Soft detection physical layer insecurity

TL;DR

This paper introduces a decoding confidence measure based on LLR during GRAND decoding, which can be used to improve decoding reliability and potentially compromise physical layer security in wiretap codes.

Contribution

It presents a novel interpretable soft output measure during GRAND decoding, enabling confidence assessment and security analysis of wiretap codes.

Findings

LLR-based confidence measure can discard erroneous decodings.

The measure can reveal parts of communication in wiretap scenarios.

Decoding confidence correlates with decoding correctness.

Abstract

We establish that during the execution of any Guessing Random Additive Noise Decoding (GRAND) algorithm, an interpretable, useful measure of decoding confidence can be evaluated. This measure takes the form of a log-likelihood ratio (LLR) of the hypotheses that, should a decoding be found by a given query, the decoding is correct versus its being incorrect. That LLR can be used as soft output for a range of applications and we demonstrate its utility by showing that it can be used to confidently discard likely erroneous decodings in favor of returning more readily managed erasures. We show that feature can be used to compromise the physical layer security of short length wiretap codes by accurately and confidently revealing a proportion of a communication when code-rate is far above the Shannon capacity of the associated hard detection channel.