Zero-Knowledge proof, deniability and their applications in blockchain, e-voting and deniable secret handshake protocols/ Somnath Panja

Thesis (Ph.D.) - Indian Statistical Institute, 2021

Includes bibliographical references

Introduction -- Background -- A secure end-to-end verifiable e-voting system using blockchain and cloud server -- A Smart contract system for decentralized borda count voting -- Deniable secret handshake protocol -revisited -- Conclusion

Guided by Prof. Bimal Roy

In this thesis, we propose a cryptographic technique for an authenticated, end-to-end verifiable and secret ballot election. Currently, almost all verifiable e-voting systems require
trusted authorities to perform the tallying process except for the DRE-i and DRE-ip systems. We have shown a weaknesses in the DRE-ip system and proposed a solution. We have
modified the DRE-ip system so that no adversary can create and post a valid ballot on the
public bulletin board without detection. We provide security proofs to prove the security
properties of the proposed scheme. We propose two methods to store these ballots using
blockchain and cloud server. To the best of our knowledge, it is the first end-to-end verifiable
Direct-Recording Electronic (DRE) based e-voting system using blockchain. We introduce
an improved non-interactive zero-knowledge proof (NIZK) that boosts the efficiency of the
system. We propose a method for publishing the final tally without revealing the tally
from individual DRE machines using secure multi-party computation and NIZK proof. The
experimental data obtained from our tests show the protocol’s potential for real-world deployment. We also propose a secure and verifiable voter registration and authentication
mechanism. The proposed system prevents ballot stuffing attack.
We also propose the first self-tallying decentralized e-voting protocol for a ranked-choice
voting system based on Borda count. Our protocol does not need any trusted setup or
tallying authority to compute the tally. The voters interact through a publicly accessible
bulletin board for executing the protocol in a way that is publicly verifiable. Our main protocol consists of two rounds. In the first round, the voters publish their public keys, and in
the second round they publish their randomized ballots. All voters provide Non-interactive
Zero-Knowledge (NIZK) proofs to show that they have been following the protocol specification honestly without revealing their secret votes. At the end of the election, anyone
including a third-party observer will be able to compute the tally without needing any tallying authority. We provide security proofs to show that our protocol guarantees the maximum
privacy for each voter. We have implemented our protocol using Ethereum’s blockchain as
a public bulletin board to record voting operations as publicly verifiable transactions. The
experimental data obtained from our tests show the protocol’s potential for the real-world deployment.
We then propose a deniable secret handshake protocol. The notion of deniability ensures
that the transcript generated in an interactive protocol does not yield any evidence of the
interaction. In the context of key-exchange protocols for secure message transmission, the
notion of deniability is well-explored. On the other hand, a secret handshake protocol enables
a group of authorized users to establish a shared secret key and authenticate each other.
Recently, a framework for deniable secret handshake is proposed by Tian et al. in ISPEC
2018. We analyze the protocol, show three flaws and give solutions to prevent them.