Black Hat Europe | LINE-Break: Cryptanalysis And Reverse Engineering Of Letter Sealing

## Overview

At Black Hat Europe, speakers Diego De Freitas Aranha (Associate Professor at Aarhus University) and Thomas Mogensen (presenting master’s research conducted alongside PhD student Adam Hansen) introduced **LINE-Break**, a security analysis and partial reverse engineering of **Letter Sealing**. Letter Sealing serves as the end-to-end encrypted (E2EE) messaging layer in LINE, a major "super app" developed by LY Corp (a merger of LINE Corporation and Yahoo Japan).

LINE functions similarly to platforms like WeChat or planned iterations of X, offering shopping, tax filing, messaging, and government interaction. The app has nearly 1 billion interacting users and dominates key regional markets, notably Japan, Thailand, Taiwan, and Indonesia. In Japan and Taiwan specifically, internet penetration reaches roughly 89% to 90% of the population. Because user threat models vary significantly across these geopolitical regions, the underlying security assurances of the messaging service are critical.

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## Evolution of Letter Sealing: V1 vs. V2

Letter Sealing has gone through multiple iterations, primarily driven by prior vulnerability disclosures:

* **Letter Sealing V1**: Employed Diffie-Hellman key exchange alongside AES in CBC mode (AES-CBC), coupled with a custom Message Authentication Code (MAC) for message integrity and authentication. Prior academic and security analysis revealed critical vulnerabilities regarding message integrity and unprotected metadata resulting from this non-standard primitive combination. * **Letter Sealing V2**: Developed and deployed by LINE to address these integrity concerns. V2 transitioned from AES-CBC with a custom MAC to standard authenticated encryption using AES in Galois/Counter Mode (AES-GCM), alongside Diffie-Hellman key exchange.

While V2 adopted more principled cryptographic primitives, the researchers emphasize that selecting standard primitives is insufficient on its own if the broader protocol design and combination of components remain flawed.

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## Coverage and Scope Limitations

Letter Sealing was deployed progressively over time across different content types and features within LINE. As a result, several communications and message formats remain entirely excluded from E2EE protection:

* **Excluded Features**: Stickers and message reactions (emoji-based reactions sent to messages) are transmitted without end-to-end encryption. * **Account and Group Constraints**: Letter Sealing is not applied to interactions with official business accounts. Additionally, it is disabled for large group communications exceeding 50 members.

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## Technical Context and Security Properties

The presentation frames the evaluation of Letter Sealing against established literature surrounding end-to-end encryption. While consensus definitions of E2EE vary across the industry, robust deployments are evaluated across four primary security properties, beginning with confidentiality against untrusted servers, with subsequent analysis examining protocol implementation, key exchange mechanics, and practical attack demonstrations against the protocol.