1. The Honey Badger of BFT Protocols 2016 BFT Blockchains CCS eprint.iacr.org
    Andrew Miller, Yu Xia, Kyle Croman, Elaine Shi, Dawn Song

    The surprising success of cryptocurrencies has led to a surge of interest in deploying large scale, highly robust, Byzantine fault tolerant (BFT) proto- cols for mission-critical applications, such as finan- cial transactions. Although the conventional wisdom is to build atop a (weakly) synchronous protocol such as PBFT (or a variation thereof), such protocols rely critically on network timing assumptions, and only guarantee liveness when the network behaves as ex- pected. We argue these protocols are ill-suited for this deployment scenario.

    We present an alternative, HoneyBadgerBFT, the first practical asynchronous BFT protocol, which guarantees liveness without making any timing as- sumptions. We base our solution on a novel atomic broadcast protocol that achieves optimal asymptotic efficiency. We present an implementation and ex- perimental results to show our system can achieve throughput of tens of thousands of transactions per second, and scales to over a hundred nodes on a wide area network. We even conduct BFT experi- ments over Tor, without needing to tune any parame- ters. Unlike the alternatives, HoneyBadgerBFT sim- ply does not care about the underlying network.

  2. A Secure Sharding Protocol For Open Blockchains 2016 Blockchains CCS CryptocurrencyScaling people.cs.georgetown.edu
    Loi Luu, Viswesh Narayanan, Chaodong Zheng, Kunal Baweja, Seth Gilbert, Prateek Saxena

    Cryptocurrencies, such as Bitcoin and 250 similar alt-coins, embody at their core a blockchain protocol — a mechanism for a distributed network of computational nodes to periodically agree on a set of new transactions. Designing a secure blockchain protocol relies on an open challenge in security, that of designing a highly-scalableagreement protocol open to manipulation by byzantine or arbitrarily malicious nodes. Bitcoin’s blockchain agreement protocol exhibits security, but does not scale: it processes 3–7 transactions per second at present, irrespective of the available computation capacity at hand.

    In this paper, we propose a new distributed agreement protocol for permission-less blockchains called ELASTICO. ELASTICO scales transaction rates almost linearly with available computation for mining: the more the computation power in the network, the higher the number of transaction blocks selected per unit time. ELASTICO is efficient in its network messages and tolerates byzantine adversaries of up to one-fourth of the total computational power. Technically, ELASTICO uniformly partitions or parallelizes the mining network (securely) into smaller committees, each of which processes a disjoint set of transactions (or “shards”). While sharding is common in non-byzantine settings, ELASTICO is the first candidate for a secure sharding protocol with presence of byzantine adversaries. Our scalability experiments on Amazon EC2 with up to $1, 600$ nodes confirm ELASTICO’s theoretical scaling properties.

  3. On the Security and Performance of Proof of Work Blockchains 2016 Blockchains CCS ProofOfWork eprint.iacr.org
    Arthur Gervais, Ghassan O. Karame, Karl Wüst, Vasileios Glykantzis, Hubert Ritzdorf, Srdjan Capkun

    Proof of Work (PoW) powered blockchains currently account for more than 90% of the total market capitalization of existing digital currencies. Although the security provisions of Bitcoin have been thoroughly analysed, the security guarantees of variant (forked) PoW blockchains (which were instantiated with different parameters) have not received much attention in the literature.

    In this paper, we introduce a novel quantitative framework to analyse the security and performance implications of various consensus and network parameters of PoW blockchains. Based on our framework, we devise optimal adversarial strategies for double-spending and selfish mining while taking into account real world constraints such as network propagation, different block sizes, block generation intervals, information propagation mechanism, and the impact of eclipse attacks. Our framework therefore allows us to capture existing PoW-based deployments as well as PoW blockchain variants that are instantiated with different parameters, and to objectively compare the tradeoffs between their performance and security provisions.

  4. Protecting the 4G and 5G Cellular Paging Protocols against Security and Privacy Attacks 2020 CellularProtocols PETS petsymposium.org
    Ankush Singla, Syed Rafiul Hussain, Omar Chowdhury, Elisa Bertino, Ninghui Li

    This paper focuses on protecting the cellular paging protocol — which balances between the quality-of-service and battery consumption of a device— against security and privacy attacks. Attacks against this protocol can have severe repercussions, for instance,allowing attacker to infer a victim’s location, leak a victim’s IMSI, and inject fabricated emergency alerts.To secure the protocol, we first identify the underlying design weaknesses enabling such attacks and then pro-pose efficient and backward-compatible approaches to address these weaknesses. We also demonstrate the deployment feasibility of our enhanced paging protocol by implementing it on an open-source cellular protocol library and commodity hardware. Our evaluation demonstrates that the enhanced protocol can thwart attacks without incurring substantial overhead.

  5. Discontinued Privacy: Personal Data Leaks in Apple Bluetooth-Low-Energy Continuity Protocols 2020 Bluetooth PETS WirelessProtocols petsymposium.org
    Guillaume Celosia, Mathieu Cunche

    Apple Continuity protocols are the underlying network component of Apple Continuity services which allow seamless nearby applications such as activity and file transfer, device pairing and sharing a network connection. Those protocols rely on Bluetooth Low Energy (BLE) to exchange information between devices: Apple Continuity messages are embedded in the pay-load of BLE advertisement packets that are periodically broadcasted by devices. Recently, Martin et al. identified [1] a number of privacy issues associated with Apple Continuity protocols; we show that this was just the tip of the iceberg and that Apple Continuity protocols leak a wide range of personal information. In this work, we present a thorough reverse engineering of Apple Continuity protocols that we use to uncover a collection of privacy leaks. We introduce new artifacts, including identifiers, counters and battery levels, that can be used for passive tracking, and describe a novel active tracking attack based on Handoff messages. Beyond tracking issues, we shed light on severe privacy flaws. First, in addition to the trivial exposure of device characteristics and status, we found that HomeKit accessories betray human activities in a smarthome. Then, we demonstrate that AirDrop and Nearby Action protocols can be leveraged by passive observers to recover e-mail addresses and phone numbers of users. Finally, we exploit passive observations on the advertising traffic to infer Siri voice commands of a user.

  6. Computation on Encrypted Data using Dataflow Authentication 2020 AuthenticatedEncryption PETS petsymposium.org
    Andreas Fischer, Benny Fuhry, Florian Kerschbaum, and Eric Bodden

    Encrypting data before sending it to the cloud protects it against hackers and malicious insiders, but requires the cloud to compute on encrypted data. Trusted (hardware) modules, e.g., secure enclaves like Intel’s SGX, can very efficiently run entire programs in encrypted memory. However, it already has been demonstrated that software vulnerabilities give an attacker ample opportunity to insert arbitrary code into the program. This code can then modify the data flow of the program and leak any secret in the program to an observer in the cloud via SGX side-channels. Since any larger program is rife with software vulnerabilities, it is not a good idea to outsource entire programs to an SGX enclave. A secure alternative with a small trusted code base would be fully homomorphic encryption (FHE) – the holy grail of encrypted computation. However, due to its high computational complexity it is unlikely to be adopted in the near future. As a result researchers have made several proposals for transforming programs to perform encrypted computations on less powerful encryption schemes. Yet, current approaches fail on programs that make control-flow decisions based on encrypted data. In this paper, we introduce the concept of data flow authentication (DFAuth). DFAuth prevents an adversary from arbitrarily deviating from the data flow of a program. Hence, an attacker cannot perform an attack as outlined before on SGX. This enables that all programs, even those including operations on control-flow decision variables, can be computed on encrypted data. We implemented DFAuth using a novel authenticated homomorphic encryption scheme, a Java bytecode-to-bytecode compiler producing fully executable programs, and SGX enclaves. A transformed neural network that performs machine learning on sensitive medical data can be evaluated on encrypted inputs and encrypted weights in 0.86 seconds.

  7. Economy Class Crypto: Exploring Weak Cipher Usage in Avionic Communications via ACARS 2017 Attacks FinancialCryptography PhysicalSystems fc17.ifca.ai
    Matthew Smith, Daniel Moser, Martin Strohmeier, Vincent Lenders, Ivan Martinovic

    Recent research has shown that a number of existing wireless avionic systems lack encryption and are thus vulnerable to eavesdropping and message injection attacks. The Aircraft Communications Addressing and Reporting System (ACARS) is no exception to this rule with 99% of the traffic being sent in plaintext. However, a small portion of the traffic coming mainly from privately-owned and government aircraft is encrypted, indicating a stronger requirement for security and privacy by those users. In this paper, we take a closer look at this protected communication and analyze the cryptographic solution being used. Our results show that the cipher used for this encryption is a mono-alphabetic substitution cipher, broken with little effort. We assess the impact on privacy and security to its unassuming users by characterizing months of real-world data, decrypted by breaking the cipher and recovering the keys. Our results show that the decrypted data leaks privacy sensitive information including existence, intent and status of aircraft owners.

  8. A Simpler Rate-Optimal CPIR Protocol 2017 FinancialCryptography PIR fc17.ifca.ai
    Helger Lipmaa, Kateryna Pavlyk

    In PETS 2015, Kiayias, Leonardos, Lipmaa, Pavlyk, and Tang proposed the first (n, 1)-CPIR protocol with rate 1−𝑜(1). They use advanced techniques from multivariable calculus (like the Newton-Puiseux algorithm) to establish optimal rate among a large family of different CPIR protocols. It is only natural to ask whether one can achieve similar rate but with a much simpler analysis. We propose parameters to the earlier (n, 1)-CPIR protocol of Lipmaa (ISC 2005), obtaining a CPIR protocol that is asymptotically almost as communication-efficient as the protocol of Kiayias et al. However, for many relevant parameter choices, it is slightly more communication-efficient, due to the cumulative rounding errors present in the protocol of Kiayias et al. Moreover, the new CPIR protocol is simpler to understand, implement, and analyze. The new CPIR protocol can be used to implement (computationally inefficient) FHE with rate 1−𝑜(1).

  9. Why Banker Bob (still) can't get TLS right: A Security Analysis of TLS in Leading UK Banking Apps 2017 FinancialCryptography TLS fc17.ifca.ai
    Tom Chothia, Flavio Garcia, Christopher Heppel, Christopher McMahon-Stone

    This paper presents a security review of the mobile apps provided by the UK’s leading banks; we focus on the connections the apps make, and the way in which TLS is used. We apply existing TLS testing methods to the apps which only find errors in legacy apps. We then go on to look at extensions of these methods and find five of the apps have serious vulnerabilities. In particular, we find an app that pins a TLS root CA certificate, but do not verify the hostname. In this case, the use of certificate pinning means that all existing test methods would miss detecting the hostname verification flaw. We also find one app that doesn’t check the certificate hostname, but bypasses proxy settings, resulting in failed detection by pentesting tools. We find that three apps load adverts over insecure connections, which could be exploited for in-app phishing attacks. Some of the apps used the users’ PIN as authentication, for which PCI guidelines require extra security, so these apps use an additional cryptographic protocol; we study the underlying protocol of one banking app in detail and show that it provides little additional protection, meaning that an active man-in-the-middle attacker can retrieve the user’s credentials, login to the bank and perform every operation the legitimate user could.

  10. Formal Modeling and Verification for Domain Validation and ACME 2017 FinancialCryptography FormalVerification fc17.ifca.ai
    Karthikeyan Bhargavan, Antoine Delignat-Lavaud, Nadim Kobeissi

    Web traffic encryption has shifted from applying only to highly sensitive websites (such as banks) to a majority of all Web requests. Until recently, one of the main limiting factors for enabling HTTPS is the requirement to obtain a valid certificate from a trusted certification authority, a tedious process that typically involves fees and ad-hoc key generation, certificate request and domain validation procedures. To remove this barrier of entry, the Internet Security Research Group created Let’s Encrypt, a new non-profit certificate authority which uses a new protocol called Automatic Certificate Management Environment (ACME) to automate certificate management at all levels (request, validation , issuance, renewal, and revocation) between clients (website operators) and servers (certificate authority nodes). Let’s Encrypt’s success is measured by its issuance of over 12 million free certificates since its launch in April 2016. In this paper, we survey the existing process for issuing domain-validated certificates in major certification authorities to build a security model of domain-validated certificate issuance. We then model the ACME protocol in the applied pi-calculus and verify its stated security goals against our threat model of domain validation. We compare the effective security of different domain validation methods and show that ACME can be secure under a stronger threat model than that of traditional CAs. We also uncover weaknesses in some flows of ACME 1.0 and propose verified improvements that have been adopted in the latest protocol draft submitted to the IETF.

  11. Unilaterally-Authenticated Key Exchange 2017 FinancialCryptography KeyExchange fc17.ifca.ai
    Yevgeniy Dodis, Dario Fiore

    Key Exchange (KE), which enables two parties (e.g., a client and a server) to securely establish a common private key while communicating over an insecure channel, is one of the most fundamental cryptographic primitives. In this work, we address the setting of unilaterally-authenticated key exchange (UAKE), where an unauthenticated (unkeyed) client establishes a key with an authenticated (keyed) server. This setting is highly motivated by many practical uses of KE on the Internet, but received relatively little attention so far.

    Unlike the prior work, defining UAKE by downgrading a relatively complex definition of mutually authenticated key exchange (MAKE), our definition follows the opposite approach of upgrading existing definitions of public key encryption (PKE) and signatures towards UAKE. As a result, our new definition is short and easy to understand. Nevertheless, we show that it is equivalent to the UAKE definition of Bellare-Rogaway (when downgraded from MAKE), and thus captures a very strong and widely adopted security notion, while looking very similar to the simple ``one-oracle’’ definition of traditional PKE/signature schemes. As a benefit of our intuitive framework, we show two exactly-as-you-expect (i.e., having no caveats so abundant in the KE literature!) UAKE protocols from (possibly interactive) signature and encryption. By plugging various one- or two-round signature and encryption schemes, we derive provably-secure variants of various well-known UAKE protocols (such as a unilateral variant of SKEME with and without perfect forward secrecy, and Shoup’s A-DHKE-1), as well as new protocols, such as the first 2-round UAKE protocol which is both (passively) forward deniable and forward-secure.

    To further clarify the intuitive connections between PKE/Signatures and UAKE, we define and construct stronger forms of (necessarily interactive) PKE/Signature schemes, called confirmed encryption and confidential authentication, which, respectively, allow the sender to obtain confirmation that the (keyed) receiver output the correct message, or to hide the content of the message being authenticated from anybody but the participating (unkeyed) receiver. Using confirmed PKE/confidential authentication, we obtain two concise UAKE protocols of the form: ``send confirmed encryption/confidential authentication of a random key K

  12. A Provably Secure PKCS#11 Configuration Without Authenticated Attributes 2017 FinancialCryptography fc17.ifca.ai
    Ryan Stanley-Oakes

    Cryptographic APIs like PKCS#11 are interfaces to trusted hardware where keys are stored; the secret keys should never leave the trusted hardware in plaintext. In PKCS#11 it is possible to give keys conflicting roles, leading to a number of key-recovery attacks. To prevent these attacks, one can authenticate the attributes of keys when wrapping, but this is not standard in PKCS#11. Alternatively, one can configure PKCS#11 to place additional restrictions on the commands permitted by the API.

    Bortolozzo et al. proposed a configuration of PKCS#11, called the Secure Templates Patch (STP), supporting symmetric encryption and key wrapping. However, the security guarantees for STP given by Bortolozzo et al. are with respect to a weak attacker model. STP has been implemented as a set of filtering rules in Caml Crush, a software filter for PKCS#11 that rejects certain API calls. The filtering rules in Caml Crush extend STP by allowing users to compute and verify MACs and so the previous analysis of STP does not apply to this configuration.

    We give a rigorous analysis of STP, including the extension used in Caml Crush. Our contribution is as follows:

    (i) We show that the extension of STP used in Caml Crush is insecure.

    (ii) We propose a strong, computational security model for configurations of PKCS#11 where the adversary can adaptively corrupt keys and prove that STP is secure in this model.

    (iii) We prove the security of an extension of STP that adds support for public-key encryption and digital signatures.

  13. Efficient No-dictionary Verifiable SSE 2017 FinancialCryptography SearchableEncryption fc17.ifca.ai
    Wakaha Ogata, Kaoru Kurosawa

    In the model of “no-dictionary” verifiable searchable symmetric encryption (SSE) scheme, a client does not need to keep the set of keywords W in the search phase, where W is called a dictionary. Still a malicious server cannot cheat the client by saying that ``your search word w does not exist in the dictionary W” when it exists. In the previous such schemes, it takes O(logm) time for the server to prove that w∉W, where m=|W| is the number of keywords. In this paper, we show a generic method to transform any SSE scheme (that is only secure against passive adversaries) to a no-dictionary verifiable SSE scheme. In the transformed scheme, it takes only O(1) time for the server to prove that w∉W.

  14. Secure Multiparty Computation from SGX 2017 FinancialCryptography IntelSGX MPC fc17.ifca.ai
    Bernardo Portela, Manuel Barbosa, Guillaume Scerri, Bogdan Warinschi, Raad Bahmani, Ferdinand Brasser, Ahmad-Reza Sadeghi

    Isolated Execution Environments (IEE) offered by novel commodity hardware such as Intel’s SGX deployed in Skylake processors permit executing software in a protected environment that shields it from a malicious operating system; it also permits a remote user to obtain strong interactive attestation guarantees on both the code running in an IEE and its input/output behaviour. In this paper we show how IEEs provide a new path to constructing general secure multiparty computation (MPC) protocols. Our protocol is intuitive and elegant: it uses code within an IEE to play the role of a trusted third party (TTP), and the attestation guarantees of SGX to bootstrap secure communications between participants and the TTP. In our protocol the load of communications and computations on participants only depends on the size of each party’s inputs and outputs and is thus small and independent from the intricacy of the functionality to be computed. The remaining computational load– essentially that of computing the functionality – is moved to an untrusted party running an IEE-enabled machine, an appealing feature for Cloud-based scenarios. However, as often the case even with the simplest cryptographic protocols, we found that there is a large gap between this intuitively appealing solution and a protocol with rigorous security guarantees. We bridge this gap through a comprehensive set of results that include: i. a detailed construction of a protocol for secure computation for arbitrary functionalities; ii. formal security definitions for the security of the overall protocol and that of its components; and iii. a modular security analysis of our protocol that relies on a novel notion of labeled attested computation. We implemented and extensively evaluated our solution on SGX-enabled hardware, providing detailed measurements of our protocol as well as comparisons with software-only MPC solutions. Furthermore, we show the cost induced by using constant-time, i.e., timing side channel resilient, code in our implementation.

  15. Efficient Round-Optimal Blind Signatures in the Standard Model 2017 FinancialCryptography Privacy Signatures fc17.ifca.ai
    Essam Ghadafi

    Blind signatures are at the core of e-cash systems and have numerous other applications. In this work we construct efficient blind and partially blind signature schemes over bilinear groups in the standard model. Our schemes yield short signatures consisting of only a couple of elements from the shorter source group and have very short communication overhead consisting of 1 group element on the user side and 3 group elements on the signer side. At 80-bit security, our schemes yield signatures consisting of only 40 bytes which is 67% shorter than the most efficient existing scheme with the same security in the standard model. Verification in our schemes requires only a couple of pairings. Our schemes compare favorably in every efficiency measure to all existing counterparts offering the same security in the standard model. In fact, the efficiency of our signing protocol as well as the signature size compare favorably even to many existing schemes in the random oracle model. For instance, our signatures are shorter than those of Brands’ scheme which is at the heart of the U-Prove anonymous credential system used in practice. The unforgeability of our schemes is based on new intractability assumptions of a ``one-more’’ type which we show are intractable in the generic group model, whereas their blindness holds w.r.t.~malicious signing keys in the information-theoretic sense. We also give variants of our schemes for a vector of messages.

  16. A Practical Multivariate Blind Signature Scheme 2017 FinancialCryptography Multivariate Privacy Signatures fc17.ifca.ai
    Albrecht Petzoldt, Alan Szepieniec, Mohamed Saied Emam Mohamed

    Multivariate Cryptography is one of the main candidates for creating post-quantum cryptosystems. Especially in the area of digital signatures, there exist many practical and secure multivariate schemes. However, there is a lack of multivariate signature schemes with special properties such as blind, ring and group signatures. In this paper, we propose a generic technique to transform multivariate signature schemes into blind signature schemes and show the practicality of the construction on the example of Rainbow. The resulting scheme satisfies the usual blindness criterion and a one-more-unforgeability criterion adapted to MQ signatures, produces short blind signatures and is very efficient.

  17. The Security of NTP's Datagram Protocol 2017 FinancialCryptography Network Protocols fc17.ifca.ai
    Aanchal Malhotra, Matthew Van Gundy, Mayank Varia, Haydn Kennedy, Jonathan Gardner, Sharon Goldberg

    For decades, the Network Time Protocol (NTP) has been used to synchronize computer clocks over untrusted network paths. This work takes a new look at the security of NTP’s datagram protocol. We argue that NTP’s datagram protocol in RFC5905 is both underspecified and flawed. The NTP specifications do not sufficiently respect (1) the conflicting security requirements of different NTP modes, and (2) the mechanism NTP uses to prevent off-path attacks. A further problem is that (3) NTP’s control-query interface reveals sensitive information that can be exploited in off-path attacks. We exploit these problems in several attacks that remote attackers can use to maliciously alter a target’s time. We use network scans to find millions of IPs that are vulnerable to our attacks. Finally, we move beyond identifying attacks by developing a cryptographic model and using it to prove the security of a new backwards-compatible client/server protocol for NTP.

  18. Improving Authenticated Dynamic Dictionaries, with Applications to Cryptocurrencies 2017 Blockchains FinancialCryptography fc17.ifca.ai
    Leonid Reyzin, Dmitry Meshkov, Alexander Chepurnoy, Sasha Ivanov

    We improve the design and implementation of two-party and three-party authenticated dynamic dictionaries and apply these dictionaries to cryptocurrency ledgers.

    A public ledger (blockchain) in a cryptocurrency needs to be easily verifiable. However, maintaining a data structure of all account balances, in order to verify whether a transaction is valid, can be quite burdensome: a verifier who does not have the large amount of RAM required for the data structure will perform slowly because of the need to continually access secondary storage. We demonstrate experimentally that authenticated dynamic dictionaries can considerably reduce verifier load. On the other hand, per-transaction proofs generated by authenticated dictionaries increase the size of the blockchain, which motivates us to find a solution with most compact proofs.

    Our improvements to the design of authenticated dictionaries reduce proof size and speed up verification by 1.4-2.5 times, making them better suited for the cryptocurrency application. We further show that proofs for multiple transactions in a single block can compressed together, reducing their total length by approximately an additional factor of 2.

    We simulate blockchain verification, and show that our verifier can be about 20 times faster than a disk-bound verifier under a realistic transaction load.

  19. A Smart Contract for Boardroom Voting with Maximum Voter Privacy 2017 Blockchains FinancialCryptography Privacy SmartContracts fc17.ifca.ai
    Patrick McCorry, Siamak Shahandashti, Feng Hao

    We present the first implementation of a decentralised and self-tallying internet voting protocol with maximum voter privacy using the Blockchain. The Open Vote Network is suitable for boardroom elec- tions and is written as a smart contract for Ethereum. Unlike previously proposed Blockchain e-voting protocols, this is the first implementation that does not rely on any trusted authority to compute the tally or to protect the voter’s privacy. Instead, the Open Vote Network is a self- tallying protocol, and each voter is in control of the privacy of their own vote such that it can only be breached by a full collusion involving all other voters. The execution of the protocol is enforced using the consensus mechanism that also secures the Ethereum blockchain. We tested the implementation on Ethereum’s official test network to demonstrate its feasibility. Also, we provide a financial and computational breakdown of its execution cost.

  20. Trust Is Risk: A Decentralized Financial Trust Platform 2017 Bitcoin Blockchains FinancialCryptography fc17.ifca.ai
    Orfeas Stefanos, Thyfronitis Litos, Dionysis Zindros

    Centralized reputation systems use stars and reviews and thus require algorithm secrecy to avoid manipulation. In autonomous open source decentralized systems this luxury is not available. We create a reputation network for decentralized marketplaces where the trust each user gives to the other users is quantifiable and expressed in monetary terms. We introduce a new model for bitcoin wallets in which user coins are split among trusted associates. Direct trust is defined using shared bitcoin accounts via bitcoin’s 1-of-2 multisig. Indirect trust is subsequently defined transitively. This enables formal game theoretic arguments pertaining to risk analysis. We prove that risk and maximum flows are equivalent in our model and that our system is Sybil-resilient. Our system allows for concrete financial decisions on the subjective monetary amount a pseudonymous party can be trusted with. Risk remains invariant under a direct trust redistribution operation followed by a purchase.

  21. Escrow protocols for cryptocurrencies: How to buy physical goods using Bitcoin 2017 Bitcoin Blockchains FinancialCryptography fc17.ifca.ai
    Steven Goldfeder, Joseph Bonneau, Rosario Gennaro, Arvind Narayanan

    We consider the problem of buying physical goods with cryptocurrencies. There is an inherent circular dependency: should be the buyer trust the seller and pay before receiving the goods or should the seller trust the buyer and ship the goods before receiving payment? This dilemma is addressed in practice using a third party escrow service. However, we show that naive escrow protocols introduce both privacy and security issues. We formalize the escrow problem and present a suite of schemes with improved security and privacy properties. Our schemes are compatible with Bitcoin and similar blockchain-based cryptocurrencies.

  22. Attacks on Secure Logging Schemes 2017 FinancialCryptography TamperResistance fc17.ifca.ai
    Gunnar Hartung

    We present four attacks on three cryptographic schemes in-tended for securing log files against illicit retroactive modification. Our first two attacks regard the LogFAS scheme by Yavuz et al. (FinancialCryptography 2012), whereas our third and fourth attacks break the BM-and AR-FssAgg schemes by Ma (AsiaCCS 2008).All schemes have an accompanying security proof, seemingly contradicting the existence of attacks. We point out flaws in these proofs, resolvingthe contradiction.

  23. Economically Optimal Variable Tag Length Authentication 2017 Authentication FinancialCryptography fc17.ifca.ai
    Reihaneh Safavi-Naini, Viliam Lisy, Yvo Desmedt

    Cryptographic authentication protects messages against forgeries. In real life, messages carry information of different value and the gain of the adversary in a successful forgery and the corresponding cost of the system designers, depend on the “meaning” of the message. This is easy to see by comparing the successful forgery of a $1,000 transaction with the forgery of a $1 one. Cryptographic protocols require computation and increase communication cost of the system, and an economically optimal system must optimize these costs such that message protection be commensurate to their values. This is especially important for resource limited devices that rely on battery power. A MAC (Message Authentication Code) provides protection by appending a cryptographic tag to the message. For secure MACs, the tag length is the main determinant of the security level: longer tags provide higher protection and at the same time increase the communication cost of the system. Our goal is to find the economically optimal tag lengths when messages carry information of different values.

    We propose a novel approach to model the cost and benefit of information authentication as a two-party extensive-form game, show how to find a Nash equilibrium for the game, and determine the optimal tag lengths for messages. We prove that computing an optimal solution for the game is NP-complete, and then show how to find an optimal solution using single Mixed Integer Linear Program (MILP). We apply the approach to the protection of messages in an industrial control system using realistic messages, and give our analysis with numerical results obtained using off-the-shelf IBM CPLEX solver.

  24. Optimally Sound Sigma Protocols Under DCRA 2017 FinancialCryptography NIZK ZK fc17.ifca.ai
    Helger Lipmaa

    Given a well-chosen additively homomorphic cryptosystem and a Σ protocol with a linear answer, Damgård, Fazio, and Nicolosi proposed a non-interactive designated-verifier zero knowledge argument in the registered public key model that is sound under non-standard complexity-leveraging assumptions. In 2015, Chaidos and Groth showed how to achieve the weaker yet reasonable culpable soundness notion under standard assumptions but only if the plaintext space order is prime. It makes use of Σ protocols that satisfy what we call the \emph{optimal culpable soundness}. Unfortunately, most of the known additively homomorphic cryptosystems (like the Paillier Elgamal cryptosystem that is secure under the standard Decisional Composite Residuosity Assumption) have composite-order plaintext space. We construct optimally culpable sound Σ protocols and thus culpably sound non-interactive designated-verifier zero knowledge protocols for NP under standard assumptions given that the least prime divisor of the plaintext space order is large.

  25. A Post-Quantum Digital Signature Scheme Based on Supersingular Isogenies 2017 FinancialCryptography Isogenies PQC Signatures fc17.ifca.ai
    Youngho Yoo, Reza Azarderakhsh, Amir Jalali, David Jao, Vladimir Soukharev

    We present the first general-purpose digital signature scheme based on supersingular elliptic curve isogenies secure against quantum adversaries in the quantum random oracle model with small key sizes. This scheme is an application of Unruh’s construction of non-interactive zero-knowledge proofs to an interactive zero-knowledge proof proposed by De Feo, Jao, and Plût. We implement our proposed scheme on an x86-64 PC platform as well as an ARM-powered device. We exploit the state-of-the-art techniques to speed up the computations for general C and assembly. Finally, we provide timing results for real world applications.