Mario Larangeira

Tokyo Institute of Technology/Input Output HK · Department of Mathematical and Computing Sciences/Research Division

During the pandemic, the limited functionality of existing privacy-preserving contact tracing systems highlights the need for new designs. Wang et al. proposed an environmental-adaptive framework (CSS ’21) but failed to formalize the security. The similarity between their framework and attribute-based credentials (ABC) inspires us to reconsider con...

With the ever greater adaptation of blockchain systems, smart contract based ecosystems have formed to provide financial services and other utility. This results in an ever increasing demand for transactions on blockchains, however, the amount of transactions per second on a given ledger is limited. Layer-2 systems attempt to improve scalability by...

The proof-of-stake paradigm is used to implement a robust ledger, i.e., immutable and decentralized collection of records via the blockchain data structure, achieving similar security properties of the proof-of-work paradigm without the drawback of centralization risks and energy high costs and, at the same time, allowing a richer and more complex...

Proof-of-useful-work (PoUW), an alternative to the widely used proof-of-work (PoW), aims to re-purpose the network’s computing power. Namely, users evaluate meaningful computational problems, e.g., solving optimization problems, instead of computing numerous hash function values as in PoW. A recent approach utilizes the training process of deep lea...

DAG-based consensus has attracted significant interest due to its high throughput in asynchronous network settings. However, existing protocols such as DAG-rider (Keidar et al., PODC 2021) and ``Narwhal and Tusk'' (Danezis et al., Eurosys 2022) face two undesired practical issues: (1) high transaction latency and (2) high cost to verify transaction...

The efficiency of blockchain systems is often compared to popular credit card networks with respect to the transactions per second rate. This seems to be an unfair comparison since these networks do not complete a transaction from beginning to end. Rather they buy the risk and settle it much later. Typically transactions have only two players, the...

The \(\mathcal {S}_{\text{ leeve }}\) construction proposed by Chaum et al. (ACNS’21) introduces an extra security layer for digital wallets by allowing users to generate a “back up key” securely nested inside the secret key of a signature scheme, i.e., ECDSA. The “back up key”, which is secret, can be used to issue a “proof of ownership”, i.e., on...

This work introduces, to the best of our knowledge, the first stake based reputation and trust layer to proof of stake (PoS) system. Namely, we show that the delegation framework, introduced by Karakostas et al. (SCN’20) to provide a delegation framework, can be extended and repurposed to construct a trust layer over a PoS consensus protocol in add...

The S leeve construction proposed by Chaum et al. (ACNS'21) introduces an extra security layer for digital wallets by allowing users to generate a "back up key" securely nested inside the secret key of a signature scheme, i.e., ECDSA. The "back up key", which is secret, can be used to issue a "proof of ownership", i.e., only the real owner of this...

This work leverages on the framework of Karakostas et al. (SCN'20) by extending it to the realm of reputation and trust. At the best of our knowledge, it is the first to introduce reputation and trust to proof of stake systems. Namely, we show that their delegation framework can be repurposed to construct a trust layer over a proof of stake consens...

Recently, Chaum et al. (ACNS'21) introduced S leeve , which describes an extra security layer for signature schemes, i.e., ECDSA. This distinctive feature is a new key generation mechanism, allowing users to generate a "back up key" securely nested inside the secret key of a signature scheme. Using this novel construction, the "back up key", which...

Distributed ledger are maintained through consensus protocols executed by mutually distrustful parties. However, these consensus protocols have inherent limitations thus resulting in scalability issues of the ledger. Layer-2 protocols operate on channels and allow parties to interact with another without going through the consensus protocol albeit...

Distributed ledger are maintained through consensus protocols which have inherent limitations to their scalability. Layer-2 protocols operate on channels and allow parties to interact with another without going through the consensus protocol albeit relying on its security as fall-back. Channels can be concatenated into networks using techniques suc...

Recently, Chaum et al. (ACNS’21) introduced Sleeve, which describes an extra security layer for signature schemes, i.e., ECDSA. This distinctive feature is a new key generation mechanism, allowing users to generate a “back up key” securely nested inside the secret key of a signature scheme. Using this novel construction, the “back up key”, which is...

The COVID-19 pandemic has made the scientific community devise means to implement “contact tracing” mechanisms to mitigate the spread of the infection. The crucial idea is to scan and record close contacts between users using mobile devices, in order to notify persons when their close contact(s) is diagnosed positive. Current contact tracing system...

The COVID-19 pandemic has made the scientific community devise means to implement “contact tracing” mechanisms to mitigate the spread of the infection. The crucial idea is to scan and record close contacts between users using mobile device, in order to notify persons when their close contact(s) is diagnosed positive. First, the ability granted to s...

Proposed by Dwork and Naor (Crypto’ 92) as an anti-spam technique, proof-of-work is attracting more attention with the boom of cryptocurrencies. A proof-of-work scheme involves two types of participants, i .e., provers and verifiers. Provers are asked to solve a computational puzzle, and verifiers need to check the solution’s correctness. A widely...

We introduce the concept of turn-based communication channel between two mutually distrustful parties with communication consistency, i.e. both parties have the same message history, and happens in sets of exchanged messages across a limited number of turns. Our construction leverages on timed primitives. Namely, we consider a \(\mathsf{\Delta }\)-...

The security of blockchain based decentralized ledgers relies on consensus protocols executed between mutually distrustful parties. Such protocols incur delays which severely limit the throughput of such ledgers. Payment and state channels enable execution of offchain protocols that allow interaction between parties without involving the consensus...

Proof-of-Stake (PoS) distributed ledgers are the most common alternative to Bitcoin’s Proof-of-Work (PoW) paradigm, replacing the hardware dependency with stake, i.e., assets that a party controls. Similar to PoW’s mining pools, PoS’s stake pools, i.e., collaborative entities comprising of multiple stakeholders, allow a party to earn rewards more r...

We introduce a new key generation mechanism where users can generate a “back up key”, securely nested inside the secret key of a signature scheme. Our main motivation is that in case of leakage of the secret key, established techniques based on zero-knowledge proofs of knowledge are void since the key becomes public. On the other hand, the “back up...

Blockchain systems have severe scalability limitations e.g., long confirmation delays. Layer-2 protocols are designed to address such limitations. The most prominent class of such protocols are payment channel networks e.g., the Lightning Network for Bitcoin where pairs of participants create channels that can be concatenated into networks. These a...

Blockchain protocols based on Proof-of-Stake (PoS) depend—by nature—on the active participation of stakeholders. If users are offline and abstain from the PoS consensus mechanism, the system’s security is at risk, so it is imperative to explore ways to both maximize the level of participation and minimize the effects of non-participation. One such...

Blockchain protocols based on Proof-of-Stake (PoS) depend-by nature-on the active participation of stakeholders. If users are offline and abstain from the PoS consensus mechanism, the system's security is at risk, so it is imperative to explore ways to both maximize the level of participation and minimize the effects of non-participation. One such...

While many tailor made card game protocols are known, the vast majority of those lack three important features: mechanisms for distributing financial rewards and punishing cheaters, composability guarantees and flexibility, focusing on the specific game of poker. Even though folklore holds that poker protocols can be used to play any card game, thi...

The research on secure poker protocols without trusted intermediaries has a long history that dates back to modern cryptography's infancy. Two main challenges towards bringing it into real-life are enforcing the distribution of the rewards, and penalizing misbehaving/aborting parties. Using recent advances on cryptocurrencies and blockchain technol...

While many tailor made card game protocols are known, the vast majority of those lack three important features: mechanisms for distributing financial rewards and punishing cheaters, composability guarantees and flexibility , focusing on the specific game of poker. Even though folklore holds that poker protocols can be used to play any card game, th...

Blockchain based systems, in particular cryptocurren-cies, face a serious limitation: scalability. This holds, especially, in terms of number of transactions per second. Several alternatives are currently being pursued by both the research and practitioner communities. One venue for exploration is on protocols that do not constantly add transaction...

The two main challenges in deploying real world secure poker protocols lie in enforcing the distribution of rewards and dealing with misbehaving/aborting parties. Using recent advances in cryptocurrencies and blockchain techniques, Kumaresan et al. (CCS 2015) and Bentov et al. (ASIACRYPT 2017) were able to solve those problems for the general case...

A Mobile Ad-Hoc Network (MANET) automatically reorganizes itself, allowing moving nodes to join or leave the network at any point in time without disrupting the communication. An essential element of such systems is a routing protocol able to quickly restructure existing routes when nodes leave and provide routes to new nodes. In most MANET routing...

While many cryptographic protocols for card games have been proposed, all of them focus on card games where players have some state that must be kept secret from each other, e.g closed cards and bluffs in Poker. This scenario poses many interesting technical challenges, which are addressed with cryptographic tools that introduce significant computa...

While many cryptographic protocols for card games have been proposed, all of them focus on card games where players have some state that must be kept secret from each other, e.g closed cards and bluffs in Poker. This scenario poses many interesting technical challenges, which are addressed with cryptographic tools that introduce significant computa...

Bitcoin is the first successful decentralized global digital cash system. Usefulness of the mining process requiring a lot of computational resources to be wasted, though, remains disputable. One of possible alternatives for useful Proof-of-Work schemes, Permacoin, is using non-interactive proofs of a static dataset retrievability thus providing a...

The security proofs which do not rely on the programmability of the Random Oracle Model (ROM) have the advantage that the reduction of the proof can be actually constructed. This feature contrasts with reductions that rely on the programmability, because hash functions with programmability capacities are not known to exist in the level required in...

The programmability has long been used as a tool to prove security of schemes in the random oracle model (ROM) even in the cases where schemes do not seem to have a security proof in the standard model [3, 8, 10]. On the other hand, it seems that a similar property has never been studied in the generic models, i.e., the generic ring and group model...