Onion-Location makes it easy for websites offering onion service access to support automatic discovery in Tor Browser of the random-looking onion address associated with their domain. We provide the first measurement study of how many websites are currently using Onion-Location. We also describe the open-source tools we created to conduct the study. Onion-Location has been criticized elsewhere for its lack of transparency and vulnerability to blocking. Perhaps even more troubling, we show that Onion-Location is vulnerable to very accurate fingerprinting. We present recommended changes to and alternatives to Onion-Location as well as steps towards even more secure onion discovery and association.

We introduce a logic for reasoning about contextual trust for web addresses, provide a Kripke semantics for it, and prove its soundness under reasonable assumptions about principals' policies. Self-Authenticating Traditional Addresses (SATAs) are valid DNS addresses or URLs that are generally meaningful—to both humans and web infrastructure—and contain a commitment to a public key in the address itself. Trust in web addresses is currently established via domain name registration, TLS certificates, and other hierarchical elements of the internet infrastructure. SATAs support such structural roots of trust but also complementary contextual roots associated with descriptive properties. The existing structural roots leave web connections open to a variety of well-documented and significant hijack vulnerabilities. Contextual trust roots provide, among other things, stronger resistance to such vulnerabilities. We also consider labeled SATAs, which include descriptive properties such as that a SATA is an address for a news organization, a site belonging to a particular government or company, a site with information about a certain topic, etc. Our logic addresses both trust in the bound together identity of the address and trust in the binding of labels to it. Our logic allows reasoning about delegation of trust with respect to specified labels, relationships between labels that provide more or less specific information, and the interaction between these two aspects. In addition to soundness, we prove that if a principal trusts a particular identity (possibly with label), then either this trust is initially assumed, or there is a trust chain of delegations to this from initial trust assumptions. We also present an algorithm that effectively derives all possible trust statements from the set of initial trust assumptions and show it to be sound, complete, and terminating.

We analyze deficiencies in Tor's positional weighting system, identifying cases in which the system either fails to produce valid weights or fails to properly load balance across positions. We describe how an attacker can take advantage of these failures to reduce Tor's performance, thereby also easing censorship and surveillance through a denial-of-service attack. Our attacks exploit incorrectly determined positional-weight equations by adding new capacity to the network or, for even more covertness, by just minor changes in the status of existing malicious relays. Our analysis of past Tor consensuses shows that these attacks could have reduced the throughput of the network by as much as 45% due only to their triggering of Tor's flawed position weights. Rather than a mere patch to Tor's currently ad hoc scheme, we then propose a new, systematic method for deriving positional weights and propose two goal sets generated using that method. We derive new sets of weights, prove that they satisfy these goal sets, and give examples of how they would change the weights from the current system. Tor could use our results to quickly fix the main deficiencies of its positional weights as well as adopt a better approach long-term.

Abstract The security of the web improved greatly throughout the last couple of years. A large majority of the web is now served encrypted as part of HTTPS, and web browsers accordingly moved from positive to negative security indicators that warn the user if a connection is insecure. A secure connection requires that the server presents a valid certificate that binds the domain name in question to a public key. A certificate used to be valid if signed by a trusted Certificate Authority (CA), but web browsers like Google Chrome and Apple’s Safari have additionally started to mandate Certificate Transparency (CT) logging to overcome the weakest-link security of the CA ecosystem. Tor and the Firefox-based Tor Browser have yet to enforce CT. In this paper, we present privacy-preserving and incrementally-deployable designs that add support for CT in Tor. Our designs go beyond the currently deployed CT enforcements that are based on blind trust: if a user that uses Tor Browser is man-in-the-middled over HTTPS, we probabilistically detect and disclose cryptographic evidence of CA and/or CT log misbehavior. The first design increment allows Tor to play a vital role in the overall goal of CT: detect mis-issued certificates and hold CAs accountable. We achieve this by randomly cross-logging a subset of certificates into other CT logs. The final increments hold misbehaving CT logs accountable, initially assuming that some logs are benign and then without any such assumption. Given that the current CT deployment lacks strong mechanisms to verify if log operators play by the rules, exposing misbehavior is important for the web in general and not just Tor. The full design turns Tor into a system for maintaining a probabilistically-verified view of the CT log ecosystem available from Tor’s consensus. Each increment leading up to it preserves privacy due to and how we use Tor.

The 19th Workshop on Privacy in the Electronic Society (WPES 2020) was held as a virtual conference on 9 November, 2020, in conjunction with the 27th ACM Conference on Computer and Communication Security (CCS 2020). The goal of WPES is to bring together privacy researchers and practitioners to discuss the privacy problems that arise in an interconnected society and solutions to those problems. The program for the workshop contains 12 full papers and 3 short papers selected from a total of 34 submissions. Specific topics covered in the program include but are not limited to: communication privacy, data anonymization, differential privacy, medical privacy, mobile privacy, privacy engineering, privacy policies, user perception of privacy, and Web privacy.

Tor’s growing popularity and user diversity has resulted in network performance problems that are not well understood, though performance is understood to be a significant factor in Tor’s security. A large body of work has attempted to solve performance problems without a complete understanding of where congestion occurs in Tor. In this article, we first study congestion in Tor at individual relays as well as along the entire end-to-end Tor path and find that congestion occurs almost exclusively in egress kernel socket buffers. We then analyze Tor’s socket interactions and discover two major contributors to Tor’s congestion: Tor writes sockets sequentially, and Tor writes as much as possible to each socket. To improve Tor’s performance, we design, implement, and test KIST: a new socket management algorithm that uses real-time kernel information to dynamically compute the amount to write to each socket while considering all circuits of all writable sockets when scheduling cells. We find that, in the medians, KIST reduces circuit congestion by more than 30%, reduces network latency by 18%, and increases network throughput by nearly 10%. We also find that client and relay performance with KIST improves as more relays deploy it and as network load and packet loss rates increase. We analyze the security of KIST and find an acceptable performance and security tradeoff, as it does not significantly affect the outcome of well-known latency, throughput, and traffic correlation attacks. KIST has been merged and configured as the default socket scheduling algorithm in Tor version 0.3.2.1-alpha (released September 18, 2017) and became stable in Tor version 0.3.2.9 (released January 9, 2018). While our focus is Tor, our techniques and observations should help analyze and improve overlay and application performance, both for security applications and in general.

Abstract We present PeerFlow, a system to securely load balance client traffic in Tor. Security in Tor requires that no adversary handle too much traffic. However, Tor relays are run by volunteers who cannot be trusted to report the relay bandwidths, which Tor clients use for load balancing. We show that existing methods to determine the bandwidths of Tor relays allow an adversary with little bandwidth to attack large amounts of client traffic. These methods include Tor’s current bandwidth-scanning system, TorFlow, and the peer-measurement system EigenSpeed. We present an improved design called PeerFlow that uses a peer-measurement process both to limit an adversary’s ability to increase his measured bandwidth and to improve accuracy. We show our system to be secure, fast, and efficient. We implement PeerFlow in Tor and demonstrate its speed and accuracy in large-scale network simulations.

Abstract Motivated by the effectiveness of correlation attacks against Tor, the censorship arms race, and observations of malicious relays in Tor, we propose that Tor users capture their trust in network elements using probability distributions over the sets of elements observed by network adversaries. We present a modular system that allows users to efficiently and conveniently create such distributions and use them to improve their security. To illustrate this system, we present two novel types of adversaries. First, we study a powerful, pervasive adversary that can compromise an unknown number of Autonomous System organizations, Internet Exchange Point organizations, and Tor relay families. Second, we initiate the study of how an adversary might use Mutual Legal Assistance Treaties (MLATs) to enact surveillance. As part of this, we identify submarine cables as a potential subject of trust and incorporate data about these into our MLAT analysis by using them as a proxy for adversary power. Finally, we present preliminary experimental results that show the potential for our trust framework to be used by Tor clients and services to improve security.

We present the first analysis of the popular Tor anonymity network that indicates the security of typical users against reasonably realistic adversaries in the Tor network or in the underlying Internet. Our results show that Tor users are far more susceptible to compromise than indicated by prior work. Specific contributions of the paper include(1)a model of various typical kinds of users,(2)an adversary model that includes Tor network relays, autonomous systems(ASes), Internet exchange points (IXPs), and groups of IXPs drawn from empirical study,(3) metrics that indicate how secure users are over a period of time,(4) the most accurate topological model to date of ASes and IXPs as they relate to Tor usage and network configuration,(5) a novel realistic Tor path simulator (TorPS), and(6)analyses of security making use of all the above. To show that our approach is useful to explore alternatives and not just Tor as currently deployed, we also analyze a published alternative path selection algorithm, Congestion-Aware Tor. We create an empirical model of Tor congestion, identify novel attack vectors, and show that it too is more vulnerable than previously indicated.

We perform a probabilistic analysis of onion routing. The analysis is presented in a black-box model of anonymous communication in the Universally Composable (UC) framework that abstracts the essential properties of onion routing in the presence of an active adversary who controls a portion of the network and knows all a priori distributions on user choices of destination. Our results quantify how much the adversary can gain in identifying users by exploiting knowledge of their probabilistic behavior. In particular, we show that, in the limit as the network gets large, a user u 's anonymity is worst either when the other users always choose the destination u is least likely to visit or when the other users always choose the destination u chooses. This worst-case anonymity with an adversary that controls a fraction b of the routers is shown to be comparable to the best-case anonymity against an adversary that controls a fraction √ b .

No abstract available.

Onion routing was invented more than fifteen years ago to separate identification from routing in network communication. Since that time there has been much design, analysis, and deployment of onion routing systems. This has been accompanied by much confusion about what these systems do, what security they provide, how they work, who built them, and even what they are called. Here I give an overview of onion routing from its earliest conception to some of the latest research, including the design and use of Tor, a global onion routing network with about a half million users on any given day.

We introduce a novel model of routing security that incorporates the ordinarily overlooked variations in trust that users have for different parts of the network. We focus on anonymous communication, and in particular onion routing, although we expect the approach to apply more broadly.

No abstract available.

Tor is an anonymous communications network with thousands of router nodes worldwide. An intuition reflected in much of the literature on anonymous communications is that, as an anonymity network grows, it becomes more secure against a given observer because the observer will see less of the network. In particular, as the Tor network grows from volunteers operating relays all over the world, it becomes less and less likely for a single autonomous system (AS) to be able to observe both ends of an anonymous connection. Yet, as the network continues to grow significantly, no analysis has been done to determine if this intuition is correct. Further, modifications to Tor's path selection algorithm to help clients avoid an AS-level observer have not been proposed and analyzed.

We consider using trust information to improve the anonymity provided by onion-routing networks. In particular, we introduce a model of trust in network nodes and use it to design path-selection strategies that minimize the probability that the adversary can successfully control the entrance to and exit from the network. This minimizes the chance that the adversary can observe and correlate patterns in the data flowing over the path and thereby deanonymize the user. We first describe the general case in which onion routers can be assigned arbitrary levels of trust. Selecting a strategy can be formulated in a straightforward way as a linear program, but it is exponential in size. We thus analyze a natural simplification of path selection for this case. More importantly, however, when choosing routes in practice, only a very coarse assessment of trust in specific onion routers is likely to be feasible. Therefore, we focus next on the special case in which there are only two trust levels. For this more practical case we identify three optimal route-selection strategies such that at least one is optimal, depending on the trust levels of the two classes, their size, and the reach of the adversary. This can yield practical input into routing decisions. We set out the relevant parameters and choices for making such decisions.

Hidden services were deployed on the Tor anonymous communication network in 2004. Announced properties include server resistance to distributed DoS. Both the EFF and Reporters Without Borders have issued guides that describe using hidden services via Tor to protect the safety of dissidents as well as to resist censorship. We present fast and cheap attacks that reveal the location of a hidden server. Using a single hostile Tor node we have located deployed hidden servers in a matter of minutes. Although we examine hidden services over Tor, our results apply to any client using a variety of anonymity networks. In fact, these are the first actual intersection attacks on any deployed public network: thus confirming general expectations from prior theory and simulation. We recommend changes to route selection design and implementation for Tor. These changes require no operational increase in network overhead and are simple to make; but they prevent the attacks we have demonstrated. They have been implemented

We study the problem of characterizing the wormhole attack, an attack that can be mounted on a wide range of wireless network protocols without compromising any cryptographic quantity or network node. A wormhole, in essence, creates a communication link between an origin and a destination point that could not exist with the use of the regular communication channel. Hence, a wormhole modifies the connectivity matrix of the network, and can be described by a graph abstraction of the ad hoc network. Making use of geometric random graphs induced by the communication range constraint of the nodes, we present the necessary and sufficient conditions for detecting and defending against wormholes. Using our theory, we also present a defense mechanism based on local broadcast keys. We believe our work is the first one to present analytical calculation of the probabilities of detection. We also present simulation results to illustrate our theory.

Although research has been going on in the formal analysis of cryptographic protocols for a number of years, they are only slowly being integrated into the protocol design process. In this paper we describe how we furthered the integration of analysi

Although there is a substantial amount of work on formal requirements for two and three-party key distribution protocols, very little has been done on requirements for group protocols. However, since the latter have security requirements that can differ in important but subtle ways, we believe that a rigorous expression of these requirements can be useful in determining whether a given protocol can satisfy an application's needs. In this paper we make a first step in providing a formal understanding of security requirements for group key distribution by using the NPATRL language, a temporal requirement specification language for use with the NRL Protocol Analyzer. We specify the requirements for GDOI, a protocol being proposed as an IETF standard, which we are formally specifying and verifying in cooperation with the MSec working group.

We present a protocol for unlinkable serial transactions suitable for a variety of network-based subscription services. It is the first protocol to use cryptographic blinding to enable subscription services. The protocol prevents the service from tracking the behavior of its customers, while protecting the service vendor from abuse due to simultaneous or cloned use by a single subscriber. Our basic protocol structure and recovery protocol are robust against failure in protocol termination. We evaluate the security of the basic protocol and extend the basic protocol to include auditing, which further deters subscription sharing. We describe other applications of unlinkable serial transactions for pay-per-use trans subscription, third-party subscription management, multivendor coupons, proof of group membership, and voting.

No abstract available.

The paper presents applications for the weak protection of secrets in which weakness is not just acceptable but desirable. For one application, two versions of a lottery scheme are presented in which the result of the lottery is determined by the ticket numbers purchased, but no one can control the outcome or determine what it is until after the lottery closes. This is because the outcome is kept secret in a way that is breakable after a predictable amount of time and/or computation. Another presented application is a variant on fair exchange protocols that requires no trusted third party at all.

Onion routing is an infrastructure for private communication over a public network. It provides anonymous connections that are strongly resistant to both eavesdropping and traffic analysis. Onion routing's anonymous connections are bidirectional, near real-time, and can be used anywhere a socket connection can be used. Any identifying information must be in the data stream carried over an anonymous connection. An onion is a data structure that is treated as the destination address by onion routers; thus, it is used to establish an anonymous connection. Onions themselves appear different to each onion router as well as to network observers. The same goes for data carried over the connections they establish. Proxy-aware applications, such as Web browsers and e-mail clients, require no modification to use onion routing, and do so through a series of proxies. A prototype onion routing network is running between our lab and other sites. This paper describes anonymous connections and their implementation using onion routing. This paper also describes several application proxies for onion routing, as well as configurations of onion routing networks.

We discuss various aspects of secure distributed computation and look at weakening both the goals of such computation and the assumed capabilities of adversaries. We present a new protocol for a conditional form of probabilistic coordination and present a model of secure distributed computation in which friendly and hostile nodes are represented in competing interwoven networks of nodes. It is suggested that reasoning about goals, risks, tradeoffs, etc. for this model be done in a game theoretic framework.

Onion routing provides anonymous connections that are strongly resistant to both eavesdropping and traffic analysis. Unmodified Internet applications can use these anonymous connections by means of proxies. The proxies may also make communication anonymous by removing identifying information from the data stream. Onion routing has been implemented on Sun Solaris 2.X with proxies for Web browsing, remote logins and e-mail. This paper's contribution is a detailed specification of the implemented onion routing system, a vulnerability analysis based on this specification, and performance results.

This paper describes a communications primitive, anonymous connections, that supports bidirectional and near real-time channels that are resistant to both eavesdropping and traffic analysis. The connections are made anonymous, although communication

Using traffic analysis, it is possible to infer who is talking to whom over a public network. This paper describes a flexible communications infrastructure, called onion routing, which is resistant to traffic analysis. Onion routing lies just beneath the application layer, and is designed to interface with a wide variety of unmodified Internet services by means of proxies. Onion routing has been implemented on a Sun Solaris 2.4; in addition, proxies for World Wide Web browsing (HTTP), remote logins (RLOGIN), e-mail (SMTP) and file transfers (FTP) have been implemented. Onion routing provides application-independent, real-time and bi-directional anonymous connections that are resistant to both eavesdropping and traffic analysis. Applications making use of onion routing's anonymous connections may (and usually should) identify their users over the anonymous connection. User anonymity may be layered on top of the anonymous connections by removing identifying information from the data stream. Our goal is anonymous connections, not anonymous communication. The use of a packet-switched public network should not automatically reveal who is talking to whom; this is the traffic analysis that onion routing complicates.

The goal of this panel is to discuss what we mean by `attack´ and to discuss what sorts of assumptions and definitions are necessary before we can give a clear answer to that question.

Recent papers have taken a new look at cryptographic protocols from the perspective of proposing design principles. For years, the main approach to cryptographic protocols has been logical, and a number of papers have examined the limitations of those logics. This paper takes a similar cautionary look at the design principle approach. Limitations and exceptions are offered on some of the previously given basic design principals. The focus is primarily on public key protocols, especially on the order of signature and encryption, but other principles are discussed as well. Apparently secure protocols that fail to meet principles are presented. Also presented are new attacks on protocols as well as previously claimed attacks which are not.

We set out a logic for reasoning about multilevel security of probabilistic systems. This logic includes modalities for time, knowledge, and probability. In earlier work we gave syntactic definitions of multilevel security and showed that their semantic interpretations are equivalent to independently motivated information-theoretic definitions. This paper builds on that earlier work in two ways. First, it substantially recasts the language and model of computation into the more standard Halpern-Tuttle framework for reasoning about knowledge and probability. Second, it brings together two distinct characterizations of security from that work. One was equivalent to the information-theoretic security criterion for a system to be free of covert channels but was difficult to prove. The other was a verification condition that implied the first; it was more easily provable but was too strong. This paper presents a characterization that is syntactically very similar to our previous verification condition but is proven to be semantically equivalent to the security criterion. The new characterization also means that our security criterion is expressible in a simpler logic and model.

This paper presents a taxonomy of replay attacks on cryptographic protocols in terms of message origin and destination. The taxonomy is independent of any method used to analyze or prevent such attacks. It is also complete in the sense that any replay attack is composed entirely of elements classified by the taxonomy. The classification of attacks is illustrated using both new and previously known attacks on protocols. The taxonomy is also used to discuss the appropriateness of particular countermeasures and protocol analysis methods to particular kinds of replays.>

We present a logic for analyzing cryptographic protocols. This logic encompasses a unification of four of its predecessors in the BAN family of logics, namely those given by Li Gong et al. (1990); M. Abadi, M. Tuttle (1991); P.C. van Oorschot (1993); and BAN itself (M. Burrows et al., 1989). We also present a model-theoretic semantics with respect to which the logic is sound. The logic presented captures all of the desirable features of its predecessors and more; nonetheless, it accomplishes this with no more axioms or rules than the simplest of its predecessors.>

: In [BAN89] Burrows, Abadi, and Needham presented a logic (BAN) for analyzing cryptographic protocols in terms of belief. This logic is quite useful in uncovering flaws in protocols; however, it also has produced confusion and controversy. Much of the confusion was cleared up when Abadi and Tuttle provided a semantics for a version of that logic (AT) in [AT91]. In this paper we present a protocol to show that both BAN and AT are not expressive enough to capture all of the kinds of flaws that appear to be within their scope. We then present a logic that adds temporal formalisms to AT and that is rich enough to reveal the flaws in the presented protocol; nonetheless, this logic is sound with respect to the same semantics that was given in [AT91]. Finally, we argue that any approach of this type is inadequate by itself to demonstrate the absence of such flaws. We must supplement the formal logic with semantic analysis techniques. 1 Introduction This paper presents a class of attacks on...

A formal language is presented for specifying and reasoning about cryptographic protocol requirements. Examples of simple sets of requirements in that language are given. The authors examine two versions of a protocol that might meet those requirements and show how to specify them in the language of the NRL Protocol Analyzer. They also show how to map one of the sets of formal requirements to the language of the NRL Protocol Analyzer and use the Analyzer to show that one version of the protocol meets those requirements. The Analyzer is used as a model checker to assess the validity of the formulas that make up the requirements.>

A second-order modal logic for reasoning about multilevel security in probabilistic systems is proposed. A possible world semantics is presented, and it is proved that the logic is sound with respect to it. The semantics is novel in treating probability measures themselves as possible worlds. After giving a syntatic definition of security, it is shown that the semantic interpretation of the syntactic definition is equivalent to an earlier independently motivated characterization called probabilistic noninterference due to J. W. Gray, III (1991). The authors examine a syntatic representation of Gray's applied flow model and discuss the relation between these characterizations of security and between their usefulness in security analysis. A syntatic description of a round-robin server and a sketch of the formal proof of its security are also provided.>

We resolve a debate over the appropriateness for cryptographic protocol analysis of formalisms representing knowledge vs. those representing belief by showing that they are equally adequate for protocol analysis on the logical level. We discuss the s

The author distinguishes between 'heuristic' and 'holistic' issues in the formal analysis of cryptographic protocols and discusses the contribution semantics can make to settling these issues.>

Logics for cryptographic protocol analysis are presented, and a study is made of the protocol features that they are appropriate for analyzing: some are appropriate for analyzing trust, others security. It is shown that both features can be adequately captured by a single properly designed logic. The goals and capabilities of M. Burrows, M. Abadi and R. Needham's (1989) BAN logic are examined. It is found that there is confusion about these. While the logic is extremely useful heuristically, as a formal method it is seen to be ultimately unacceptable. Formal semantics is explored as a reasoning tool and the importance of soundness and completeness for protocol security is discussed. The KPL logic is used to resolve a debate over an alleged flaw in BAN logic and is shown to be uniquely capable of dealing with certain protocol security issues.>

A logic and associated formal semantics specifically designed to represent and analyze cryptographic protocols are presented. A language is given with distinct means to represent knowledge of an individual word (e.g., the ability to recognize or produce a decryption key) and propositional knowledge. A sample analysis of a protocol is given to demonstrate the potential usefulness of the system.>