bug: 🐛 login fails for accounts with letter sealing OFF

[clins1994]

Summary

The bridge login fails for LINE accounts that have "Letter Sealing" (E2EE) disabled in their privacy settings. The bridge requires Letter Sealing to be enabled to function properly.

Problem

When attempting to log in with an account that has Letter Sealing disabled:

• The PIN notification is sent to the LINE mobile app (indicating the API call reaches LINE servers)
• However, the login fails with an "Error when logging in: Internal error" response
• The bridge state remains UNCONFIGURED after the login attempt

Root Cause

The issue is not a bug in the bridge itself, but rather a requirement of the LINE API: the bridge cannot function with accounts that have Letter Sealing disabled. End-to-end encryption (E2EE) is a hard requirement for this bridge to work as is.

Solution

Users experiencing this issue must enable "Letter Sealing" in their LINE app's privacy settings (LINE app → Settings → Privacy → Letter Sealing → ON) before attempting to log in to the bridge. Once enabled, the login completes successfully and the bridge state changes to CONNECTED.

Notes on Letter Sealing

1. According to LINE's documentation: "Letter Sealing only works in chats where all members have enabled it."
2. The option to turn it on only exists on the LINE mobile app (verified on ios, probably same for android)

Affected Users

This primarily affects older LINE accounts (created before E2EE became mandatory). New accounts have Letter Sealing forced enabled by default.

According to LINE Encryption Report (2024) some accounts made before 2022 may have the setting turned off by default. The "Letter Sealing" feature was released in 2015. See About Letter Sealing | LINE Help Center

Improvement Needed

While support for accounts with Letter Sealing disabled may be investigated in the future, currently:

• The error message should be more specific (e.g., "Letter Sealing must be enabled in privacy settings on the LINE mobile app")
• Error handling could display a helpful message directing users to enable this setting rather than showing a generic "Internal error"

Testing Needed

In the settings it says: "Letter Sealing only works in chats where all members have enabled it". What does that actually mean? I think we should sanity check then:

1. Test chat on groups including both users who have it on or off (Beeper user has it on)
2. Same as 1 but Beeper user has it off
3. Same as 1 but 1:1 chat
4. Same as 2 but 1:1 chat
5. After groups test case where all users in group become "Letter Sealed" after some were not. Follow-up test to 1 and 2
6. Business chats ?

We'll need to test various things to make sure it is supported

References

• About Letter Sealing | LINE Help Center
• October, 2015 LINE Introduces Letter Sealing Feature for Advanced Security
• LINE Encryption Report（2024）

[clins1994]

@highesttt thanks for making this. can you help me get it to work? 🆘

[clins1994]

edit after getting it to work i realized this was wrong but it might be that pin for non-e2ee are 4 digits and with e2ee are 6

got the pin to show up after changing the code a bit

it hangs indefinitely waiting for the pin and the bot stopped working ...

i'll continue having a go at it to see if i can get it to work and hopefully submit a pr with a fix soon

[clins1994]

Tried many things but couldn't leave hanging state that eventually fails after 2 minutes. Hanging state happened on both beeper mobile and beeper desktop apps :(

[highesttt]

Hey,
I do not think its related to your LINE account being Japanese. However I have suspicions that it's related to this:

Can you tell me if you have this option enabled or disabled

If it is disabled, do you, per chance, have a spare LINE account that you're okay with giving for testing. Since all new accounts i make have this forced enabled by default
Or even better, would you be willing to help fix me fix this by actively testing the changes that I'll make

[clins1994]

@highesttt that option was indeed disabled! i am definitely interested in helping you test and fix things. as for spare
accounts ... i don't have any but if i can help create from Japan sure. my account is old though and as expected ... I can't turn that setting back off =/

well i don't think we need to support it although a better error message would of been nice

i made my account in 2015 so i imagine many users have it off

i take back what i said, if this becomes an official bridge i think it is a hard requirement to either

1. only allow login success if the user has enabled letter sealing
2. add support for letter sealing off

[highesttt]

Does the bridge work now that it is enabled?

If so, do you know anyone else who might have this switch disabled and is willing to help test it out so we can try and get it working

[clins1994]

@highesttt

Does the bridge work now that it is enabled?

testing ...

do you know anyone else who might have this switch disabled and is willing to help test it out so we can try and get it working

yes, my wife has it off 😅 also i believe most of my friends here have it off so meanwhile i'll ask around

can we keep in touch on matrix: @clins777:beeper.com or other sns ?

[highesttt]

Sure. I sent a request

I'll just post important updates here in case anyone else has this issue

[clins1994]

@highesttt worked after turning on letter sealing

[clins1994]

Memo

"Letter Sealing only works in chats where all members have enabled it"

1. Test chat on groups including both users who have it on or off (Beeper user has it on)
2. Same as 1 but Beeper user has it off
3. Same as 1 but 1:1 chat
4. Same as 2 but 1:1 chat
5. After groups test case where all users in group become "Letter Sealed" after some were not. Follow-up test to 1 and 2
6. Business chats ?

Also: updated description to match the reality

We'll need to test various things to make sure it is supported