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CBC without a MAC is unauthenticated — anyone who can write rows to
whatsapp_config can flip bits in the stored access_token or
verify_token without the decrypt throwing. You'd silently get garbled
bytes; worst case, if the mutation happens to produce a valid token,
messages go out under a spoofed account. GCM appends a 16-byte
authentication tag; any tampering fails the decrypt.
## What changed
src/lib/whatsapp/encryption.ts
- encrypt() now emits AES-256-GCM ciphertexts with a 12-byte IV,
formatted as `<iv>:<ciphertext>:<authTag>` (three colons).
- decrypt() auto-detects format by splitting on `:`:
3 parts → GCM, validated via auth tag.
2 parts → legacy CBC, read-only compatibility path.
Callers are unchanged; same signature, same return type.
- isLegacyFormat() exposes the format check so hot paths can
opportunistically upgrade old rows.
- IV length is validated on decrypt (12 for GCM, 16 for CBC) to
fail fast on corrupted input.
## Self-healing existing rows
Two call sites opportunistically upgrade a legacy row to GCM after a
successful decrypt — fire-and-forget, no latency impact on the request
path, idempotent under concurrent writes.
- src/app/api/whatsapp/send/route.ts (hot path)
After decrypting access_token, if it was CBC, we schedule an
UPDATE that writes a fresh GCM ciphertext back to the same row.
The first send after deploy upgrades the token; subsequent
sends are no-ops because isLegacyFormat returns false.
- src/app/api/whatsapp/webhook/route.ts (subscribe handshake)
When a config's verify_token matches the inbound
hub.verify_token, we upgrade it in place if it was legacy.
Runs during Meta's webhook re-subscription events, which
happen on config changes and periodic refreshes.
No schema migration required — the DB column holds whatever string
encrypt() produces. Operators who want to force an upgrade for a
particular user can clear and re-save the WhatsApp settings form.
## Verified (tsx smoke test)
- GCM roundtrip works (plaintext in → same plaintext out).
- Legacy CBC decrypt works on 2-part format.
- isLegacyFormat distinguishes new/legacy correctly.
- Bit-flipped GCM ciphertext throws on decrypt.
- Forged auth tag throws on decrypt.
- Same plaintext produces different ciphertexts across calls
(IV randomness).
## Rollback
Revert this file. Existing GCM rows would then fail to decrypt
because the previous encrypt.ts has no branch for the 3-part format.
Safer rollback: keep the decrypt branches, revert only the encrypt()
body to CBC — but since GCM roundtrips cleanly and the PR's bug
surface is small (pure crypto utility), a straight revert should only
matter if a forker had time between merging this PR and reverting to
accumulate GCM rows.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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