Payment channels (PC) are a promising solution to the scalability issue of cryptocurrencies, allowing users to perform the bulk of the transactions off-chain without needing to post everything on the blockchain. Many PC proposals however, suffer from a severe limitation: Both parties need to constantly monitor the blockchain to ensure that the other party did not post an outdated transaction. If this event happens, the honest party needs to react promptly and engage in a punishment procedure. This means that prolonged absence periods (e.g., a power outage) may be exploited by malicious users. As a mitigation, the community has introduced watchtowers, a third-party monitoring the blockchain on behalf of off-line users. Unfortunately, watchtowers are either trusted, which is critical from a security perspective, or they have to lock a certain amount of coins, called collateral, for each monitored PC in order to be held accountable, which is financially infeasible for a large network. We present Sleepy Channels, the first bi-directional PC protocol without watchtowers (or any other third party) that supports an unbounded number of payments and does not require parties to be persistently online. The key idea is to confine the period in which PC updates can be validated on-chain to a short, pre-determined time window, which is when the PC parties have to be online. This behavior is incentivized by letting the parties lock a collateral in the PC, which can be adjusted depending on their mutual trust and which they get back much sooner if they are online during this time window. Our protocol is compatible with any blockchain that is capable of verifying digital signatures (e.g., Bitcoin), as shown by our proof of concept. Moreover, our experimental results show that Sleepy Channels impose a communication and computation overhead similar to state-of-the-art PC protocols while removing watchtower's collateral and fees for the monitoring service.

Most blockchain-based cryptocurrencies suffer from a heavily limited transaction throughput, which is a barrier to their growing adoption. Payment channel networks (PCNs) are one of the promising solutions to this problem. PCNs reduce the on-chain load of transactions and increase the throughput by processing many payments off-chain. In fact, any two users connected via a path of payment channels (i.e., joint addresses between the two channel end-points) can perform payments, and the underlying blockchain is used only when there is a dispute between users. Unfortunately, payments in PCNs can only be conducted securely along a path, which prevents the design of many interesting applications. Moreover, the most widely used implementation, the Lightning Network in Bitcoin, suffers from a collateral lock time linear in the path length, it is affected by security issues, and it relies on specific scripting features called Hash Timelock Contracts that hinders the applicability of the underlying protocol in other blockchains. In this work, we present Thora, the first Bitcoin-compatible off-chain protocol that enables the atomic update of arbitrary channels (i.e., not necessarily forming a path). This enables the design of a number of new off-chain applications, such as payments across different PCNs sharing the same blockchain, secure and trustless crowdfunding, and channel rebalancing. Our construction requires no specific scripting functionalities other than digital signatures and timelocks, thereby being applicable to a wider range of blockchains. We formally define security and privacy in the Universal Composability framework and show that our cryptographic protocol is a realization thereof. In our performance evaluation, we show that our construction requires only constant collateral, independently from the number of channels, and has only a moderate off-chain communication as well as computation overhead.

This report documents the program and the outcomes of Dagstuhl Seminar 21431 “Rigorous Methods for Smart Contracts”. Blockchain technologies have emerged as an exciting field for both researchers and practitioners focusing on formal guarantees for software. It is arguably a “once in a lifetime” opportunity for rigorous methods to be integrated in audit processes for parties deploying smart contracts, whether for fund raising, securities trading, or supply-chain management. Smart contracts are programs managing cryptocurrency accounts on a blockchain. Research in the area of smart contracts includes a fascinating combination of formal methods, programming-language semantics, and cryptography. First, there is vibrant development of verification and program-analysis techniques that check the correctness of smart-contract code. Second, there are emerging designs of programming languages and methodologies for writing smart contracts such that they are more robust by construction or more amenable to analysis and verification. Programming-language abstraction layers expose low-level cryptographic primitives enabling developers to design high-level cryptographic protocols. Automated-reasoning mechanisms present a common underlying enabler; and the specific needs of the smart-contract world offer new challenges. This workshop brought together stakeholders in the aforementioned areas related to advancing reliable smart-contract technologies.

Coin mixing services allow users to mix their cryptocurrency coins and thus enable unlinkable payments in a way that prevents tracking of honest users' coins by both the service provider and the users themselves. The easy bootstrapping of new users and backwards compatibility with cryptocurrencies (such as Bitcoin) with limited support for scripts are attractive features of this architecture, which has recently gained considerable attention in both academia and industry. A recent work of Tairi et al. [IEEE S&P 2021] formalizes the notion of a coin mixing service and proposes A2L, a new cryptographic protocol that simultaneously achieves high efficiency and interoperability. In this work, we identify a gap in their formal model and substantiate the issue by showing two concrete counterexamples: we show how to construct two encryption schemes that satisfy their definitions but lead to a completely insecure system. To amend this situation, we investigate secure constructions of coin mixing services. First, we develop the notion of blind conditional signatures (BCS), which acts as the cryptographic core for coin mixing services. We propose game-based security definitions for BCS and propose A2L+, a modified version of the protocol by Tairi et al. that satisfies our security definitions. Our analysis is in an idealized model (akin to the algebraic group model) and assumes the hardness of the one-more discrete logarithm problem. Finally, we propose A2L-UC, another construction of BCS that achieves the stronger notion of UC-security (in the standard model), albeit with a significant increase in computation cost. This suggests that constructing a coin mixing service protocol secure under composition requires more complex cryptographic machinery than initially thought.

Payment-channel networks (PCN) are the most prominent approach to tackle the scalability issues of current permissionless blockchains. A PCN reduces the load on-chain by allowing arbitrarily many off-chain multi-hop payments (MHPs) between any two users connected through a path of payment channels. Unfortunately, current MHP protocols are far from satisfactory. One-round MHPs (e.g., Interledger) are insecure as a malicious intermediary can steal the payment funds. Two-round MHPs (e.g., Lightning Network (LN)) follow the 2-phase-commit paradigm as in databases to overcome this issue. However, when tied with economical incentives, 2-phase-commit brings other security threats (i.e., wormhole attacks), staggered collateral (i.e., funds are locked for a time proportional to the payment path length) and dependency on specific scripting language functionality (e.g., Hash Time-Lock Contracts) that hinders a wider deployment in practice. We present Blitz, a novel MHP protocol that demonstrates for the first time that we can achieve the best of the two worlds: a single round MHP where no malicious intermediary can steal coins. Moreover, Blitz provides the same privacy for sender and receiver as current MHP protocols do, is not prone to the wormhole attack and requires only constant collateral. Additionally, we construct MHPs using only digital signatures and a timelock functionality, both available at the core of virtually every cryptocurrency today. We provide the cryptographic details of Blitz and we formally prove its security. Furthermore, our experimental evaluation on a LN snapshot shows that (i) staggered collateral in LN leads to in between 4x and 33x more unsuccessful payments than the constant collateral in Blitz; (ii) Blitz reduces the size of the payment contract by 26%; and (iii) Blitz prevents up to 0.3 BTC (3397 USD in October 2020) in fees being stolen over a three day period as it avoids wormhole attacks by design.

Current permissionless cryptocurrencies such as Bitcoin suffer from a limited transaction rate and slow confirmation time, which hinders their large scale adoption. Payment channels are one of the most promising solutions to address these problems, as they allow two end-points of the channel to perform arbitrarily many payments in a peer-to-peer fashion while uploading only two transactions on the blockchain. This concept has been generalized into payment-channel networks where a path of payment channels is used to settle the payment between two users that might not share a channel between them. However, this approach requires the active involvement of each user in the path, making the system less reliable (they might be offline), more expensive (they charge fees per payment) and slower (intermediaries need to be actively involved in the payment). To mitigate this issue, recent work has introduced the concept of virtual channels, which involve intermediaries only in the initial creation of a bridge between payer and payee, who can later on independently perform arbitrarily many off-chain transactions. Unfortunately, existing constructions are only available for Ethereum, as they rely on its account model and Turing-complete scripting language. The realization of virtual channels in other blockchain technologies with limited scripting capabilities, like Bitcoin, was considered so far an open challenge. In this work, we present the first virtual channel protocols that are built on the UTXO-model and require a script language supporting only a digital signature scheme and a timelock functionality, being thus backwards compatible with virtually every cryptocurrency, including Bitcoin. We formalize the security properties of virtual channels as an ideal functionality in the Universal Composability framework, and prove that our protocol constitutes a secure realization thereof. We have prototyped and evaluated our protocol on the Bitcoin blockchain, demonstrating its efficiency: for n sequential payments, they require an off-chain exchange of 11+2⋅(n−1) transactions or a total of 4219+695⋅(n−1) bytes, with no on-chain footprint in the optimistic case.

Adaptor signatures (AS) are an extension of digital signatures that enable the encoding of a cryptographic hard problem (e.g., discrete logarithm) within the signature itself. An AS scheme ensures that (i) the signature can be created only by the user knowing the solution to the cryptographic problem; (ii) the signature reveals the solution itself; (iii) the signature can be verified with the standard verification algorithm. These properties have made AS a salient building block for many blockchain applications, in particular, off-chain payment systems such as payment-channel networks, payment-channel hubs, atomic swaps or discrete log contracts. Current AS constructions, however, are not secure against adversaries with access to a quantum computer. In this work, we present IAS, a construction for adaptor signatures that relies on standard cryptographic assumptions for isogenies, and builds upon the isogeny-based signature scheme CSI-FiSh. We formally prove the security of IAS against a quantum adversary. We have implemented IAS and our evaluation shows that IAS can be incorporated into current blockchains while requiring ∼1500 bytes of storage size on-chain and ∼140 milliseconds for digital signature verification. We also show how IAS can be seamlessly leveraged to build post-quantum off-chain payment applications without harming their security and privacy.

Payment channel hubs (PCHs) constitute a promising solution to the inherent scalability problems of blockchain technologies, allowing for off-chain payments between sender and receiver through an intermediary, called the tumbler. While state-of-the-art PCHs provide security and privacy guarantees against a malicious tumbler, they do so by relying on the scripting-based functionality available only at few cryptocurrencies, and they thus fall short of fundamental properties such as backwards compatibility and efficiency. In this work, we present Trilero, the first PCH protocol to achieve all aforementioned properties. Trilero builds upon A2L, a novel cryptographic primitive that realizes a three-party protocol for conditional transactions, where the tumbler pays the receiver only if the latter solves a cryptographic challenge with the help of the sender, which implies the sender has paid the tumbler. We prove the security and privacy guarantees of A2L (which carry over to Trilero) in the Universal Composability framework and present a provably secure instantiation based on adaptor signatures. We implemented A2L and compared it to TumbleBit, the state-of-the-art Bitcoin-compatible PCH. Asymptotically, A2L has a communication complexity that is constant, as opposed to linear in the security parameter like in TumbleBit. In practice, A2L requires ∼33x less bandwidth than TumleBit, while retaining the computational cost (or providing 2x speedup with a preprocessing technique). This demonstrates that A2L (and thus Trilero) is ready to be deployed today. In theory, we demonstrate for the first time that it is possible to design a secure and privacy-preserving PCH while requiring only digital signatures and timelock functionality from the underlying scripting language. In practice, this result makes Trilero backwards compatible with virtually all cryptocurrencies available today, even those offering a highly restricted form of scripting language such as Ripple or Stellar. The practical appealing of Trilero has resulted in a proof-of-concept implementation in the COMIT Network, a blockchain technology focused on cross-currency payments.

Related-domain attackers control a sibling domain of their target web application, e.g., as the result of a subdomain takeover. Despite their additional power over traditional web attackers, related-domain attackers received only limited attention by the research community. In this paper we define and quantify for the first time the threats that related-domain attackers pose to web application security. In particular, we first clarify the capabilities that related-domain attackers can acquire through different attack vectors, showing that different instances of the related-domain attacker concept are worth attention. We then study how these capabilities can be abused to compromise web application security by focusing on different angles, including: cookies, CSP, CORS, postMessage and domain relaxation. By building on this framework, we report on a large-scale security measurement on the top 50k domains from the Tranco list that led to the discovery of vulnerabilities in 887, sites, where we quantified the threats posed by related-domain attackers to popular web applications.

Service workers boost the user experience of modern web applications by taking advantage of the Cache API to improve responsiveness and support offline usage. In this paper, we present the first security analysis of the threats posed by this programming practice, identifying an attack with major security implications. In particular, we show how a traditional XSS attack can abuse the Cache API to escalate into a person-in-the-middle attack against cached content, thus compromising its confidentiality and integrity. Remarkably, this attack enables new threats which are beyond the scope of traditional XSS. After defining the attack, we study its prevalence in the wild, finding that the large majority of the sites which register service workers using the Cache API are vulnerable as long as a single webpage in the same origin of the service worker is affected by an XSS. Finally, we propose a browser-side countermeasure against this attack, and we analyze its effectiveness and practicality in terms of security benefits and backward compatibility with existing web applications.

Bitcoin (BTC) pseudonyms (layer 1) can effectively be deanonymized using heuristic clustering techniques. However, while performing transactions off-chain (layer 2) in the Lightning Network (LN) seems to enhance privacy, a systematic analysis of the anonymity and pri-vacy leakages due to the interaction between the two layers is missing. We present clustering heuristics that group BTC addresses, based on their in-teraction with the LN, as well as LN nodes, based on shared naming and hosting information. We also present linking heuristics that link 45.97% of all LN nodes to 29.61% BTC addresses interacting with the LN. These links allow us to attribute information (e.g., aliases, IP addresses) to 21.19% of the BTC addresses contributing to their deanonymization. Further, these deanonymization results suggest that the security and privacy of LN payments are weaker than commonly believed, with LN users being at the mercy of as few as five actors that control 36 nodes and over 33% of the total capacity. Overall, this is the first paper to present a method for linking LN nodes with BTC addresses across layers and to discuss privacy and security implications.

Decentralized and permissionless ledgers offer an inherently low transaction rate, as a result of their consensus protocol demanding the storage of each transaction on-chain. A prominent proposal to tackle this scalability issue is to utilize off-chain protocols, where parties only need to post a limited number of transactions on-chain. Existing solutions can roughly be categorized into: (i) application-specific channels (e.g., payment channels), offering strictly weaker functionality than the underlying blockchain; and (ii) state channels, supporting arbitrary smart contracts at the cost of being compatible only with the few blockchains having Turing-complete scripting languages (e.g., Ethereum). In this work, we introduce and formalize the notion of generalized channels allowing users to perform any operation supported by the underlying blockchain in an off-chain manner. Generalized channels thus extend the functionality of payment channels and relax the definition of state channels. We present a concrete construction compatible with any blockchain supporting transaction authorization, time-locks and constant number of Boolean ∧ and ∨ operations -- requirements fulfilled by many (non-Turing-complete) blockchains including the popular Bitcoin. To this end, we leverage adaptor signatures -- a cryptographic primitive already used in the cryptocurrency literature but formalized as a standalone primitive in this work for the first time. We formally prove the security of our generalized channel construction in the Universal Composability framework. As an important practical contribution, our generalized channel construction outperforms the state-of-the-art payment channel construction, the Lightning Network, in efficiency. Concretely, it halves the off-chain communication complexity and reduces the on-chain footprint in case of disputes from linear to constant in the number of off-chain applications funded by the channel. Finally, we evaluate the practicality of our construction via a prototype implementation and discuss various applications including financially secured fair two-party computation.

The widespread adoption of decentralized cryptocurrencies, such as Bitcoin or Ethereum, is currently hindered by their inherently limited transaction rate. One of the most prominent proposals to tackle this scalability issue are payment channels which allow mutually distrusted parties to exchange an arbitrary number of payments in the form of off-chain authenticated messages while posting only a limited number of transactions onto the blockchain. Specifically, two transactions suffice, unless a dispute between these parties occurs, in which case more on-chain transactions are required to restore the correct balance. Unfortunately, popular constructions, such as the Lightning network for Bitcoin, suffer from heavy communication complexity both off-chain and on-chain in case of dispute. Concretely, the communication overhead grows exponentially and linearly, respectively, in the number of applications that run in the channel. In this work, we introduce and formalize the notion of generalized channels for Bitcoin-like cryptocurrencies. Generalized channels significantly extend the concept of payment channels so as to perform off-chain any operation supported by the underlying blockchain. Besides the gain in expressiveness, generalized channels outperform state-of-the-art payment channel constructions in efficiency, reducing the communication complexity and the on-chain footprint in case of disputes to linear and constant, respectively. We provide a cryptographic instantiation of generalized channels that is compatible with Bitcoin, leveraging adaptor signatures -- a cryptographic primitive already used in the cryptocurrency literature but formalized as a standalone primitive in this work for the first time. We formally prove the security of our construction in the Universal Composability framework. Furthermore, we conduct an experimental evaluation, demonstrating the expressiveness and performance of generalized channels when used as building blocks for popular off-chain applications, such as channel splitting and payment-channel networks.

Session management is a fundamental component of web applications: despite the apparent simplicity, correctly implementing web sessions is extremely tricky, as witnessed by the large number of existing attacks. This motivated the design of formal methods to rigorously reason about web session security which, however, are not supported at present by suitable automated verification techniques. In this paper we introduce the first security type system that enforces session security on a core model of web applications, focusing in particular on server-side code. We showcase the expressiveness of our type system by analyzing the session management logic of HotCRP, Moodle, and phpMyAdmin, unveiling novel security flaws that have been acknowledged by software developers.

Ethereum has emerged as the most popular smart contract platform, with hundreds of thousands of contracts stored on the blockchain and covering diverse application scenarios, such as auctions, trading platforms, or elections. Given the financial nature of smart contracts, security vulnerabilities may lead to catastrophic consequences and, even worse, can hardly be fixed as data stored on the blockchain, including the smart contract code itself, are immutable. An automated security analysis of these contracts is thus of utmost interest, but at the same time technically challenging. This is as e.g., Ethereum's transaction-oriented programming mechanisms feature a subtle semantics, and since the blockchain data at execution time, including the code of callers and callees, are not statically known. In this work, we present eThor, the first sound and automated static analyzer for EVM bytecode, which is based on an abstraction of the EVM bytecode semantics based on Horn clauses. In particular, our static analysis supports reachability properties, which we show to be sufficient for capturing interesting security properties for smart contracts (e.g., single-entrancy) as well as contract-specific functional properties. Our analysis is proven sound against a complete semantics of EVM bytecode, and a large-scale experimental evaluation on real-world contracts demonstrates that eThor is practical and outperforms the state-of-the-art static analyzers: specifically, eThor is the only one to provide soundness guarantees, terminates on 94% of a representative set of real-world contracts, and achieves an F-measure (which combines sensitivity and specificity) of 89%.

Ethereum smart contracts are distributed programs running on top of the Ethereum blockchain. Since program flaws can cause significant monetary losses and can hardly be fixed due to the immutable nature of the blockchain, there is a strong need of automated analysis tools which provide formal security guarantees. Designing such analyzers, however, proved to be challenging and error-prone. We review the existing approaches to automated, sound, static analysis of Ethereum smart contracts and highlight prevalent issues in the state of the art. Finally, we overview eThor, a recent static analysis tool that we developed following a principled design and implementation approach based on rigorous semantic foundations to overcome the problems of past works

Payment channel networks have been introduced to mitigate the scalability issues inherent to permissionless decentralized cryptocurrencies such as Bitcoin. Launched in 2018, the Lightning Network (LN) has been gaining popularity and consists today of more than 5000 nodes and 35000 payment channels that jointly hold 965 bitcoins (9.2M USD as of June 2020). This adoption has motivated research from both academia and industryPayment channels suffer from security vulnerabilities, such as the wormhole attack [39], anonymity issues [38], and scalability limitations related to the upper bound on the number of concurrent payments per channel [28], which have been pointed out by the scientific community but never quantitatively analyzedIn this work, we first analyze the proneness of the LN to the wormhole attack and attacks against anonymity. We observe that an adversary needs to control only 2% of nodes to learn sensitive payment information (e.g., sender, receiver, and amount) or to carry out the wormhole attack. Second, we study the management of concurrent payments in the LN and quantify its negative effect on scalability. We observe that for micropayments, the forwarding capability of up to 50% of channels is restricted to a value smaller than the channel capacity. This phenomenon hinders scalability and opens the door for denial-of-service attacks: we estimate that a network-wide DoS attack costs within 1.6M USD, while isolating the biggest community costs only 238k USDOur findings should prompt the LN community to consider the issues studied in this work when educating users about path selection algorithms, as well as to adopt multi-hop payment protocols that provide stronger security, privacy and scalability guarantees.

Cloud storage has rapidly become a cornerstone of many IT infrastructures, constituting a seamless solution for the backup, synchronization, and sharing of large amounts of data. Putting user data in the direct control of cloud service providers, however, raises security and privacy concerns related to the integrity of outsourced data, the accidental or intentional leakage of sensitive information, the profiling of user activities and so on. Furthermore, even if the cloud provider is trusted, users having access to outsourced files might be malicious and misbehave. These concerns are particularly serious in sensitive applications like personal health records and credit score systems. To tackle this problem, we present ΠGORAM, a definitional framework for Group Oblivious RAM, in which we formalize several security and privacy properties such as secrecy, integrity, anonymity, and obliviousness. ΠGORAM allows per entry access control, as selected by the data owner. ΠGORAM is the first framework to define such a wide range of security and privacy properties for outsourced storage. Regarding obliviousness, we tackle two different attacker models: our first definition protects against an honest-but-curious server while our second definition protects against such a server colluding with malicious clients. In the latter model, we prove a server-side computational lower bound of Ω(n) where n is the number of entries in the database, i.e., every operations requires to process a constant fraction of the database. Furthermore, we present two constructions: a pure cryptographic instantiation, which achieves an O(n) amortized communication and computation complexity and a construction based on a trusted proxy with logarithmic communication and server-side computational complexity. The second construction bypasses the previously established lower bound leveraging a trusted party. Both schemes achieve secrecy, integrity, and obliviousness with respect to a server colluding with malicious clients, but not anonymity due to the deployed access control mechanism. In the former model, we present a cryptographic system that achieves secrecy, integrity, obliviousness, and anonymity. In the process of designing an efficient construction, we developed three new, generally applicable cryptographic schemes, namely, batched zero-knowledge proof of shuffle correctness, the hash-and-proof paradigm, which even improves upon the former, and an accountability technique based on chameleon signatures, which we consider of independent interest. We implemented our constructions in Amazon Elastic Compute Cloud (EC2) and ran a performance evaluation demonstrating the scalability and efficiency of our construction.

Tremendous growth in cryptocurrency usage is exposing the inherent scalability issues with permis- sionless blockchain technology. Payment-channel networks (PCNs) have emerged as the most widely deployed solution to mitigate the scalability issues, allowing the bulk of payments between two users to be carried out off-chain. Unfortunately, as reported in the literature and further demonstrated in this paper, current PCNs do not provide meaningful security and privacy guarantees [30], [40]. In this work, we study and design secure and privacy- preserving PCNs. We start with a security analysis of exist- ing PCNs, reporting a new attack that applies to all major PCNs, including the Lightning Network, and allows an attacker to steal the fees from honest intermediaries in the same payment path. We then formally define anonymous multi-hop locks (AMHLs), a novel cryptographic primitive that serves as a cornerstone for the design of secure and privacy-preserving PCNs. We present several provably secure cryptographic instantiations that make AMHLs compatible with the vast majority of cryptocurrencies. In particular, we show that (linear) homomorphic one-way functions suffice to construct AMHLs for PCNs supporting a script language (e.g., Ethereum). We also propose a construction based on ECDSA signatures that does not require scripts, thus solving a prominent open problem in the field. AMHLs constitute a generic primitive whose useful- ness goes beyond multi-hop payments in a single PCN and we show how to realize atomic swaps and interoper- able PCNs from this primitive. Finally, our performance evaluation on a commodity machine finds that AMHL operations can be performed in less than 100 millisec- onds and require less than 500 bytes of communication overhead, even in the worst case. In fact, after acknowl- edging our attack, the Lightning Network developers have implemented our ECDSA-based AMHLs into their PCN. This demonstrates the practicality of our approach and its impact on the security, privacy, interoperability, and scalability of today´s cryptocurrencies.

Smart contracts are programs running on cryptocurrency (e.g., Ethereum) blockchains, whose popularity stem from the possibility to perform financial transactions, such as payments and auctions, in a distributed environment without need for any trusted third party. Given their financial nature, bugs or vulnerabilities in these programs may lead to catastrophic consequences, as witnessed by recent attacks. Unfortunately, programming smart contracts is a delicate task that requires strong expertise: Ethereum smart contracts are written in Solidity, a dedicated language resembling JavaScript, and shipped over the blockchain in the EVM bytecode format. In order to rigorously verify the security of smart contracts, it is of paramount importance to formalize their semantics as well as the security properties of interest, in particular at the level of the bytecode being executed. In this paper, we present the first complete small-step semantics of EVM bytecode, which we formalize in the F* proof assistant, obtaining executable code that we successfully validate against the official Ethereum test suite. Furthermore, we formally define for the first time a number of central security properties for smart contracts, such as call integrity, atomicity, and independence from miner controlled parameters. This formalization relies on a combination of hyper- and safety properties. Along this work, we identified various mistakes and imprecisions in existing semantics and verification tools for Ethereum smart contracts, thereby demonstrating once more the importance of rigorous semantic foundations for the design of security verification techniques.

In this work we introduce the notion of Subset Predicate Encryption, a form of attribute-based encryption (ABE) in which a message is encrypted with respect to a set s′ and the resulting ciphertext can be decrypted by a key that is associated with a set s if and only if s⊆s′. We formally define our primitive and identify several applications. We also propose two new constructions based on standard assumptions in bilinear groups; the constructions have very efficient decryption algorithms (consisting of one and two pairing computations, respectively) and small keys: in both our schemes, private keys contain only two group elements. We prove selective security of our constructions without random oracles. We demonstrate the usefulness of Subset Predicate Encryption by describing several black-box transformations to more complex primitives, such as identity-based encryption with wildcards and ciphertext-policy ABE for DNF formulas over a small universe of attributes. All of the resulting schemes are as efficient as the base Subset Predicate Encryption scheme in terms of decryption and key generation.

An ongoing challenge with differentially private database systems is that of maximizing system utility while staying within a certain privacy budget. One approach is to maintain per-user budgets instead of a single global budget, and to silently drop users whose budget is depleted. This, however, can lead to very misleading analyses because the system cannot provide the analyst any information about which users have been dropped. This paper presents UniTraX, the first differentially private system that allows per-user budgets while providing the analyst information about the budget state. The key insight behind UniTraX is that it tracks budget not only for actual records in the system, but at all points in the domain of the database, including points that could exist but do not. UniTraX can safely report the budget state because the analyst does not know if the state refers to actual records or not. We prove that UniTraX is differentially private. UniTraX is compatible with existing differentially private analyses and our implementation on top of PINQ shows only moderate runtime overheads on a realistic workload.

Recently, many tools have been proposed for automatically analysing, in symbolic models, equivalence of security protocols. Equivalence is a property needed to state privacy properties or game-based properties like strong secrecy. Tools for a bounded number of sessions can decide equivalence but typically suffer from efficiency issues. Tools for an unbounded number of sessions like Tamarin or ProVerif prove a stronger notion of equivalence (diff-equivalence) that does not properly handle protocols with else branches. Building upon a recent approach, we propose a type system for reasoning about branching protocols and dynamic keys. We prove our type system to entail equivalence, for all the standard primitives. Our type system has been implemented and shows a significant speedup compared to the tools for a bounded number of sessions, and compares similarly to ProVerif for an unbounded number of sessions. Moreover, we can also prove security of protocols that require a mix of bounded and unbounded number of sessions, which ProVerif cannot properly handle.

Relational properties describe multiple runs of one or more programs. They characterize many useful notions of security, program refinement, and equivalence for programs with diverse computational effects, and they have received much attention in the recent literature. Rather than developing separate tools for special classes of effects and relational properties, we advocate using a general purpose proof assistant as a unifying framework for the relational verification of effectful programs. The essence of our approach is to model effectful computations using monads and to prove relational properties on their monadic representations, making the most of existing support for reasoning about pure programs. We apply this method in F* and evaluate it by encoding a variety of relational program analyses, including information flow control, program equivalence and refinement at higher order, correctness of program optimizations and game-based cryptographic security. By relying on SMT-based automation, unary weakest preconditions, user-defined effects, and monadic reification, we show that, compared to unary properties, verifying relational properties requires little additional effort from the F* programmer.

A functional credential allows a user to anonymously prove possession of a set of attributes that fulfills a certain policy. The policies are arbitrary polynomially computable predicates that are evaluated over arbitrary attributes. The key feature of this primitive is the delegation of verification to third parties, called designated verifiers. The delegation protects the privacy of the policy : A designated verifier can verify that a user satisfies a certain policy without learning anything about the policy itself. We illustrate the usefulness of this property in different applications, including outsourced databases with access control. We present a new framework to construct functional credentials that does not require (non-interactive) zero-knowledge proofs. This is important in settings where the statements are complex and thus the resulting zero-knowledge proofs are not efficient. Our construction is based on any predicate encryption scheme and the security relies on standard assumptions. A complexity analysis and an experimental evaluation confirm the practicality of our approach.

Passwords and access control remain the popular choice for protecting sensitive data stored online, despite their well-known vulnerability to brute-force attacks. A natural solution is to use encryption. Although standard practices of using encryption somewhat alleviate the problem, decryption is often needed for utility, and keeping the decryption key within reach is obviously dangerous. To address this seemingly unavoidable problem in data security, we propose password-hardened encryption (PHE). With the help of an external crypto server, a service provider can recover the user data encrypted by PHE only when an end user supplied a correct password. PHE inherits the security features of passwordhardening (Usenix Security ´15), adding protection for the user data. In particular, the crypto server does not learn any information about any user data. More importantly, both the crypto server and the service provider can rotate their secret keys, a proactive security mechanism mandated by the Payment Card Industry Data Security Standard (PCI DSS). We build an extremely simple password-hardened encryption scheme. Compared with the state-of-the-art password-hardening scheme (Usenix Security ´17), our scheme only uses minimal number-theoretic operations and is, therefore, 30% - 50% more efficient. In fact, our extensive experimental evaluation demonstrates that our scheme can handle more than 525 encryption and (successful) decryption requests per second per core, which shows that it is lightweight and readily deployable in large-scale systems. Regarding security, our scheme also achieves a stronger soundness property, which puts less trust on the good behavior of the crypto server.

We present WPSE, a browser-side security monitor for web protocols designed to ensure compliance with the intended protocol flow, as well as confidentiality and integrity properties of messages. We formally prove that WPSE is expressive enough to protect web applications from a wide range of protocol implementation bugs and web attacks. We discuss concrete examples of attacks which can be prevented by WPSE on OAuth 2.0 and SAML 2.0, including a novel attack on the Google implementation of SAML 2.0 which we discovered by formalizing the protocol specification in WPSE. Moreover, we use WPSE to carry out an extensive experimental evaluation of OAuth 2.0 in the wild. Out of 90 tested websites, we identify security flaws in 55 websites (61.1%), including new critical vulnerabilities introduced by tracking libraries such as Facebook Pixel, all of which fixable by WPSE. Finally, we show that WPSE works flawlessly on 83 websites (92.2%), with the 7 compatibility issues being caused by custom implementations deviating from the OAuth 2.0 specification, one of which introducing a critical vulnerability.

The recent growth of the blockchain technology market puts its main cryptocurrencies in the spotlight. Among them, Ethereum stands out due to its virtual machine (EVM) supporting smart contracts, i.e., distributed programs that control the flow of the digital currency Ether. Being written in a Turing complete language, Ethereum smart contracts allow for expressing a broad spectrum of financial applications. The price for this expressiveness, however, is a significant semantic complexity, which increases the risk of programming errors. Recent attacks exploiting bugs in smart contract implementations call for the design of formal verification techniques for smart contracts. This, however, requires rigorous semantic foundations, a formal characterization of the expected security properties, and dedicated abstraction techniques tailored to the specific EVM semantics. This work will overview the state-of-the-art in smart contract verification, covering formal semantics, security definitions, and verification tools. We will then focus on EtherTrust [1], a framework for the static analysis of Ethereum smart contracts which includes the first complete small-step semantics of EVM bytecode, the first formal characterization of a large class of security properties for smart contracts, and the first static analysis for EVM bytecode that comes with a proof of soundness.

This book constitutes the proceedings of the 6th International Conference on Principles of Security and Trust, POST 2017, which took place in Uppsala, Sweden in April 2017, held as Part of the European Joint Conferences on Theory and Practice of Software, ETAPS 2017. The 14 papers presented in this volume were carefully reviewed and selected from 40 submissions. They were organized in topical sections named: information flow; security protocols; security policies; and information leakage.

PermissionlessblockchainsprotocolssuchasBitcoinareinherently limitedintransactionthroughputandlatency.Currenteffortsto address this key issue focus on off-chain payment channels that canbecombinedinaPayment-ChannelNetwork(PCN)toenable anunlimitednumberofpaymentswithoutrequiringtoaccessthe blockchainotherthantoregistertheinitialandfinalcapacityof eachchannel.Whilethisapproachpavesthewayforlowlatency andhighthroughputofpayments,itsdeploymentinpracticeraises severalprivacyconcernsaswellastechnicalchallengesrelatedto theinherentlyconcurrentnatureofpaymentsthathavenotbeen sufficientlystudiedsofar. In this work, we lay the foundations for privacy and concurrency in PCNs, presenting a formal definition in the Universal Composability framework as well as practical and provably securesolutions.Inparticular,wepresentFulgorandRayo.Fulgor isthefirstpaymentprotocolforPCNsthatprovidesprovableprivacyguaranteesforPCNsandisfullycompatiblewiththeBitcoin scriptingsystem.However,Fulgorisablockingprotocolandtherefore prone to deadlocks of concurrent payments as in currently available PCNs. Instead, Rayo is the first protocol for PCNs that enforcesnon-blocking progress (i.e.,atleastoneoftheconcurrent payments terminates). We show through a new impossibility result that non-blocking progress necessarily comes at the cost of weakerprivacy.AtthecoreofFulgorandRayoisMulti-HopHTLC, anewsmartcontract,compatiblewiththeBitcoinscriptingsystem, thatprovidesconditionalpaymentswhilereducingrunningtime andcommunicationoverheadwithrespecttopreviousapproaches. OurperformanceevaluationofFulgorandRayoshowsthatapaymentwith10intermediateuserstakesasfewas5seconds,thereby demonstratingtheirfeasibilitytobedeployedinpractice.

Maturepushbuttontoolshaveemergedforcheckingtraceproperties(e.g.secrecyorauthentication)ofsecurityprotocols.Thecase ofindistinguishability-basedprivacyproperties(e.g.ballotprivacy oranonymity)ismorecomplexandconstitutesanactiveresearch topicwithseveralrecentpropositionsoftechniquesandtools. Weexploreanovelapproachbasedontypesystemsandprovide a (sound) type system for proving equivalence of protocols, for a bounded or an unbounded number of sessions. The resulting prototype implementation has been tested on various protocols of the literature. It provides a significant speed-up (by orders of magnitude)comparedtotoolsforaboundednumberofsessions andcomplementsintermsofexpressivenessotherstate-of-the-art tools,suchasProVerifandTamarin:e.g.,weshowthatouranalysis techniqueisthefirstonetohandleafaithfulencodingoftheHelios e-votingprotocolinthecontextofanuntrustedballotbox.

Abstract-Credit networks model transitive trust (or credit) between users in a distributed environment and have recently seen a rapid increase of popularity due to their flexible design and robustness against intrusion. They serve today as a backbone of real-world IOweYou transaction settlement networks such as Ripple and Stellar, which are deployed by various banks worldwide, as well as several other systems, such as spamresistant communication protocols and Sybil-tolerant social networks.Currentsolutions,however,raiseseriousprivacyconcerns, asthenetworktopologyaswellasthecreditvalueofthelinksare madepublicforapparenttransparencypurposesandanychanges are logged. In payment scenarios, for instance, this means that all transactions have to be public and everybody knows who paid what to whom. In this work, we question the necessity of a privacy-invasive transaction ledger. In particular, we present SilentWhispers, the first distributed, privacy-preserving credit network that does not require any ledger to protect the integrity of transactions. Yet, SilentWhispers guarantees integrity and privacy of link values and transactions even in the presence of distrustful users and malicious neighbors, whose misbehavior in changing link values is detected and such users can be held accountable. We formalize these properties as ideal functionalities in the universal composability framework and present a secure realization based on a novel combination of secret-sharing-based multiparty computation and digital signature chains. SilentWhispers can handle network churn, and it is efficient as demonstrated with a prototype implementation evaluated using payments data extracted from the currently deployed Ripple payment system.

Oblivious RAM (ORAM) has emerged as an enabling technology to secure cloud-based storage services. The goal of this cryptographic primitive is to conceal not only the data but also the access patterns from the server. While the early constructions focused on a single client scenario, a few recent works have focused on a setting where multiple clients may access the same data, which is crucial to support data sharing applications. All these works, however, either do not consider malicious clients or they significantly constrain the definition of obliviousness and the system´s practicality. It is thus an open question whether a natural definition of obliviousness can be enforced in a malicious multi-client setting and, if so, what the communication and computational lower bounds are. In this work, we formalize the notion of maliciously secure multi-client ORAM, we prove that the server-side computational complexity of any secure realization has to be Ω(n), and we present a cryptographic instantiation of this primitive based on private information retrieval techniques, which achieves an O(√N) communication complexity. We further devise an efficient access control mechanism, built upon a novel and generally applicable realization of plaintext equivalence proofs for ciphertext vectors. Finally, we demonstrate how our lower bound can be bypassed by leveraging a trusted proxy, obtaining logarithmic communication and server-side computational complexity. We implemented our scheme and conducted an experimental evaluation, demonstrating the feasibility of our approach.

Android is today the most popular operating system for mobile phones and tablets, and it boasts the largest application market among all its competitors. Though the huge number of available applications is arguably one of the main reasons for the success of Android, it also poses an important security challenge: there are way too many applications to ensure that they go through a timely and thorough security vetting before their publication on the market. Automated analysis tools thus play a critical role in ensuring that security verification does not fall behind with respect to the release of malicious (or buggy) applications. There are many relevant security concerns for Android applications, e.g., privilege escalation [12], [5] and component hijacking [26], but the most important challenge in the area is arguably information flow control, since Android applications are routinely granted access to personal information and other sensitive data stored on the device where they are installed. To counter the threats posed by malicious applications, the research community has proposed a plethora of increasingly sophisticated (static) information flow control frameworks for Android [41], [42], [27], [14], [22], [3], [40], [15], [7]. Despite all this progress, however, none of these static analysis tools is able to properly reconcile soundness and precision in its treatment of heap-allocated data structures

Progressive Web Applications (PWAs) are the new trend in web development, promising several features and similar advantages as native applications. They heavily rely on modern web APIs to offer an engaging user experience. Service Workers are one of the core technologies employed by PWAs. They work as a proxy server for websites, allowing requests and responses to be modified, cached and served to the browser even when the user is offline. In this work we showcase a number of flaws in the Cache API that allow an attacker to void the security policies put in place by web developers, posing serious security and privacy threats. Given that these attacks are enabled by the presence of Service Workers, we demonstrate the impact of our findings by performing a large-scale analysis on the top 110K websites. Finally, we propose a redesign of the Cache API that prevents all the attacks discussed in the paper.

Current cryptocurrencies provide a heavily limited transaction throughput that is clearly insufficient to cater their growing adop- tion. Payment-channel networks (PCNs) have emerged as an inter- esting solution to the scalability issue and are currently deployed by popular cryptocurrencies such as Bitcoin and Ethereum. While PCNs do increase the transaction throughput by processing pay- ments off-chain and using the blockchain only as a dispute arbitra- tor, they unfortunately require high collateral (i.e., they lock coins for a non-constant time along the payment path) and are restricted to payments in a path from sender to receiver. These issues have severe consequences in practice. The high collateral enables denial- of-service attacks that hamper the throughput and utility of the PCN. Moreover, the limited functionality hinders the applicability of current PCNs in many important application scenarios. Unfortu- nately, current proposals do not solve either of these issues, or they require Turing-complete language support, which severely limit their applicability. In this work, we present AMCU, the first protocol for atomic multi-channel updates and reduced collateral that is compatible with Bitcoin (and other cryptocurrencies with reduced scripting ca- pabilities). We provide a formal model in the Universal Composabil- ity framework and show that AMCU realizes it, thus demonstrating that AMCU achieves atomicity and value privacy. Moreover, the reduced collateral mitigates the consequences of griefing attacks in PCNs while the (multi-payment) atomicity achieved by AMCU opens the door to new applications such as credit rebalancing and crowdfunding that are not possible otherwise. Moreover, our eval- uation results demonstrate that AMCU has a performance in line with that of the Lightning Network (the most widely deployed PCN) and thus is ready to be deployed in practice.

Tremendous growth in cryptocurrency usage is exposing the inherent scalability issues with permis- sionless blockchain technology. Payment-channel networks (PCNs) have emerged as the most widely deployed solution to mitigate the scalability issues, allowing the bulk of payments between two users to be carried out off-chain. Unfortunately, as reported in the literature and further demonstrated in this paper, current PCNs do not provide meaningful security and privacy guarantees [30], [40]. In this work, we study and design secure and privacy- preserving PCNs. We start with a security analysis of exist- ing PCNs, reporting a new attack that applies to all major PCNs, including the Lightning Network, and allows an attacker to steal the fees from honest intermediaries in the same payment path. We then formally define anonymous multi-hop locks (AMHLs), a novel cryptographic primitive that serves as a cornerstone for the design of secure and privacy-preserving PCNs. We present several provably secure cryptographic instantiations that make AMHLs compatible with the vast majority of cryptocurrencies. In particular, we show that (linear) homomorphic one-way functions suffice to construct AMHLs for PCNs supporting a script language (e.g., Ethereum). We also propose a construction based on ECDSA signatures that does not require scripts, thus solving a prominent open problem in the field. AMHLs constitute a generic primitive whose useful- ness goes beyond multi-hop payments in a single PCN and we show how to realize atomic swaps and interoper- able PCNs from this primitive. Finally, our performance evaluation on a commodity machine finds that AMHL operations can be performed in less than 100 millisec- onds and require less than 500 bytes of communication overhead, even in the worst case. In fact, after acknowl- edging our attack, the Lightning Network developers have implemented our ECDSA-based AMHLs into their PCN. This demonstrates the practicality of our approach and its impact on the security, privacy, interoperability, and scalability of today's cryptocurrencies.