Cryptanalysis of a privacy-preserving behavior-oriented authentication scheme

TL;DR

This paper critically analyzes a privacy-preserving behavioral authentication scheme, revealing vulnerabilities that allow data and key recovery, thus questioning its security assumptions and effectiveness.

Contribution

It demonstrates that the proposed homomorphic encryption-based scheme is insecure against honest-but-curious servers and active eavesdroppers, providing concrete attack methods.

Findings

The scheme leaks secret user keys and behavior data to the server.

Active eavesdroppers can recover plaintext behavior data from encrypted transmissions.

The proposed attacks compromise the privacy guarantees of the original scheme.

Abstract

Continuous authentication has been proposed as a complementary security mechanism to password-based authentication for computer devices that are handled directly by humans, such as smart phones. Continuous authentication has some privacy issues as certain user features and actions are revealed to the authentication server, which is not assumed to be trusted. Wei et al. proposed in 2021 a privacy-preserving protocol for behavioral authentication that utilizes homomorphic encryption. The encryption prevents the server from obtaining sampled user features. In this paper, we show that the Wei et al. scheme is insecure regarding both an honest-but-curious server and an active eavesdropper. We present two attacks: The first attack enables the authentication server to obtain the secret user key, plaintext behavior template and plaintext authentication behavior data from encrypted data. The second attack enables an active eavesdropper to restore the plaintext authentication behavior data from the transmitted encrypted data.