Financial Cryptography and Data Security

An attribute-based credential scheme allows a user, given a set of attributes, to prove ownership of these attributes to a verifier, voluntarily disclosing some of them while keeping the others secret. A number of such schemes exist, of which some additionally provide unlinkability: that is, when the same attributes were disclosed in two transactions, it is not possible to tell if one and the same or two different credentials were involved. Recently full-fledged implementations of such schemes on smart cards have emerged; however, these need to compromise the security level to achieve reasonable transaction speeds. In this paper we present a new unlinkable attribute-based credential scheme with a full security proof, using a known hardness assumption in the standard model. Defined on elliptic curves, the scheme involves bilinear pairings but only on the verifier’s side, making it very efficient both in terms of speed and size on the user’s side.

Group signature allows members to issue signatures on behalf of the group anonymously in normal circumstances. When the need arises, an opening authority (OA) can open a signature and reveal its true signer. Yet, many constructions require not only the secret key of the OA but also a member database (cf. a public-key repository) for this opening. This “secret members list” put the anonymity of members at risk as each of them is a potential signer.To resolve this “anonymity catch-22” issue, Kiayias and Zhou proposed hidden identity-based signatures (Financial Crypt. 2007 and IET Information Security 2009), where the opening just takes in the secret key of the OA and directly outputs the signer identity. The membership list can be hidden from the OA since there is no membership list whatsoever. However, their constructions suffer from efficiency problem.This paper aims to realize the vision of Kiayias and Zhou for real, that is, an efficient construction which achieves the distinctive feature of hidden identity-based signatures. Moreover, our construction is secure against concurrent attack, and easily extensible with linkability such that any double authentication can be publicly detected. Both features are especially desirable in Internet-based services which allow anonymous authentication with revocation to block any misbehaving user. We believe our work will improve the usability of group signature and its variant.

We propose that by integrating behavioural biometric gestures—such as drawing figures on a touch screen—with challenge-response based cognitive authentication schemes, we can benefit from the properties of both. On the one hand, we can improve the usability of existing cognitive schemes by significantly reducing the number of challenge-response rounds by (partially) relying on the hardness of mimicking carefully designed behavioural biometric gestures. On the other hand, the observation resistant property of cognitive schemes provides an extra layer of protection for behavioural biometrics; an attacker is unsure if a failed impersonation is due to a biometric failure or a wrong response to the challenge. We design and develop a prototype of such a “hybrid” scheme, named BehavioCog. To provide security close to a 4-digit PIN—one in 10,000 chance to impersonate—we only need two challenge-response rounds, which can be completed in less than 38 s on average (as estimated in our user study), with the advantage that unlike PINs or passwords, the scheme is secure under observation.

Tokenization is the process of consistently replacing sensitive elements, such as credit cards numbers, with non-sensitive surrogate values. As tokenization is mandated for any organization storing credit card data, many practical solutions have been introduced and are in commercial operation today. However, all existing solutions are static yet, i.e., they do not allow for efficient updates of the cryptographic keys while maintaining the consistency of the tokens. This lack of updatability is a burden for most practical deployments, as cryptographic keys must also be re-keyed periodically for ensuring continued security. This paper introduces a model for updatable tokenization with key evolution, in which a key exposure does not disclose relations among tokenized data in the past, and where the updates to the tokenized data set can be made by an untrusted entity and preserve the consistency of the data. We formally define the desired security properties guaranteeing unlinkability of tokens among different time epochs and one-wayness of the tokenization process. Moreover, we construct two highly efficient updatable tokenization schemes and prove them to achieve our security notions.

In the era of big data, graph databases have become increasingly important for NoSQL technologies, and many systems can be modeled as graphs for semantic queries. Meanwhile, with the advent of cloud computing, data owners are highly motivated to outsource and store their massive potentially-sensitive graph data on remote untrusted servers in an encrypted form, expecting to retain the ability to query over the encrypted graphs.To allow effective and private queries over encrypted data, the most well-studied class of structured encryption schemes are searchable symmetric encryption (SSE) designs, which encrypt search structures (e.g., inverted indexes) for retrieving data files. In this paper, we tackle the challenge of designing a Secure Graph DataBase encryption scheme (SecGDB) to encrypt graph structures and enforce private graph queries over the encrypted graph database. Specifically, our construction strategically makes use of efficient additively homomorphic encryption and garbled circuits to support the shortest distance queries with optimal time and storage complexities. To achieve better amortized time complexity over multiple queries, we further propose an auxiliary data structure called query history and store it on the remote server to act as a “caching” resource. We prove that our construction is adaptively semantically-secure in the random oracle model and finally implement and evaluate it on various representative real-world datasets, showing that our approach is practically efficient in terms of both storage and computation.

We explore the possible ways modern cryptographic methods can be applied to the field of medical data analysis. Current systems require large computational facilities owned by the data owners or excessive trust given to the researchers. We implement one possible solution in which researchers operate directly on homomorphically encrypted data and the data owner decrypts the results. We test our implementation on large datasets and show that it is sufficiently practical that it could be a helpful tool for modern researchers. We also perform a heuristic analysis of the security of our system.

We introduce the notion of homomorphic proxy re-authenticators, a tool that adds security and verifiability guarantees to multi-user data aggregation scenarios. It allows distinct sources to authenticate their data under their own keys, and a proxy can transform these single signatures or message authentication codes (MACs) to a MAC under a receiver’s key without having access to it. In addition, the proxy can evaluate arithmetic circuits (functions) on the inputs so that the resulting MAC corresponds to the evaluation of the respective function. As the messages authenticated by the sources may represent sensitive information, we also consider hiding them from the proxy and other parties in the system, except from the receiver.We provide a general model and two modular constructions of our novel primitive, supporting the class of linear functions. On our way, we establish various novel building blocks. Most interestingly, we formally define the notion and present a construction of homomorphic proxy re-encryption, which may be of independent interest. The latter allows users to encrypt messages under their own public keys, and a proxy can re-encrypt them to a receiver’s public key (without knowing any secret key), while also being able to evaluate functions on the ciphertexts. The resulting re-encrypted ciphertext then holds an evaluation of the function on the input messages.

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.

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.

Given a well-chosen additively homomorphic cryptosystem and a $$\varSigma $$ 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 $$\varSigma $$ protocols that satisfy what we call the 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 $$\varSigma $$ 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.

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 o 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.

New emerging devices open up immense opportunities for everyday users. At the same time, they may raise significant security and privacy threats. One such device, forming the central focus of this work, is an EEG headset, which allows a user to control her computer only using her thoughts.In this paper, we show how such a malicious EEG device or a malicious application having access to EEG signals recorded by the device can be turned into a new form of a keylogger, called PEEP, that passively eavesdrops over user’s sensitive typed input, specifically numeric PINs and textual passwords, by analyzing the corresponding neural signals. PEEP works because user’s input is correlated with user’s innate visual processing as well as hand, eye, and head muscle movements, all of which are explicitly or implicitly captured by the EEG device.Our contributions are two-fold. First, we design and develop PEEP against a commodity EEG headset and a higher-end medical-scale EEG device based on machine learning techniques. Second, we conduct the comprehensive evaluation with multiple users to demonstrate the feasibility of PEEP for inferring PINs and passwords as they are typed on a physical keyboard, a virtual keyboard, and an ATM-style numeric keypad. Our results show that PEEP can extract sensitive input with an accuracy significantly higher than a random guessing classifier. Compared to prior work on this subject, PEEP is highly surreptitious as it only requires passive monitoring of brain signals, not deliberate, and active strategies that may trigger suspicion and be detected by the user. Also, PEEP achieves orders of magnitude higher accuracies compared to prior active PIN inferring attacks. Our work serves to raise awareness to a potentially hard-to-address threat arising from EEG devices which may remain attached to the users almost invariably soon.

Research showed that microarchitectural attacks like cache attacks can be performed through websites using JavaScript. These timing attacks allow an adversary to spy on users secrets such as their keystrokes, leveraging fine-grained timers. However, the W3C and browser vendors responded to this significant threat by eliminating fine-grained timers from JavaScript. This renders previous high-resolution microarchitectural attacks non-applicable.We demonstrate the inefficacy of this mitigation by finding and evaluating a wide range of new sources of timing information. We develop measurement methods that exceed the resolution of official timing sources by 3 to 4 orders of magnitude on all major browsers, and even more on Tor browser. Our timing measurements do not only re-enable previous attacks to their full extent but also allow implementing new attacks. We demonstrate a new DRAM-based covert channel between a website and an unprivileged app in a virtual machine without network hardware. Our results emphasize that quick-fix mitigations can establish a dangerous false sense of security.

We present four attacks on three cryptographic schemes intended for securing log files against illicit retroactive modification. Our first two attacks regard the LogFAS scheme by Yavuz et al. (Financial Cryptography 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, resolving the contradiction.

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.

Apps in the FINANCE category constitute approximately 2% of the 2,000,000 apps in the Google Play Store. These apps handle extremely sensitive data, such as online banking credentials, budgets, salaries, investments and the like. Although apps are automatically vetted for malicious activity before being admitted to the Google Play Store, it remains unclear whether app developers themselves check their apps for vulnerabilities before submitting them to be published. Additionally, it is not known how financial apps compare to other apps in terms of dangerous permission usage or how they evolve as they are updated. We analyse 10,400 apps to understand how apps in general and financial apps in particular have evolved over the past two years in terms of dangerous permission usage and the vulnerabilities they contain. Worryingly, we discover that both financial and non-financial apps are getting more vulnerable over time. Moreover, we discover that while financial apps tend to have less vulnerabilities, the rate of increase in vulnerabilities in financial apps is three times as much as that of other apps.

We argue that as email attacks continue to increase in sophistication, error rates and filter processing times are both likely to increase. We address the problem at its root by introducing the notion of open quarantine, an approach that avoids tradeoffs between filtering precision and delivery delays. This is achieved using a multi-phase filtering approach, combined with the neutralization of messages with undetermined security posture.

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.

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.

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 elections 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.

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.

The rise of blockchain-based cryptocurrencies has led to an explosion of services using distributed ledgers as their underlying infrastructure. However, due to inherently single-service oriented blockchain protocols, such services can bloat the existing ledgers, fail to provide sufficient security, or completely forego the property of trustless auditability. Security concerns, trust restrictions, and scalability limits regarding the resource requirements of users hamper the sustainable development of loosely-coupled services on blockchains.This paper introduces Aspen, a sharded blockchain protocol designed to securely scale with increasing number of services. Aspen shares the same trust model as Bitcoin in a peer-to-peer network that is prone to extreme churn containing Byzantine participants. It enables introduction of new services without compromising the security, leveraging the trust assumptions, or flooding users with irrelevant messages.

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.

Although the Domain Name System (DNS) was designed as a naming system, its features have made it appealing to repurpose it for the deployment of novel systems. One important class of such systems are security enhancements, and this work sheds light on their deployment. We show the characteristics of these solutions and measure reliability of DNS in these applications. We investigate the compatibility of these solutions with the Tor network, signal necessary changes, and report on surprising drawbacks in Tor’s DNS resolution.

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 the Rainbow multivariate signature scheme into a blind signature schemes. 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.

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.

In this paper we show how Isolated Execution Environments (IEE) offered by novel commodity hardware such as Intel’s SGX 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. 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 intricacies 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 attractive feature for Cloud-based scenarios.Our rigorous modular security analysis relies on the novel notion of labeled attested computation which we put forth in this paper. This notion is a convenient abstraction of the kind of attestation guarantees one can obtain from trusted hardware in multi-user scenarios.Finally, we present an extensive experimental evaluation of our solution on SGX-enabled hardware. Our implementation is open-source and it is functionality agnostic: it can be used to securely outsource to the Cloud arbitrary off-the-shelf collaborative software, such as the one employed on financial data applications, enabling secure collaborative execution over private inputs provided by multiple parties.

In the model of no-dictionary verifiable searchable symmetric encryption (SSE) scheme, a client does not need to keep the set of keywords $$\mathcal{W}$$ in the search phase, where $$\mathcal{W}$$ 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 $$\mathcal{W}$$” when it exists. In the previous such schemes, it takes $$O(\log m)$$ time for the server to prove that $$w \not \in \mathcal{W}$$, where $$m=|\mathcal{W}|$$ 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 $$w \not \in \mathcal{W}$$.

We present a methodology to achieve low latency homomorphic operations on approximations to complex numbers, by encoding a complex number as an evaluation of a polynomial at a root of unity. We then use this encoding to evaluate a Discrete Fourier Transform (DFT) on data which has been encrypted using a Somewhat Homomorphic Encryption (SHE) scheme, with up to three orders of magnitude improvement in latency over previous methods. We are also able to deal with much larger input sizes than previous methods. Due to the fact that the entire DFT algorithm is an algebraic operation over the underlying ring of the SHE scheme (for a suitably chosen ring), our method for the DFT utilizes exact arithmetic over the complex numbers, as opposed to approximations.

Recently, high-speed IPv4 scanners, such as ZMap, have enabled rapid and timely collection of TLS certificates and other security-sensitive parameters. Such large datasets led to the development of the Censys search interface, facilitating comprehensive analysis of TLS deployments in the wild. Several recent studies analyzed TLS certificates as deployed in web servers. Beyond public web servers, TLS is deployed in many other Internet-connected devices, at home and enterprise environments, and at network backbones. In this paper, we report the results of a preliminary analysis using Censys on TLS deployments in such devices (e.g., routers, modems, NAS, printers, SCADA, and IoT devices in general). We compare certificates and TLS connection parameters from a security perspective, as found in common devices with Alexa 1M sites. Our results highlight significant weaknesses, and may serve as a catalyst to improve TLS security for these devices.

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.”

Web traffic encryption has shifted from applying only to sensitive websites (such as banks) to a majority of all Web requests. Until recently, one of the main limiting factors for enabling HTTPS was the requirement to obtain a valid certificate from a trusted certification authority. This process traditionally involves steps such as paying a certificate issuance fee, ad-hoc private key and certificate request generation, and domain validation procedures. To remove this barrier of entry, the Internet Security Research Group (ISRG) introduced “Let’s Encrypt”, a new non-profit certificate authority that 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 27 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. Based on our findings, we 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 security model. 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.

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.

As distributed storage systems grow in popularity, there is now a demand for a reliable incentive and payment system to guarantee and reward the pristine storage of documents. However, many existing proof-of-retrieval and micropayment protocols are not secure in a censorship resistance setting, in which powerful adversaries may infiltrate a system or coerce the original publisher to remove content. Additionally, most existing censorship resistance systems lack a rigorous game-theoretic analysis. We propose Lavinia, an audit and payment protocol for censorship-resistant storage. Lavinia incentivizes document availability by providing micropayments to participating servers in exchange for honestly storing and serving content. Our protocol enables the implementation of a digital printing press as described in Anderson’s Eternity Service: allowing the publisher, as opposed to public interest or an appointed editorial board, to decide whether a document is worth storing, and for how long. In addition to proving the security of our protocol, we provide an in-depth game-theoretic analysis and show that self-interested participants of our system will faithfully implement the desired behaviour and continue to store documents until their expiration date.

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 $$1 - o (1)$$.