feat: column-level encryption codec over fjall store (declarative encrypted-fields map)

[forkwright]

Context

Area

akroasis fjall codec layer — declarative, config-driven middleware that consults a static BTreeSet<(table, column)> map and applies ChaCha20-Poly1305 to specified fields before write / after read. Closes the gap where filesystem access to the fjall store reveals unencrypted signal metadata.

Severity

low — incremental hardening. Today akroasis has at-rest passphrase encryption on the fjall store but no per-field encryption; an attacker with filesystem-level access can read mesh signal metadata directly. Filing as a small enhancement, not a structural redesign.

Evidence

External design prior: thunderbird/thunderbolt src/db/powersync/middleware/EncryptionMiddleware.ts + src/db/encryption/config.ts:encryptedColumnsMap.

encryptedColumnsMap: {
  "signals":   ["payload", "metadata"],
  "vault":     ["secret"],
  "messages":  ["body"]
}

Middleware intercepts each write, looks up the table-column pair, applies AES-GCM (Thunderbolt) or ChaCha20-Poly1305 (akroasis equivalent — already in the akroasis crypto stack) before passing through to the storage layer. Reads do the inverse.

The key insight: which columns are encrypted is a single map separate from the middleware. Adding an encrypted column is a one-line change to the map; no code modification.

Conflict

• Adjacent to AK1 (akroasis#17 hybrid PQ key wrapping) — that issue handles distributing keys across devices; this issue handles per-field encryption with the resulting keys.
• No conflict with existing akroasis issues.

Why it matters

1. Filesystem-level threat surface. A laptop seizure scenario with akroasis running locally exposes signal metadata if the fjall store is at-rest only. Per-field encryption mitigates.
2. Pattern is generic. A BTreeSet<(TableId, ColumnId)> + a ColumnCodec trait covers this in <100 LOC of Rust.
3. akroasis already has ChaCha20-Poly1305. No new crypto crate; just a codec layer over existing storage.

Done criteria

1. akroasis (likely the fjall-store wrapper crate): ColumnCodec trait with encrypt(field: &[u8], context: &EncryptionContext) -> Vec<u8> and inverse.
2. Static ENCRYPTED_FIELDS: BTreeSet<(TableId, FieldId)> lives in a single canonical place; not scattered across modules.
3. Storage-layer write path consults the map; no opt-in per-call API.
4. Read path does the inverse before returning to the consumer.
5. Migration: existing un-encrypted data is re-encrypted on first write after the codec lands; or a one-shot migration tool wraps existing rows.
6. Test: a synthetic fjall write of a mapped field produces ciphertext on disk, plaintext through the codec read path.

Source

thunderbird/thunderbolt src/db/powersync/middleware/EncryptionMiddleware.ts (read 2026-04-25). Sister to akroasis#17 (AK1 hybrid key wrapping).

Filed 2026-04-25 from thunderbolt deep-dive.

Provenance

Originally filed on the kanon forge as issue #18 on 2026-04-25T22:22:43.405368579-05:00[America/Chicago]. Recovered from 2026-05-09 pre-brick restic backup. Forge URL no longer reachable post firmware brick.

Severity

P1