Drawing the LINE: Cryptographic Analysis and Security Improvements for the LINE E2EE Protocol

TL;DR

This paper provides the first cryptographic security analysis of LINEv2, revealing its security strengths and weaknesses, and proposes enhancements to include forward secrecy and post-compromise security.

Contribution

It introduces a formal security analysis of LINEv2 using a modified MSKE model and proposes improvements to enhance its cryptographic guarantees.

Findings

LINEv2 achieves key indistinguishability and message authentication

Lacks forward secrecy and post-compromise security

Proposes a stronger protocol with improved security features

Abstract

LINE has emerged as one of the most popular communication platforms in many East Asian countries, including Thailand and Japan, with millions of active users. Therefore, it is essential to understand its security guarantees. In this work, we present the first provable security analysis of the LINE version two (LINEv2) messaging protocol, focusing on its cryptographic guarantees in a real-world setting. We capture the architecture and security of the LINE messaging protocol by modifying the Multi-Stage Key Exchange (MSKE) model, a framework for analysing cryptographic protocols under adversarial conditions. While LINEv2 achieves basic security properties such as key indistinguishability and message authentication, we highlight the lack of forward secrecy (FS) and post-compromise security (PCS). To address this, we introduce a stronger version of the LINE protocol, introducing FS and PCS to LINE, analysing and benchmarking our results.