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.

翻译：LINE已成为包括泰国和日本在内的许多东亚国家中最受欢迎的通信平台之一，拥有数百万活跃用户。因此，理解其安全保证至关重要。在本工作中，我们首次对LINE第二版（LINEv2）消息协议进行了可证明的安全分析，重点关注其在真实环境中的密码学保证。我们通过修改多阶段密钥交换（MSKE）模型——一种在对抗条件下分析密码协议的理论框架——来刻画LINE消息协议的架构与安全性。虽然LINEv2实现了密钥不可区分性和消息认证等基本安全属性，但我们指出其缺乏前向安全性（FS）与后泄露安全性（PCS）。为解决这一问题，我们提出了一个增强版的LINE协议，为其引入FS与PCS，并对结果进行了分析与基准测试。