title: Distributed Computation Meets Blockchain: Advanced Cryptographic Services from Blockchain Features
Creators: name: Kretzler, David
Publisher: name: UNSPECIFIED
Year: 2024

description: Today's blockchain systems are no longer just about financial transactions within decentralized networks. Instead, they offer a wide range of additional features. A recent trend in cryptography leverages the rich functionality provided by blockchains to implement new cryptographic services and enhance existing ones. However, the potential of blockchain systems is far from exhausted and there is still significant room for improvement in existing blockchain-based cryptographic solutions. This thesis, therefore, aims to identify and unlock further potential for providing more advanced cryptographic services by identifying and closing gaps in prior work on blockchain-based cryptography.
Covert security, introduced by Aumann and Lindell (TCC'07), is a security notion for cryptographic protocols that allows an adversary to successfully cheat and break the protocol's security with a fixed probability 1-e, while honest parties are guaranteed to detect the cheating attempt with probability e. Zhu et al. (CCS'19) proposed strengthening this notion by financially punishing detected cheaters via a smart contract. However, their work focuses on a specific two-party protocol. This thesis advances their ideas by demonstrating how to transform an arbitrary semi-honest secure protocol into a financially-backed covert secure protocol combining cheating detection with immediate financial punishment.
Witness encryption, a primitive introduced by Garg et al. (STOC'13), allows a party to encrypt a message under a statement x from an NP-language L with relation R, such that the ciphertext can only be decrypted by a party knowing the corresponding witness w for which R(x,w) holds. Unfortunately, known instantiations of general-purpose witness encryption are based on strong assumptions and lack efficiency. Moreover, the standard notion of witness encryption does not consider the need to keep the statement used for encryption private. Goyal et al. (PKC'22) addressed the former shortcoming by demonstrating how a committee elected by a blockchain can provide a service equivalent to witness encryption but with significant higher efficiency and without the need of strong cryptographic assumptions. We advance on this idea by showing how such a committee-based approach to witness encryption can be adopted without disclosing the statement used for encryption, thereby addressing the latter shortcoming.
We envision a virtual trusted third party (V-TTP) as a service that is continuously available, strictly adheres to expected behavior, keeps its state and communication secret, and is capable of performing complex computations. A promising approach to implementing a V-TTP is through a smart contract deployed on a blockchain. Smart contracts inherit excellent liveness guarantees from the underlying blockchain and ensure the correct execution of their code. However, traditional smart contracts are inherently public and limited in their complexity. While numerous proposals address these limitations, they often focus on only one aspect or introduce new shortcomings, such as requiring locked collateral. In this thesis, we propose a new smart contract platform that addresses the limitations of previous smart contract systems in one holistic solution.
Naturally, utilization and improvement of blockchain features go hand in hand. During our work with smart contracts, we identified several shortcomings in the prevalent approach to smart contract development. We address these shortcomings by proposing a new programming language for smart contracts, which reduces the risk of security critical programming errors and increases the usability of smart contracts.