Secure Multiparty Protocols on Blockchain with Fairness and
Scalability
Authors: Holmes Joshua 
Published: January 2025

Secure multiparty computation is a major field of research in modern
cryptography. It allows for the creation of a protocol that maintains the
privacy of the inputs and ensures that violation of the protocol results in no
undue benefit to the violator or detriment to an honest party. These protocols
can be used in many fields. In this dissertation, we explore the application of
the mechanisms of secure party computation in the context of peer-to-peer
lending, fair exchange with cryptocurrencies, consensus, and electronic voting.
In all these areas, honesty of execution and fairness in the outcome should be
assured or verifiable, especially if the other parties are not trusted. In this
dissertation, we designed protocols to solve the above-mentioned problems,
analyzed their efficiency and scalability, and proved their security.

First, we present a platform called ZeroLender for peer-to-peer lending in
Bitcoin. Our protocol utilizes zero-knowledge proofs to achieve unlinkability
between lenders and borrowers while securing payments in both directions against
potential malicious behavior of the ZeroLender as well as the lenders and covert
action by the borrowers. We prove by simulation that our protocol is
privacy-preserving. Based on our experiments, we show that the runtime and
transcript size of our protocol scale linearly with respect to the number of
lenders and repayments.

Second, we propose a generic framework for atomic swap, called PolySwap, that
enables fair exchange of assets between two {heterogeneous sets of blockchains}.
Our construction preserves the anonymity of the swap by preventing transactions
from being linked to each other or be distinguishable from other transactions on
the blockchain and does not require any scripting capability in the blockchain,
all without requiring a third party. We provide construction details of secret
sharing signatures for ECDSA, Schnorr, and CryptoNote-style Ring signatures.
Additionally, we provide an alternative contingency protocol, allowing parties
to exchange to and from blockchains that do not support any form of time-locked
escape transactions. We prove that PolySwap is secure against malicious
adversaries, and is privacy-preserving against passive observers. We conducted
experiments to demonstrate the efficiency of the protocol.

Third, we propose ACCORD, a consensus protocol consisting of three distinct
components: an asynchronous quorum selection procedure to designate the creators
of future blocks, a block creation protocol run by the quorum to prevent
omissions in the presence of honest quorum members, and a decentralized
arbitration protocol to ensure consensus by voting. We implemented the protocol
and conducted experiments to demonstrate scalability, robustness, and
fairness.

Finally, we introduce ORBIT, a cryptographic voting protocol that uses hidden
credentials and mutable identities through ciphertext manipulation to prevent
coercion. This enables voters to submit dummy ballots that are indistinguishable
from genuine ones, thus enabling them to evade potential coercers, as well as
preventing a fully compromised government from determining their voting
preferences. ORBIT is blockchain-based, allowing government verifiers to process
incoming ballots as they are submitted. We implemented ORBIT and performed
experiments that illustrate its linear scaling in relation to three key
variables: election size, the number of ring members within the anonymity set,
and the number of ballots.