Secrecy Communication with Security Rate Measure

TL;DR

This paper introduces the concept of security rate based on rate-distortion theory, providing a new measure of secrecy that quantifies the minimum additional rate needed for a wiretapper to reconstruct a source within a certain distortion level.

Contribution

It extends the traditional equivocation measure to a lossy decryption scenario and characterizes the region of key rate, coding rate, and wiretapper distortion in a Shannon cipher system.

Findings

Defines security rate as a new secrecy measure.

Establishes the equivalence between security rate and distortion-based equivocation.

Characterizes the admissible region of system parameters.

Abstract

We introduce a new measure on secrecy, which is established based on rate-distortion theory. It is named \emph{security rate}, which is the minimum (infimum) of the additional rate needed to reconstruct the source within target distortion level with any positive probability for wiretapper. It denotes the minimum distance in information metric (bits) from what wiretapper has received to any decrypted reconstruction (where decryption is defined as reconstruction within target distortion level with some positive possibility). By source coding theorem, it is equivalent to a distortion-based equivocation $\mathop{\min}\limits _{p\left(v^{n}|s^{n},m\right):Ed\left(S^{n},V^{n}\right)\le D_{E}}\frac{1}{n}I\left(S^{n};V^{n}|M\right)$ which can be seen as a direct extension of equivocation $\frac{1}{n}H\left(S^{n}|M\right)$ to lossy decryption case, given distortion level $D_{E}$ and the received (encrypted) message $M$ of wiretapper. In this paper, we study it in Shannon cipher system with lossless communication, where a source is transmitted from sender to legitimate receiver secretly and losslessly, and also eavesdropped by a wiretapper. We characterize the admissible region of secret key rate, coding rate of the source, wiretapper distortion, and security rate (distortion-based equivocation). Since the security rate equals the distortion-based equivocation, and the equivocation is a special case of the distortion-based equivocation (with Hamming distortion measure and $D_{E}=0$), this gives an answer for the meaning of the maximum equivocation.