Paper 2026/2649

LINE-Break: Cryptanalysis and Reverse Engineering of Letter Sealing

Abstract

We present a security analysis of the messaging service known as LINE, a popular platform used daily by millions of users in East and Southeast Asia – most notably Japan, Taiwan, Thailand, and Indonesia. More specifically, we focus on its underlying custom end-to-end encryption (E2EE) protocol, known as Letter Sealing v2. Our findings show that Letter Sealing allows a TLS Machine-in-the- Middle attacker or malicious server to violate integrity, authenticity, and confidentiality of communications. The stateless design of the protocol allows message replay, reordering, and blocking attacks without the user being notified. The lack of origin authentication facilitates impersonation attacks, in which the authorship of mes- sages in one-to-one or group chats can be forged by malicious users colluding with the adversary. Lastly, stickers and URL previews present a notable leakage of plaintext, which leads to a violation of confidentiality. To verify the correctness of our findings, we mounted a Machine-in-the-Middle attack on an iOS device, yielding the device’s outgoing traffic and the corresponding server responses. Utilizing this setup, we experimentally verified our attacks against the authentic LINE application and an independent implementation. We discuss our findings in comparison to the state-of-the-art E2EE protocols, and conclude that Letter Sealing does not satisfy the requirements expected from a modern E2EE messaging protocol.

Note: Minor revision of proceedings version, with some typos corrected.