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Letter Sealing

This note summarizes what issue #42 and issue #54 currently tell us about Letter Sealing support in this bridge, and how that lines up with the current implementation.

TL;DR

  • The bridge supports both LSON and LSOFF accounts for login and messaging.
  • Text messages work in all combinations: LSON/LSOFF direct messages, mixed groups, and business/bot accounts.
  • Image, video, file, and audio sending works for all user types. E2EE media uses encrypted OBS upload; plain media uses the r/talk/m post-send upload path.
  • Transparent PNGs are composited onto a white background before upload, matching LINE native client behavior.

Terms

  • LSON: LINE user with Letter Sealing enabled.
  • LSOFF: LINE user with Letter Sealing disabled.

Current behavior matrix

Scenario Current status Notes
Bridge account is LSOFF and tries to log in WORKS Uses non-E2EE login flow: fresh RSA key, type 0 retry, JQ polling.
Bridge account is LSON and tries to log in WORKS Uses E2EE login flow: LF1 polling, ConfirmE2EELogin, key chain export.
Bridge account is LSON, receives direct message from LSOFF WORKS Confirmed in issue testing.
Bridge account is LSON, receives group message in a chat that includes LSOFF WORKS Confirmed in issue testing.
Bridge account is LSON, sends direct message to LSOFF WORKS Detects missing peer E2EE key and falls back to plain text send.
Bridge account is LSON, sends group message to a room that includes LSOFF WORKS Detects missing group shared key and falls back to plain text send.
Bridge account is LSON, sends to LSON only chats WORKS Uses full E2EE encryption path.
Bridge account is LSOFF, sends direct message to LSON WORKS Sends as plain text (E2EE not initialized).
Bridge account is LSOFF, sends to mixed groups WORKS Sends as plain text.
Bridge account is LSOFF, sends to bot/business account WORKS Sends as plain text.
Bridge account sends images/media to LSON WORKS E2EE encrypted media with keyMaterial in payload.
Bridge account sends images/media to LSOFF or plain chats WORKS Plain media uploaded via r/talk/m/{msgId} after sending.

How it works

Login path

The bridge supports two login flows, selected automatically based on whether the LINE account has Letter Sealing enabled:

LSON accounts (E2EE login):

  1. loginV2 with type: 2 and E2EE secret returns a verifier and PIN.
  2. Background goroutine polls LF1 endpoint until the user confirms on their phone.
  3. LF1 returns EncryptedKeyChain and PublicKey.
  4. confirmE2EELogin completes the E2EE handshake.
  5. loginV2WithVerifier finalizes and returns access tokens + E2EE key material.

LSOFF accounts (non-E2EE login):

  1. loginV2 with type: 2 and E2EE secret fails with code 89 "not supported".
  2. Bridge fetches a fresh RSA key (LINE invalidates the previous one after the failed attempt).
  3. Retry loginV2 with type: 0, no secret, and the new RSA key returns a verifier and PIN.
  4. Background goroutine polls JQ endpoint (not LF1) until the user confirms on their phone.
  5. JQ returns authPhase: "QRCODE_VERIFIED".
  6. loginV2WithVerifier finalizes and returns access tokens (no E2EE data).

Send path

The bridge determines E2EE capability per-chat before sending:

  • If lc.E2EE == nil (LSOFF bridge account): all messages sent as plain text.
  • For 1:1 chats: ensurePeerKey probes the peer's E2EE public key. If the peer is LSOFF, falls back to plain text.
  • For groups: fetchAndUnwrapGroupKey attempts to get the group shared key. If not available (mixed group), falls back to plain text.
  • E2EE capability is cached per-peer and per-group with a 1-hour TTL.

Plain text media uses a different upload flow than E2EE media:

  • E2EE: encrypt data, upload to r/talk/emi/{id} (OBS), send message with OID in metadata.
  • Plain: send message first, then upload raw data to r/talk/m/{serverMessageId}.

Receive path

  • pkg/connector/handle_message.go lazily fetches peer keys or group keys based on key IDs in incoming message chunks.
  • Incoming messages from LSOFF users arrive as plain text and are handled without decryption.

Test matrix

Login

Test case Status
LSOFF bridge account login Verified
LSON bridge account login Verified

Text messages

Test case Path Status
LSON bridge -> LSON peer (DM) E2EE Verified
LSON bridge -> LSOFF peer (DM) Plain Verified
LSON bridge -> LSON-only group E2EE Verified
LSON bridge -> mixed group Plain Verified
LSON bridge -> bot/business account Plain Verified
LSOFF bridge -> LSON peer (DM) Plain Verified
LSOFF bridge -> LSOFF peer (DM) Plain Not tested
LSOFF bridge -> mixed group (LSOFF created) Plain Verified
LSOFF bridge -> mixed group (LSON created) Plain Verified
LSOFF bridge -> bot/business account Plain Verified

Receiving messages

Test case Status
Incoming E2EE text from LSON peer Verified
Incoming plain text from LSOFF peer Verified
Incoming E2EE group message (LSON-only) Verified
Incoming group message (mixed group) Verified

Media (images)

Test case Path Status
Image to LSON peer E2EE (emi upload) Verified
Image to LSOFF peer Plain (m upload) Verified
Image to mixed group Plain (m upload) Verified
Image to bot/business account Plain (m upload) Verified
Transparent PNG White background compositing Verified

Media (audio, video, files)

Test case Path Status
Video to LSON peer E2EE (emv upload) Verified
Video to LSOFF / plain chats Plain (m upload) Verified
File to LSON peer E2EE (emf upload) Verified
File to LSOFF / plain chats Plain (m upload) Verified
Audio to LSON peer E2EE (ema upload) Verified
Audio to LSOFF / plain chats Plain (m upload) Verified

Media receive (incoming to Beeper)

Test case Path Status
Incoming image from plain chat Plain (m download) Verified
Incoming image from E2EE chat E2EE (emi download) Verified
Incoming video from plain chat Plain (m download) Not tested
Incoming video from E2EE chat E2EE (emv download) Not tested
Incoming file from plain chat Plain (m download) Not tested
Incoming file from E2EE chat E2EE (emf download) Not tested
Incoming audio from plain chat Plain (m download) Verified
Incoming audio from E2EE chat E2EE (ema download) Verified

Re-login and token refresh

Test case Status
LSON re-login after token expiry Not tested
LSOFF re-login after token expiry Not tested

References