In New generation of safe messaging: “Letter Sealing”, we announced that end-to-end encryption (E2EE) has been made available on LINE messages.

But we've made even more improvements to safe messaging over the past few months, expanding Letter Sealing to features other than one-on-one chats. We'd like to share some of them here in this post.

What changed?

Letter Sealing is now enabled on one-on-one chats by default

In the previous version of LINE, you had to opt in to use Letter Sealing. With the latest version we've made it so that it's enabled by default. You can still opt out if you wish to do so.

One reason we couldn't enable Letter Sealing by default for everyone was because of an issue we had with the iOS push notification system. LINE displays a small part of the messages you receive through push notifications. Unlike on Android, the push notifications on iOS gives apps much less room to work with. When someone sends a message to you, the servers normally only send part of the message to you through a push notification. The LINE server cannot read these Letter Sealing-enabled messages and thus can't display the contents of the message on the notification.

Fortunately, a new type of notification called VoIP push notification was added to iOS 8. We were able to display the contents of messages using VoIP notifications to run the LINE app to decrypt encrypted messages. The team confirmed that VoIP notifications were very stable starting with iOS 9.3.1, and decided that Letter Sealing wass ready to be enabled as a default option.

Letter Sealing can now be enabled in group chats

Your messages are now safer even in group chats.

Messages inside a group chat with Letter Sealing enabled can only be encrypted by users in the same chat. When someone leaves the group, that user is no longer be able to decrypt the messages sent in the group.

Another thing to consider is that users in a group chat are not always online at any given time, and can't always receive messages right away in real time. This limitation made applying Letter Sealing to groups much harder than applying it to one-on-one chats. We took these issues into consideration and spent a whole year tweaking and testing Letter Sealing so it is finally ready to be available on group chats in the latest version of the LINE app (LINE 6.5 for iOS and Android). For now, it's only available for group chats that have up to 50 members, but we're working on increasing this number to support larger group chats.

I'll explain more on how LINE engineers dealt with these problems in an upcoming post.

Letter Sealing is now available on free calls (voice/video)

LINE has offered free voice and video calls since 2011. After discussing what features we should provide Letter Sealing to next, the team decided that we should start work on developing Letter Sealing for free calls.

Voice and video calls are now protected by an encryption key only known and exchanged between participants of the call. Even LINE employees cannot see what's going on in these calls as the calls are protected by an encryption key, which is only stored on user devices and not on LINE servers.

No additional manual settings are required for using this feature. Users on LINE 6.5 (iOS, Android) or LINE 4.8 (Windows) and above can enjoy the extra protection provided by this security upgrade if they keep Letter Sealing enabled.

You can now easily tell if Letter Sealing is enabled

Your messages on LINE are still safe even without Letter Sealing, as they are encrypted when they travel between your device and our servers. No one can know about your messages even if they are monitoring your network. The reason we developed Letter Sealing was to add an extra layer of protection so that your messages can only be seen on your devices and kept safely encrypted on our servers.

After introducing Letter Sealing last year, the team hotly debated about adding a status indicator for Letter Sealing. We didn't want to give our users the impression that chats without Letter Sealing are unsafe nor did we want to cause confusion on who you can use Letter Sealing with and who you cannot, since Letter Sealing was disabled by default when it was first introduced.

Now that Letter Sealing is enabled by default, we decided that it's more important to give correct information to our users rather than dwell on these concerns. Starting with LINE 6.5 (iOS, Android) and LINE 4.8 (Windows, Mac), you will notice a padlock icon next to the name of the chat when you're in chats or calls with Letter Sealing enabled.

The status indicator will be gradually rolled out to the various regions around the world. All users using supported versions of the LINE app should be able to see the status indicator displayed on Letter Sealing-enabled chats by the beginning of September.

We plan to bring all the security features mentioned above to platforms other than iOS, Android, Windows, and OS X¹ as well. Our mission is to keep messaging safe on LINE, and we will continue to do so.

1: The status indicator will be displayed on one-on-one chats and group chats on LINE 4.8 for Windows and OS X. Status indicators for free calls will be available with the next update.

About the author

Shin, Ki Bin: A LINE engineer working on the messaging server.