# Semi-quantum communication: Protocols for key agreement, controlled secure direct communication and dialogue

## TL;DR

This paper introduces semi-quantum protocols enabling classical users to participate in secure communication tasks, expanding quantum security methods to more practical, less quantum-dependent scenarios with new protocols and security analyses.

## Contribution

It proposes the first semi-quantum protocols for key agreement, controlled secure communication, and dialogue, demonstrating their equivalence to fully quantum protocols in secure tasks.

## Key findings

- Semi-quantum protocols for key agreement, secure communication, and dialogue are feasible.
- Some schemes are based solely on orthogonal states, differing from conjugate-coding schemes.
- Security and efficiency of the protocols are thoroughly analyzed.

## Abstract

Semi-quantum protocols that allow some of the users to remain classical are proposed for a large class of problems associated with secure communication and secure multiparty computation. Specifically, first time semi-quantum protocols are proposed for key agreement, controlled deterministic secure communication and dialogue, and it is shown that the semi-quantum protocols for controlled deterministic secure communication and dialogue can be reduced to semi-quantum protocols for e-commerce and private comparison (socialist millionaire problem), respectively. Complementing with the earlier proposed semi-quantum schemes for key distribution, secret sharing and deterministic secure communication, set of schemes proposed here and subsequent discussions have established that almost every secure communication and computation tasks that can be performed using fully quantum protocols can also be performed in semi-quantum manner. Further, it addresses a fundamental question in context of a large number problems- how much quantumness is (how many quantum parties are) required to perform a specific secure communication task? Some of the proposed schemes are completely orthogonal-state-based, and thus, fundamentally different from the existing semi-quantum schemes that are conjugate-coding-based. Security, efficiency and applicability of the proposed schemes have been discussed with appropriate importance.