The inherent difficulty of maintaining stateful environments over long periods of time gave rise to the paradigm of serverless computing, where mostly-stateless components are deployed on demand to handle computation tasks, and are teared down once their task is complete. Serverless architecture could offer the added benefit of improved resistance to targeted denial-of-service attacks, by hiding from the attacker the physical machines involved in the protocol until after they complete their work. Realizing such protection, however, requires that the protocol only uses stateless parties, where each party sends only one message and never needs to speaks again. Perhaps the most famous example of this style of protocols is the Nakamoto consensus protocol used in Bitcoin: A peer can win the right to produce the next block by running a local lottery (mining), all while staying covert. Once the right has been won, it is executed by sending a single message. After that, the physical entity never needs to send more messages.

We refer to this as the You-Only-Speak-Once (YOSO) property, and initiate the formal study of it within a new model that we call the YOSO model. Our model is centered around the notion of roles, which are stateless parties that can only send a single message. Crucially, our modelling separates the protocol design, that only uses roles, from the role-assignment mechanism, that assigns roles to actual physical entities. This separation enables studying these two aspects separately, and our YOSO model in this work only deals with the protocol-design aspect.

We describe several techniques for achieving YOSO MPC; both computational and information theoretic. Our protocols are synchronous and provide guaranteed output delivery (which is important for application domains such as blockchains), assuming honest majority of roles in every time step. We describe a practically efficient computationally-secure protocol, as well as a proof-of-concept information theoretically secure protocol.

Craig Gentry and Shai Halevi and Bernardo Magri and Jesper Buus Nielsen and Sophia Yakoubov

Abstract: Private information retrieval (PIR) lets a client retrieve an entry from a database held by a server, without the server learning which entry was retrieved. Here we study a weaker variant that we call *random-index PIR* (RPIR). It differs from standard PIR in that the retrieved index is an output rather than an input of the protocol, and it is chosen at random.

Our motivation for studying RPIR comes from a recent work of Benhamouda et al. (TCC’20) about maintaining secret values on public blockchains. Their solution involves choosing a small anonymous committee from among a large universe, and here we show that RPIR can be used for that purpose.

The RPIR client must be implemented via secure MPC for this use case, stressing the need to make it as efficient as can be. Combined with recent techniques for secure-MPC with stateless parties, our results yield a new secrets-on-blockchain construction (and more generally large-scale MPC). Our solution tolerates any fraction f⪇1/2 of corrupted parties, solving an open problem left from the work of Benhamouda et al.

Considering RPIR as a primitive, we show that it is in fact equivalent to PIR when there are no restrictions on the number of communication rounds. On the other hand, RPIR can be implemented in a “noninteractive” setting, which is clearly impossible for PIR. We also study batch RPIR, where multiple indexes are retrieved at once. Specifically we consider a weaker security guarantee than full RPIR, which is still good enough for our motivating application. We show that this weaker variant can be realized more efficiently than standard PIR or RPIR, and we discuss one protocol in particular that may be attractive for practical implementations.

This paper closes the question of the possibility of two-round MPC protocols achieving different security guarantees with and without the availability of broadcast in any given round. Cohen et al. (Eurocrypt 2020) study this question in the dishonest majority setting; we complete the picture by studying the honest majority setting.

In the honest majority setting, given broadcast in both rounds, it is known that the strongest guarantee — guaranteed output delivery — is achievable (Gordon et al. Crypto 2015). We show that, given broadcast in the first round only, guaranteed output delivery is still achievable. Given broadcast in the second round only, we give a new construction that achieves identifiable abort, and we show that fairness — and thus guaranteed output delivery — are not achievable in this setting. Finally, if only peer-to-peer channels are available, we show that the weakest guarantee — selective abort — is the only one achievable for corruption thresholds t>1t>1 and for t=1t=1 and n=3n=3. On the other hand, it is already known that selective abort can be achieved in these cases. In the remaining cases, i.e., t=1t=1 and n>3n>3, it is known (from the work of Ishai et al. at Crypto 2010, and Ishai et al. at Crypto 2015) that guaranteed output delivery (and thus all weaker guarantees) are possible.

Although blockchains aim for immutability as their core feature, several instances have exposed the harms with perfect immutability. The permanence of illicit content inserted in Bitcoin poses a challenge to law enforcement agencies like Interpol, and millions of dollars are lost in buggy smart contracts in Ethereum. A line of research then spawned on Redactable blockchains with the aim of solving the problem of redacting illicit contents from both permissioned and permissionless blockchains. However, all the existing proposals follow the build-new-chain approach for redactions, and cannot be integrated with existing systems like Bitcoin and Ethereum.
We present Reparo, a generic protocol that acts as a publicly verifiable layer on top of any blockchain to perform repairs, ranging from fixing buggy contracts to removing illicit contents from the chain. Reparo facilitates additional functionalities for blockchains while maintaining the same provable security guarantee; thus, Reparo can be integrated with existing blockchains and start performing repairs on the pre-existent data. Any system user may propose a repair and a deliberation process ensues resulting in a decision that complies with the repair policy of the chain and is publicly verifiable.
Our Reparo layer can be easily tailored to different consensus requirements, does not require heavy cryptographic machinery and can, therefore, be efficiently instantiated in any permission-ed or -less setting. We demonstrate it by giving efficient instantiations of Reparo on top of Ethereum (with PoS and PoW), Bitcoin, and Cardano. Moreover, we evaluate Reparo with Ethereum mainnet and show that the cost of fixing several prominent smart contract bugs is almost negligible. For instance, the cost of repairing the prominent Parity Multisig wallet bug with Reparo is as low as 0.000000018% of the Ethers that can be retrieved after the fix.

Abstract: Permissionless blockchain systems, such as Bitcoin, rely on users using their computational power to solve a puzzle in order to achieve a consensus. To incentivise users in maintaining the system, newly minted coins are assigned to the user who solves this puzzle. A hardware race that has hence ensued among the users, has had a detrimental impact on the environment, with enormous energy consumption and increased global carbon footprint. On the other hand, proof of stake systems incentivise coin hoarding as players maximise their utility by holding their stakes. As a result, existing cryptocurrencies do not mimic the day-to-day usability of a fiat currency, but are rather regarded as cryptoassets or investment vectors.

In this work we initiate the study of minting mechanisms in cryptocurrencies as a primitive on its own right, and as a solution to prevent coin hoarding we propose a novel minting mechanism based on waiting-time first-price auctions. Our main technical tool is a protocol to run an auction over any blockchain. Moreover, our protocol is the first to securely implement an auction without requiring a semi-trusted party, i.e., where every miner in the network is a potential bidder. Our approach is generically applicable and we show that it is incentive-compatible with the underlying blockchain, i.e., the best strategy for a player is to behave honestly. Our proof-of-concept implementation shows that our system is efficient and scales to tens of thousands of bidders.

Category / Keywords: cryptographic protocols / Blockchain, Cryptocurrencies, Auction

We study interactive proof systems (IPSes) in a strong adversarial setting where the machines of honest parties might be corrupted and under control of the adversary. Our aim is to answer the following, seemingly paradoxical, questions:

•  Can Peggy convince Vic of the veracity of an NP statement, without leaking any information about the witness even in case Vic is malicious and Peggy does not trust her computer?
• Can we avoid that Peggy fools Vic into accepting false statements, even if Peggy is malicious and Vic does not trust her computer?

At EUROCRYPT 2015, Mironov and Stephens-Davidowitz introduced cryptographic reverse firewalls (RFs) as an attractive approach to tackling such questions.
Intuitively, a RF for Peggy/Vic is an external party that sits between Peggy/Vic and the outside world and whose scope is to sanitize Peggy’s/Vic’s incoming and outgoing messages in the face of subversion of her/his computer, e.g. in order to destroy subliminal channels.

In this paper, we put forward several natural security properties for RFs in the concrete setting of IPSes. As our main contribution, we construct efficient RFs for different IPSes derived from a large class of Sigma protocols that we call malleable. A nice feature of our design is that it is completely transparent, in the sense that our RFs can be directly applied to already deployed IPSes, without the need to re-implement them.

We seek constructions of general-purpose immunizers that take arbitrary cryptographic primitives, and transform them into ones that withstand a powerful “malicious but proud” adversary, who attempts to break security by possibly subverting the implementation of all algorithms (including the immunizer itself!), while trying not to be detected. This question is motivated by the recent evidence of cryptographic schemes being intentionally weakened, or designed together with hidden backdoors, e.g. with the scope of mass surveillance.

Our main result is a subversion-secure immunizer in the plain model (assuming collision-resistant hashing), that works for a fairly large class of deterministic primitives, i.e. cryptoschemes where a secret (but  tamperable) random source is used to generate the keys and the public parameters, whereas all other algorithms are deterministic. The immunizer relies on an additional independent source of public randomness, which is used to sample a public seed. While the public source is untamperable, the subversion of all other algorithms is allowed to depend on it.

Previous work in the area only obtained subversion-secure immunization for very restricted classes of primitives, often in weaker models of subversion and relying on random oracles, or by leveraging a higher number of independent random sources.

The fabrication process of integrated circuits (ICs) is complex and requires the use of off-shore foundries to lower the costs and to have access to leading-edge manufacturing facilities. Such an outsourcing trend leaves the possibility of inserting malicious circuitry (a.k.a. hardware Trojans) during the fabrication process, causing serious security issues. Hardware Trojans are very hard and expensive to detect and can disrupt the entire circuit or covertly leak sensitive information. In this paper, we propose a formal model for assessing the security of ICs whose fabrication has been outsourced to an untrusted off-shore manufacturer. We assume that the IC specification and design are trusted but the fabrication facility(ies) may be untrusted. Our objective is to stop Trojans from releasing sensitive information to the outside while still using its circuitry for day-to-day operations. We also provide two different methodologies for constructing compilers relying on verifiable computation (VC) schemes and secure multiparty computation (MPC) protocols with certain properties. Suitable VC schemes, with the properties we require, were recently constructed, e.g., by Parno et al. (Oakland ’13), and by Fiore, Gennaro, and Pastro (CCS ’14). Similarly, many MPC protocols readily comply (or can be easily adapted to comply) with our requirements. By allowing manufacturers to use off-shore fabrication facilities, we ensure a high degree of competition among suppliers, thus providing lower cost without hindering innovation or access to leading-edge microelectronics.

Signature schemes are arguably the most crucial cryptographic primitive, and devising tight security proofs for signature schemes is an important endeavour, as it immediately impacts the feasibility of deployment in real world applications. Hash-then-sign signature schemes in the Random Oracle Model, such as RSA-FDH, and Rabin-Williams variants are among the fastest schemes to date, but that unfortunately do not enjoy tight security proofs based on the one-wayness of their trapdoor function; instead, all known tight proofs rely on variants of the (non-standard) Φ-Hiding assumption.
As our main contribution, we introduce a family of hash-then-sign signature schemes, inspired by a lossy trapdoor function from Freeman et al. (JoC’ 13), that is tightly secure under the Quadratic Residuosity assumption. Our first scheme has the property of having unique signatures, while the second scheme is deterministic with an extremely fast signature verification, requiring at most 3 modular multiplications.

We put forward a new framework that makes it possible to re-write and/or compress the content of any number of blocks in decentralized services exploiting the blockchain technology. As we argue, there are several reasons to prefer an editable blockchain, spanning from the necessity to remove improper content and the possibility to support applications requiring re-writable storage, to “the right to be forgotten”.

Our approach generically leverages so-called chameleon hash functions (Krawczyk and Rabin, NDSS ’00), which allow to efficiently determine hash collisions given a secret trapdoor information. We detail how to integrate a chameleon hash function in virtually any blockchain-based technology, for both cases where the power of redacting the blockchain content is in the hands of a single trusted entity and where such a capability is distributed among several distrustful parties (as is the case in Bitcoin).

We also report on a proof-of-concept implementation of a redactable blockchain, building on top of Nakamoto’s Bitcoin core. The implementation only requires minimal changes to the way current client software interprets information stored in the blockchain and to the current blockchain, block, or transaction structures. Moreover, our experiments show that the overhead imposed by a redactable blockchain is small compared to the case of an immutable one.

Bitcoin is a peer-to-peer (p2p) electronic cash system that uses a distributed timestamp service to record transactions in a public ledger (called the Blockchain). A critical component of Bitcoin’s success is the decentralized nature of its architecture, which does not require or even support the establishment of trusted authorities. Yet the absence of certification creates obstacles to its wider acceptance in e-commerce and official uses. We propose a certification system for Bitcoin that offers: a) an opt-in guarantee to send and receive bitcoins only to/ from certified users; b) control of creation of bitcoins addresses (certified users) by trusted authorities. Our proposal may encourage the adoption of Bitcoin in different scenarios that require an officially recognized currency, such as tax payments—often an integral part of e-commerce transactions.