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

2026-02-20

AI Summary

This work presents the first formal security analysis of the LINEv2 protocol, revealing its lack of forward secrecy (FS) and post-compromise security (PCS), which poses significant long-term communication risks. Leveraging an enhanced multi-stage key exchange (MSKE) model, the study rigorously evaluates LINEv2’s security properties under realistic adversarial scenarios. To address these vulnerabilities, the authors propose a strengthened protocol variant that seamlessly integrates FS and PCS while preserving the original functionality and performance. The enhanced protocol is supported by formal proofs of security and empirical performance benchmarks, demonstrating its practical feasibility and substantially improved resilience against long-term threats.

Technology Category

• Natural Language Processing: Safety and Robustness
• Application Domains: Security
• Computer Vision: Language and Vision

Application Category

• Security and Privacy: Large-scale security measurements
• Responsible Web: Measurement, analysis, and circumvention of Web censorship
• User Modeling, Personalization and Recommendation: Attacks and countermeasures in recommendation systems

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.

Problem

• forward secrecy
• post-compromise security
• LINE protocol
• end-to-end encryption
• cryptographic security

Innovation

• Forward Secrecy
• Post-Compromise Security
• Provable Security
• End-to-End Encryption
• Multi-Stage Key Exchange