary Agreement on Food Traceability Minimize the Cost of Food Safety Incidents?

In the recent past the United States has had a number of severe food-safety outbreaks in the produce, vegetable and beef industry that greatly disrupted the food system. In all these outbreaks here were severe disruptions on sales that affected the whole industry, and it took an extended period of time to correctly locate the source of the outbreak. Traceability can be an effective tool to reduce the impact of food safety incidents my expediting the search for the origin of outbreaks. This paper investigates to what extent an industry-led voluntary agreement for providing traceability can reduce the cost of a food-safety outbreak. We find that a voluntary agreement on traceability can successfully reduce the cost of a food-safety outbreak but will unlikely achieve the optimal social level of traceability because of significant free riding.Traceability, voluntary agreements, food safety, Food Consumption/Nutrition/Food Safety,

Foundations of Fully Dynamic Group Signatures

Group signatures allow members of a group to anonymously sign on behalf of the group. Membership is administered by a designated group manager. The group manager can also reveal the identity of a signer if and when needed to enforce accountability and deter abuse. For group signatures to be applicable in practice, they need to support fully dynamic groups, i.e., users may join and leave at any time. Existing security definitions for fully dynamic group signatures are informal, have shortcomings, and are mutually incompatible. We fill the gap by providing a formal rigorous security model for fully dynamic group signatures. Our model is general and is not tailored toward a specific design paradigm and can therefore, as we show, be used to argue about the security of different existing constructions following different design paradigms. Our definitions are stringent and when possible incorporate protection against maliciously chosen keys. We consider both the case where the group management and tracing signatures are administered by the same authority, i.e., a single group manager, and also the case where those roles are administered by two separate authorities, i.e., a group manager and an opening authority. We also show that a specialization of our model captures existing models for static and partially dynamic schemes. In the process, we identify a subtle gap in the security achieved by group signatures using revocation lists. We show that in such schemes new members achieve a slightly weaker notion of traceability. The flexibility of our security model allows to capture such relaxation of traceability

Accountable infrastructure and its impact on internet security and privacy

The Internet infrastructure relies on the correct functioning of the basic underlying protocols, which were designed for functionality. Security and privacy have been added post hoc, mostly by applying cryptographic means to different layers of communication. In the absence of accountability, as a fundamental property, the Internet infrastructure does not have a built-in ability to associate an action with the responsible entity, neither to detect or prevent misbehavior. In this thesis, we study accountability from a few different perspectives. First, we study the need of having accountability in anonymous communication networks as a mechanism that provides repudiation for the proxy nodes by tracing back selected outbound traffic in a provable manner. Second, we design a framework that provides a foundation to support the enforcement of the right to be forgotten law in a scalable and automated manner. The framework provides a technical mean for the users to prove their eligibility for content removal from the search results. Third, we analyze the Internet infrastructure determining potential security risks and threats imposed by dependencies among the entities on the Internet. Finally, we evaluate the feasibility of using hop count filtering as a mechanism for mitigating Distributed Reflective Denial-of-Service attacks, and conceptually show that it cannot work to prevent these attacks.Die Internet-Infrastrutur stützt sich auf die korrekte Ausführung zugrundeliegender Protokolle, welche mit Fokus auf Funktionalität entwickelt wurden. Sicherheit und Datenschutz wurden nachträglich hinzugefügt, hauptsächlich durch die Anwendung kryptografischer Methoden in verschiedenen Schichten des Protokollstacks. Fehlende Zurechenbarkeit, eine fundamentale Eigenschaft Handlungen mit deren Verantwortlichen in Verbindung zu bringen, verhindert jedoch, Fehlverhalten zu erkennen und zu unterbinden. Diese Dissertation betrachtet die Zurechenbarkeit im Internet aus verschiedenen Blickwinkeln. Zuerst untersuchen wir die Notwendigkeit für Zurechenbarkeit in anonymisierten Kommunikationsnetzen um es Proxyknoten zu erlauben Fehlverhalten beweisbar auf den eigentlichen Verursacher zurückzuverfolgen. Zweitens entwerfen wir ein Framework, das die skalierbare und automatisierte Umsetzung des Rechts auf Vergessenwerden unterstützt. Unser Framework bietet Benutzern die technische Möglichkeit, ihre Berechtigung für die Entfernung von Suchergebnissen nachzuweisen. Drittens analysieren wir die Internet-Infrastruktur, um mögliche Sicherheitsrisiken und Bedrohungen aufgrund von Abhängigkeiten zwischen den verschiedenen beteiligten Entitäten zu bestimmen. Letztlich evaluieren wir die Umsetzbarkeit von Hop Count Filtering als ein Instrument DRDoS Angriffe abzuschwächen und wir zeigen, dass dieses Instrument diese Art der Angriffe konzeptionell nicht verhindern kann

Foundations of Fully Dynamic Group Signatures

Group signatures are a central cryptographic primitive that has received a considerable amount of attention from the cryptographic community. They allow members of a group to anonymously sign on behalf of the group. Membership is overseen by a designated group manager. There is also a tracing authority that can revoke anonymity by revealing the identity of the signer if and when needed, to enforce accountability and deter abuse. For the primitive to be applicable in practice, it needs to support fully dynamic groups, i.e. users can join and leave at any time. In this work we take a close look at existing security definitions for fully dynamic group signatures. We identify a number of shortcomings in existing security definitions and fill the gap by providing a formal rigorous security model for the primitive. Our model is general and is not tailored towards a specific design paradigm and can therefore, as we show, be used to argue about the security of different existing constructions following different design paradigms. Our definitions are stringent and when possible incorporate protection against maliciously chosen keys. In the process, we identify a subtle issue inherent to one design paradigm, where new members might try to implicate older ones by means of back-dated signatures. This is not captured by existing models. We propose some inexpensive fixes for some existing constructions to avoid the issue

MLAS: Multiple level authentication scheme for VANETs

The vehicular ad hoc network (VANET) is an emerging type of network which enables vehicles on roads to inter-communicate for driving safety. The basic idea is to allow arbitrary vehicles to broadcast ad hoc messages (e.g. traffic accidents) to other vehicles. However, this raises the concern of security and privacy. Messages should be signed and verified before they are trusted while the real identity of vehicles should not be revealed, but traceable by authorized party. Existing solutions either rely too heavily on a tamper-proof hardware device, or do not have an effective message verification scheme. In this paper, we propose a multiple level authentication scheme which still makes use of tamper-proof devices but the strong assumption that a long-term system master secret is preloaded into all tamper-proof devices is removed. Instead the master secret can be updated if needed to increase the security level. On the other hand, messages sent by vehicles are classified into two types - regular messages and urgent messages. Regular messages can be verified by neighboring vehicles by means of Hash-based Message Authentication Code (HMAC) while urgent messages can only be verified with the aid of RSUs nearby by means of a conditional privacy-preserving authentication scheme. Copyright 2011 ACM.postprintThe 6th ACM Symposium on Information, Computer and Communications Security (ASIACCS 2011), Hong Kong, China, 22-24 March 2011. In Proceedings of 6th ACM ASIACCS, 2011, p. 471-47