Key establishment \`a la Merkle in a quantum world

TL;DR

This paper introduces new key establishment schemes inspired by Merkle's work, demonstrating varying levels of security against quantum adversaries, including a classical scheme resistant to quantum attacks and quantum protocols with near-quadratic security.

Contribution

The paper presents two novel key establishment schemes, one classical and one quantum, with security levels against quantum adversaries that improve upon previous schemes.

Findings

First scheme broken by quantum effort proportional to N^{5/3}

Second scheme is classical and resistant to quantum attacks at effort level of legitimate parties

Quantum protocols achieve security close to quadratic in the query complexity model

Abstract

In 1974, Ralph Merkle proposed the first unclassified scheme for secure communications over insecure channels. When legitimate communicating parties are willing to spend an amount of computational effort proportional to some parameter N, an eavesdropper cannot break into their communication without spending a time proportional to N^2, which is quadratically more than the legitimate effort. Two of us showed in 2008 that Merkle's schemes are completely insecure against a quantum adversary, but that their security can be partially restored if the legitimate parties are also allowed to use quantum computation: the eavesdropper needed to spend a time proportional to N^{3/2} to break our earlier quantum scheme. Furthermore, all previous classical schemes could be broken completely by the onslaught of a quantum eavesdropper and we conjectured that this is unavoidable. We give now two novel key establishment schemes in the spirit of Merkle's. The first one can be broken by a quantum adversary who makes an effort proportional to N^{5/3}, which is the optimal attack against this scheme. Our second scheme is purely classical, yet it cannot be broken by a quantum eavesdropper who is only willing to expend an effort proportional to that of the legitimate parties. We then introduce two families of more elaborate protocols. The first family consists in quantum protocols whose security is arbitrarily close to quadratic in the query complexity model. The second is a family of classical protocols whose security against a quantum adversary is arbitrarily close to N^{3/2} in the same model.