LINE Encryption Report

Purpose of this Report

The purpose of this report is to describe the type, scope, and deployment status of the encryption mechanisms integrated into each of LINE's main features.

Encryption in LINE

LINE employs various encryption technologies to protect user information. In addition to transport layer encryption, which is used to protect traffic between LINE clients and servers, we also apply Letter Sealing encryption to supported message types and voice/video calls. Letter Sealing is the name of the end-to-end encryption (E2EE) protocol developed by LINE. For details about the encryption protocols and algorithms used in LINE, please refer to our Encryption Whitepaper.

Major updates

In November 2023 LINE started to roll out end-to-end encryption not only for messages and calls but now also for images, voice messages, videos, and file attachments. This feature enables higher level of privacy and security and was transparently enabled for all users worldwide as of November 2024. Under the same conditions as for existing E2EE chat rooms, as long as all chat participants have upgraded to a compatible LINE version, those additional content types will be automatically end-to-end encrypted.

Encryption Deployment Status

(1) LINE Messaging

Letter sealing (End-to-end encryption)

Text, location, media files and calls in 1-to-1 chat room using LINE's messaging feature are end-to-end encrypted with Letter Sealing if one of the following conditions is met.

• Both users have Letter Sealing enabled in 1-to-1 chats
• All users have Letter Sealing enabled in group chats (groups with up to 50 members)

If you want to see if the chat room is protected by Letter Sealing, you can check the room menu. Please refer to the guide here. Messages other than text and location messages, such as image, video, file, and audio messages in lower versions that does not support Media e2ee, are encrypted only using transport layer encryption – either by LEGY*1 encryption or TLS. The chart below shows the transport layer encryption deployment status for each type of message for the period Jul 2024 - Dec 2024.

Figure 1: Transport encryption deployment status (2024)

• *1LEGY stands for Line Event-delivery GatewaY, and it's a custom-built API gateway server. LEGY uses standard cryptography algorithms for key exchange and encryption.
• *For data before Aug 2022, please refer to the previous Encryption Report.

Transport layer encryption

The former default encryption was LEGY encryption. But we are migrating to TLS, and TLS is the default encryption protocol. Currently TLS 1.2 and 1.3 are supported.

We use either DHE or ECDHE for the key exchange to make sure forward secrecy.

Due to certain technical and environmental constraints, transport layer encryption was not sufficiently deployed for certain media types. However, starting from September 2017, we have maintained a transport layer encryption rate very close to 100%.

• Due to performance issues related to encryption, transport encryption deployment on certain platforms was delayed.
• In some countries, HTTPS could not be used on mobile networks for a certain period of time. Currently, transport encryption is deployed irrespective of network connectivity type.
• In July 2017, in order to resolve an issue with deploying transport encryption for audio files, transport encryption was scaled down for about two months. In September 2017, transport encryption scope was restored.

The following table summarizes the encryption (Letter Sealing, LEGY Encryption, TLS) deployment status for each message and media type.

Legend: ◎ Letter Sealing (end-to-end encryption) / ◯ Transport-level encryption / △ Partially protected / ☓ Not encrypted or not sufficiently encrypted

Notes

◎ Letter Sealing is enabled by default in major LINE clients.
◯ Sufficiently secure transport encryption, as evaluated at release time, is employed by major LINE clients.
△ Partial data protection was applied. The security level of the employed encryption was considered adequate both at release time, and at the time of writing of this report.
☓ The referenced message type or functionality did not have sufficient protection, as evaluated at the time of writing of this report.

• *2△ During this period, HTTP was used to download sticker packages, but messages that include stickers were themselves encrypted.
• *3△ During this period, both HTTP and HTTPS were used. Depending on usage environment (OS, region, connection type, etc.) transport encryption may not have been applied.
• *4☓ During this period, transport encryption was employed only for uploads, when connected via WiFi.
• *5Image files and video files saved in the user album are outside the scope of Letter Sealing and only client-server encryption using TLS is applied.

For users running older versions of the LINE client, or using an older or unsupported OS version, the encryption methods described above may not be applicable, and an older encryption method or algorithm may be used instead. In such cases, LINE communication may not be sufficiently protected.

(2) LINE Free Calls (Audio and Video Calls)

LINE supports several types of free calls. Of those, 1-to-1 audio calls and 1-to-1 video calls are end-to-end encrypted using Letter Sealing as detailed in the table below. Group calls, group video calls, and Line meeting are only protected with transport-level encryption.

Legend: ◎ Letter Sealing (end-to-end encryption) / ◯ Transport-level encryption / - Not implemented

(3) Media End-to-end encryption

When LINE released the initial implementation of Letter Sealing in 2016, constraints such a device performance and incompatible media codec supports across platforms forced us to limit encryption to text content only. As those constraints have now subsided and with a renewed focus on user privacy, LINE decided to expand the scope of Letter Sealing and apply it to more content types. As part of this initiative, images, voice messages, videos, and file attachments send inside Letter Sealing enabled chat rooms are now also end-to-end encrypted. This feature rolled out first in Korea, Indonesia in Nov 2023 as part of a testing phase, and was then gradually rolled out to the remaining regions. The rollout to all regions was completed in Nov 2024 and the required LINE client minimum versions are detailed in the table below.

(4) Letter Sealing (end-to-end encryption) Deployment Status

Letter Sealing is LINE's end-to-end encryption protocol. Message types that support Letter Sealing are encrypted on the LINE client before being sent, and cannot be decrypted by LINE's servers. Letter Sealing was initially released as an optional feature in 2015/8 and was enabled by default in major clients in 2016. Currently most message types are encrypted with Letter Sealing but there are some exceptions.

Letter Sealing Protection Scope

• Text messages (in 1-to-1 chats, and in group chats with up to 50 members)
• Location messages (in 1-to-1 chats, and in group chats with up to 50 members)
• Media files (Image, Voice, Video, Other file) (in 1-to-1 chats, and in group chats with up to 50 members)
• Audio calls (1-to-1 calls)
• Video calls (1-to-1 calls)

Letter Sealing Exception

In the following cases, a part of user communication can be sent to LINE servers

• Website preview(aka Pagepoker): To generate the website preview in the chat room, URLs will be sent to the Pagepoker server. Such URLs are processed solely for the purpose of preview generation. Users can opt-out of this function in the settings.(Settings → Chats → URL previews)
• Spam report: When a user wants to report a spam issue, the suspicious part of the chat message will be sent to the server for investigation. The message is sent to the server only if the reporter consents.
• Announced messages: You can "announce" things like messages you've sent and received, polls you created, and events so they permanently appear at the top of a chat. To make them available to newly joinned members, they are sent to the server.
• Sticker keyword: For LINE to be able to recommend a suitable sticker for the chat context, the LINE client will check the certain keywords in the messages. And if there's a match, it will be sent to the server anonymously.

Also, the messages will not be end-to-end encrypted when you use the cloud backup feature provided by a third party such as Apple and Google. In a such case the raw content will be stored on the cloud storage.

Deployment Status Summary by Features

• *6To generate a sticker with a custom wording of a user's choice, the user's client needs to talk to the rendering server. The text communication between the client and the rendering server is protected by E2EE. But the result image will be protected by transport layer encryption only.

Requirements for using Letter Sealing

Letter Sealing is enabled by default in all regions and cannot be manually turned it off since 2021.

Letter Sealing Versions

The original version of LINE messaging end-to-encryption protocol, released in 2015, is referred to as Letter Sealing v1. While v1 provides end-to-end encryption for both 1-to-1 and group chats, several protocol-level attacks that could potentially lead to message forgery and user impersonation were reported via our Bug Bounty Program by Takanori Isobe (University of Hyogo, Japan) and Kazuhiko Minematsu (NEC Corporation, Japan). We verified that the attacks are not practically feasible due to additional server-side checks and restrictions implemented in LINE's messaging servers. In order to improve the security of our users and avoid potential future attacks against Letter Sealing, we developed Letter Sealing v2 in collaboration with the researchers. Details about Letter Sealing v2 can be found in our Encryption Whitepaper.

Letter Sealing v2 was deployed in October 2019 to all major LINE clients. The table below shows the minimum version of each LINE client required to support Letter Sealing v2.

Letter Sealing v2 usage status

Figure 2: Letter Sealing v2 ratio in the total Letter Sealing traffic (2024)

E2EE v2 has been deployed since 2019. The graph above shows the ratio of E2EE v2 in the total E2EE traffic. For the previous data, please refer to the previous Encryption Report.

Letter Sealing key migration

When you change your phone, you need to transfer the Letter Sealing key in order to decrypt the previous messages.
In case you still have your old phone, you can transfer the Letter Sealing key. For the details, please refer to the official guide.
For the safety, we provide an end-to-end encryption for the Letter Sealing key. Each device generates an ephemeral ECDH key pair. And the old device's public key is sent via the QR code as an Out-Of-Band cryptographic data. Then the Letter Sealing key is encrypted by AES256-GCM using the derived key.

Letter Sealing key backup

It is possible to do Letter Sealing key backup to keep it safe. Then you can access the encrypted messages, even if you lost the phone. For the details enabling it, please refer to the official guide.

To keep the confidentiality, the Letter Sealing key is encrypted using "client-side encryption"; the Letter Sealing key is encrypted by the key generated from 6-digit PIN code. More over, the encrypted Letter Sealing key is protected by our SGX server; the encrypted Letter Sealing key is also end-to-end encrypted by our SGX server.

(5) Forward Secrecy Deployment Status

Forward secrecy is enabled in some LINE usage environments. Even if one of the parties' long-term private key is leaked or compromised, forward secrecy guarantees that messages encrypted before the time of compromise are still protected. Currently, only some of the LINE's communication channels support forward secrecy.

Forward Secrecy for LINE Client-Server Communication (forward-security in the case of LINE server key leak)

2021 Supporting cases reduced *7
2017/9 ○ Supported for major clients *8
2016 △ Partially supported *9

• *7TLS1.3 0-RTT(Zero Round Trip Time) has been enabled
• *8May not be supported depending on OS and LINE client version
• *9Supported for some regions and client versions

Forward Secrecy in Letter Sealing (forward-security in the case of Letter Sealing private key leak)

Not supported