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nrobi144
72c3dff870
Addresses the P0 items in the security review at
docs/plans/2026-07-01-privacy-lock-security-review.md.
## PBKDF2 iterations 100k → 600k (M1) via versioned hash format (M2)
- New PasswordHasher storage format: `v1$saltB64$hashB64` (600k
iterations, matches OWASP 2023 Password Storage Cheat Sheet for
PBKDF2-HMAC-SHA256).
- Legacy `saltB64$hashB64` (100k iterations) format still verifies
correctly — no user gets locked out by the bump.
- `hash()` always produces `v1$…`; users migrate to v1 opportunistically
when they Change or Set a new password.
- New `PasswordHasher.isLegacyFormat()` helper for callers that want
to force-migrate on next successful unlock.
- Verify cost goes from ~50ms → ~250ms on a modern laptop — well
within tolerable UX for a lock users open a handful of times per
session.
## Exponential backoff on failed unlock (M3)
- `PrivacyLockSettings` gains `failedUnlockAttempts: StateFlow<Int>`
and `lockedUntilEpochMs: StateFlow<Long?>`, both persisted via
java.util.prefs so a reboot cannot reset the backoff.
- `MessagesLockState.onFailedUnlockAttempt(nowMs)` implements the
schedule: no lockout for first 4 fails, then 30s / 60s / 120s /
300s (capped at 5 min).
- `MessagesLockState.onUnlockSuccess()` transparently clears the
attempt counter and any active lockout (also called from the
banner-enable path).
- DesktopLockScreen shows a countdown ("Try again in 27s") in the
supportingText, disables the password field and Unlock button
during lockout, ticks every 500ms via a LaunchedEffect.
- RemovePasswordDialog inherits the same protection — Settings can't
bypass the throttle by disabling the lock.
- 4 new unit tests cover threshold behavior, base trip, doubling +
cap, reset on success. All 13 tests green.
## Not in this commit
- L1/L2 (String/CharArray memory retention) — out-of-tree fix in
Compose; accepted per threat model.
- L3 (post-uninstall prefs) — release-notes item.
- M4 (Limitations copy update) — deferred; existing "does not
protect against filesystem access" line already covers.