[kosisochukwu1234]

Summary

Records the architectural decision to store one message_envelopes row per recipient device rather than a single shared ciphertext per message in docs/adr/0001-per-device-envelopes.md.

Details Covered

• Context & Motivation: Multi-device cryptographic isolation, Phase-1 sealed-box and Signal Double Ratchet compatibility, and self-sync requirements across sender sibling devices (Multi-device self-sync (sender's own other devices) #188, Migrate send_file_message and ask_assistant to ciphertext + envelopes #337).
• Decision: Atomic transaction-bound persistence via insertMessageEnvelopes, sibling-device coverage checks (fetchSiblingDeviceIds, findMissingSiblingDeviceIds) in apps/backend/src/lib/messageFanout.ts, targeted realtime delivery (deliverMessage), and uniform fan-out shapes for server-authored messages (buildBroadcastEnvelopes).
• Alternatives Considered & Rejected:
  • One shared conversation ciphertext with a static shared key.
  • User-level envelopes (encrypting only to the recipient user).
  • Dual-pipeline routing (shared ciphertext for file/bot messages vs envelopes for text).
• Consequences & Operational Costs:
  • Linear database growth ($O(\text{messages} \times \text{devices})$) mitigated by the background cleanup job in apps/backend/src/services/envelopeGc.ts.
  • Sibling-device enforcement on send and edit, rejecting payloads missing sibling envelopes with device_set_mismatch.
• Exception for MLS Groups: Notes that MLS group messages store a single ciphertext per epoch on messages without per-device envelopes, referencing the next ADRs (docs/adr/0006-mls-for-groups.md / docs/adr/0002-signal-dms-mls-groups.md).

Closes #641