Phase 2 (infra layer) of v0.3.0. Topics now generate a 32-byte
XSalsa20-Poly1305 key on creation; the broker seals one copy via
crypto_box for the topic creator using an ephemeral x25519
sender keypair (whose public half lives on
topic.encrypted_key_pubkey). Topic key plaintext leaves memory
immediately after the creator's seal — the broker can't read it.
Schema 0026:
+ topic.encrypted_key_pubkey (text, nullable for legacy v0.2.0)
+ topic_message.body_version (integer, 1=plaintext / 2=v2 cipher)
+ topic_member_key (id, topic_id, member_id,
encrypted_key, nonce, rotated_at)
API:
+ GET /v1/topics/:name/key — return the calling member's sealed
copy. 404 if no copy exists yet (joined post-creation, no peer
has re-sealed). 409 if the topic is legacy unencrypted.
Open question parked: how new joiners get their sealed copy
without ceding plaintext to the broker. Spec at
.artifacts/specs/2026-05-02-topic-key-onboarding.md picks
member-driven re-seal (Option B). Pending-seals endpoint, seal
POST, and the actual on-the-wire encryption ship in phase 3.
Mention fan-out from phase 1 (notification table) is decoupled
from ciphertext, so /v1/notifications + MentionsSection keep
working unchanged through both phases.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
