The Effective Key Length of Watermarking Schemes

TL;DR

This paper introduces the effective key length as a new measure of security for watermarking schemes, demonstrating its application to additive spread spectrum methods and revealing their vulnerabilities under certain attack scenarios.

Contribution

It proposes the effective key length as a practical security metric, extending the analysis beyond the embedding process to assess overall scheme security.

Findings

Additive spread spectrum schemes are insecure under Known Message Attack.

The effective key length can be computed theoretically and practically.

The new measure captures the difficulty for adversaries to access the watermarking channel.

Abstract

Whereas the embedding distortion, the payload and the robustness of digital watermarking schemes are well understood, the notion of security is still not completely well defined. The approach proposed in the last five years is too theoretical and solely considers the embedding process, which is half of the watermarking scheme. This paper proposes a new measurement of watermarking security, called the effective key length, which captures the difficulty for the adversary to get access to the watermarking channel. This new methodology is applied to additive spread spectrum schemes where theoretical and practical computations of the effective key length are proposed. It shows that these schemes are not secure as soon as the adversary gets observations in the Known Message Attack context.