## CryptoDB

### Sebastian Faust

#### Publications

Year
Venue
Title
2021
EUROCRYPT
Covert security has been introduced as a compromise between semi-honest and malicious security. In a nutshell, covert security guarantees that malicious behavior can be detected by the honest parties with some probability, but in case detection fails all bets are off. While the security guarantee offered by covert security is weaker than full-fledged malicious security, it comes with significantly improved efficiency. An important extension of covert security introduced by Asharov and Orlandi (ASIACRYPT'12) is \emph{public verifiability}, which allows the honest parties to create a publicly verifiable certificate of malicious behavior. Public verifiability significantly strengthen covert security as the certificate allows punishment via an external party, e.g., a judge. Most previous work on publicly verifiable covert (PVC) security focuses on the two-party case, and the multi-party case has mostly been neglected. In this work, we introduce a novel compiler for multi-party PVC secure protocols with no private inputs. The class of supported protocols includes the preprocessing of common multi-party computation protocols that are designed in the offline-online model. Our compiler leverages time-lock encryption to offer high probability of cheating detection (often also called deterrence factor) independent of the number of involved parties. Moreover, in contrast to the only earlier work that studies PVC in the multi-party setting (CRYPTO'20), we provide the first full formal security analysis.
2021
TOSC
In order to lower costs, the fabrication of Integrated Circuits (ICs) is increasingly delegated to offshore contract foundries, making them exposed to malicious modifications, known as hardware Trojans. Recent works have demonstrated that a strong form of Trojan-resilience can be obtained from untrusted chips by exploiting secret sharing and Multi-Party Computation (MPC), yet with significant cost overheads. In this paper, we study the possibility of building a symmetric cipher enabling similar guarantees in a more efficient manner. To reach this goal, we exploit a simple round structure mixing a modular multiplication and a multiplication with a binary matrix. Besides being motivated as a new block cipher design for Trojan resilience, our research also exposes the cryptographic properties of the modular multiplication, which is of independent interest.
2021
PKC
2021
CRYPTO
Proving the security of masked implementations in theoretical models that are relevant to practice and match the best known attacks of the side-channel literature is a notoriously hard problem. The random probing model is a good candidate to contribute to this challenge, due to its ability to capture the continuous nature of physical leakage (contrary to the threshold probing model), while also being convenient to manipulate in proofs and to automate with verification tools. Yet, despite recent progresses in the design of masked circuits with good asymptotic security guarantees in this model, existing results still fall short when it comes to analyze the security of concretely useful circuits under realistic noise levels and with low number of shares. In this paper, we contribute to this issue by introducing a new composability notion, the Probe Distribution Table (PDT), and a new tool (called STRAPS, for the Sampled Testing of the RAndom Probing Security). Their combination allows us to significantly improve the tightness of existing analyses in the most practical (low noise, low number of shares) region of the design space. We illustrate these improvements by quantifying the random probing security of an AES S-box circuit, masked with the popular multiplication gadget of Ishai, Sahai and Wagner from Crypto 2003, with up to six shares.
2021
ASIACRYPT
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 and or 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.
2020
JOFC
Non-malleable codes (Dziembowski et al., ICS’10 and J. ACM’18) are a natural relaxation of error correcting/detecting codes with useful applications in cryptography. Informally, a code is non-malleable if an adversary trying to tamper with an encoding of a message can only leave it unchanged or modify it to the encoding of an unrelated value. This paper introduces continuous non-malleability, a generalization of standard non-malleability where the adversary is allowed to tamper continuously with the same encoding. This is in contrast to the standard definition of non-malleable codes, where the adversary can only tamper a single time. The only restriction is that after the first invalid codeword is ever generated, a special self-destruct mechanism is triggered and no further tampering is allowed; this restriction can easily be shown to be necessary. We focus on the split-state model, where an encoding consists of two parts and the tampering functions can be arbitrary as long as they act independently on each part. Our main contributions are outlined below. We show that continuous non-malleability in the split-state model is impossible without relying on computational assumptions. We construct a computationally secure split-state code satisfying continuous non-malleability in the common reference string (CRS) model. Our scheme can be instantiated assuming the existence of collision-resistant hash functions and (doubly enhanced) trapdoor permutations, but we also give concrete instantiations based on standard number-theoretic assumptions. We revisit the application of non-malleable codes to protecting arbitrary cryptographic primitives against related-key attacks. Previous applications of non-malleable codes in this setting required perfect erasures and the adversary to be restricted in memory. We show that continuously non-malleable codes allow to avoid these restrictions.
2019
EUROCRYPT
Smart contracts are self-executing agreements written in program code and are envisioned to be one of the main applications of blockchain technology. While they are supported by prominent cryptocurrencies such as Ethereum, their further adoption is hindered by fundamental scalability challenges. For instance, in Ethereum contract execution suffers from a latency of more than 15 s, and the total number of contracts that can be executed per second is very limited. State channel networks are one of the core primitives aiming to address these challenges. They form a second layer over the slow and expensive blockchain, thereby enabling instantaneous contract processing at negligible costs.In this work we present the first complete description of a state channel network that exhibits the following key features. First, it supports virtual multi-party state channels, i.e. state channels that can be created and closed without blockchain interaction and that allow contracts with any number of parties. Second, the worst case time complexity of our protocol is constant for arbitrary complex channels. This is in contrast to the existing virtual state channel construction that has worst case time complexity linear in the number of involved parties. In addition to our new construction, we provide a comprehensive model for the modular design and security analysis of our construction.
2019
ASIACRYPT
Masking schemes are a prominent countermeasure against power analysis and work by concealing the values that are produced during the computation through randomness. The randomness is typically injected into the masked algorithm using a so-called refreshing scheme, which is placed after each masked operation, and hence is one of the main bottlenecks for designing efficient masking schemes. The main contribution of our work is to investigate the security of a very simple and efficient refreshing scheme and prove its security in the noisy leakage model (EUROCRYPT’13). Compared to earlier constructions our refreshing is significantly more efficient and uses only n random values and ${<}2n$ operations, where n is the security parameter. In addition we show how our refreshing can be used in more complex masked computation in the presence of noisy leakage. Our results are established using a new methodology for analyzing masking schemes in the noisy leakage model, which may be of independent interest.
2019
JOFC
We investigate the relationship between theoretical studies of leaking cryptographic devices and concrete security evaluations with standard side-channel attacks. Our contributions are in four parts. First, we connect the formal analysis of the masking countermeasure proposed by Duc et al. (Eurocrypt 2014) with the Eurocrypt 2009 evaluation framework for side-channel key recovery attacks. In particular, we re-state their main proof for the masking countermeasure based on a mutual information metric, which is frequently used in concrete physical security evaluations. Second, we discuss the tightness of the Eurocrypt 2014 bounds based on experimental case studies. This allows us to conjecture a simplified link between the mutual information metric and the success rate of a side-channel adversary, ignoring technical parameters and proof artifacts. Third, we introduce heuristic (yet well-motivated) tools for the evaluation of the masking countermeasure when its independent leakage assumption is not perfectly fulfilled, as it is frequently encountered in practice. Thanks to these tools, we argue that masking with non-independent leakages may provide improved security levels in certain scenarios. Eventually, we consider the tradeoff between the measurement complexity and the key enumeration time complexity in divide-and-conquer side-channel attacks and show that these complexities can be lower bounded based on the mutual information metric, using simple and efficient algorithms. The combination of these observations enables significant reductions of the evaluation costs for certification bodies.
2019
JOFC
A recent trend in cryptography is to formally show the leakage resilience of cryptographic implementations in a given leakage model. One of the most prominent leakage model—the so-called bounded leakage model—assumes that the amount of leakage that an adversary receives is a-priori bounded. Unfortunately, it has been pointed out by several works that the assumption of bounded leakages is hard to verify in practice. A more realistic assumption is to consider that leakages are sufficiently noisy, following the engineering observation that real-world physical leakages are inherently perturbed by physical noise. While already the seminal work of Chari et al. (in: CRYPTO, pp 398–412, 1999 ) study security of side-channel countermeasures in the noisy model, only recently Prouff and Rivain (in: Johansson T, Nguyen PQ (eds) EUROCRYPT, volume 7881 of lecture notes in 931 computer science, pp 142–159, Springer, 2013 ) offer a full formal analysis of the masking countermeasure in a physically motivated noise model. In particular, the authors show that a block-cipher implementation that uses the Boolean masking scheme is secure against a very general class of noisy leakage functions. While this is an important step toward better understanding the security of masking schemes, the analysis of Prouff and Rivain has several shortcomings including in particular requiring leak-free gates. In this work, we provide an alternative security proof in the same noise model that overcomes these challenges. We achieve this goal by a new reduction from noisy leakage to the important model of probing adversaries (Ishai et al. in: CRYPTO, pp 463–481, 2003 ). This reduction is the main technical contribution of our work that significantly simplifies the formal security analysis of masking schemes against realistic side-channel leakages.
2018
TCHES
Composability and robustness against physical defaults (e.g., glitches) are two highly desirable properties for secure implementations of masking schemes. While tools exist to guarantee them separately, no current formalism enables their joint investigation. In this paper, we solve this issue by introducing a new model, the robust probing model, that is naturally suited to capture the combination of these properties. We first motivate this formalism by analyzing the excellent robustness and low randomness requirements of first-order threshold implementations, and highlighting the difficulty to extend them to higher orders. Next, and most importantly, we use our theory to design and prove the first higher-order secure, robust and composable multiplication gadgets. While admittedly inspired by existing approaches to masking (e.g., Ishai-Sahai-Wagner-like, threshold, domain-oriented), these gadgets exhibit subtle implementation differences with these state-of-the-art solutions (none of which being provably composable and robust). Hence, our results illustrate how sound theoretical models can guide practically-relevant implementations.
2017
EUROCRYPT
2017
CRYPTO
2017
ASIACRYPT
2017
ASIACRYPT
2017
TCC
2017
JOFC
2016
EUROCRYPT
2016
CRYPTO
2016
PKC
2016
TCC
2016
JOFC
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
PKC
2015
EUROCRYPT
2015
EUROCRYPT
2015
EUROCRYPT
2015
CRYPTO
2014
EUROCRYPT
2014
EUROCRYPT
2014
TCC
2014
EPRINT
2014
EPRINT
2014
EPRINT
2013
CHES
2013
ASIACRYPT
2012
TCC
2012
TCC
2012
CHES
2012
ASIACRYPT
2012
ASIACRYPT
2011
ASIACRYPT
2010
TCC
2010
EUROCRYPT
2008
PKC

PKC 2020
CHES 2019
Eurocrypt 2018
CHES 2018
TCC 2018
PKC 2017
CHES 2016
Eurocrypt 2016
PKC 2015
TCC 2015