The last of the four privacy features Android was missing that iOS already had.
The sealed history file protects a phone that is off or locked; it does nothing
about the phone somebody just handed to a friend. iOS asked for Face ID; on
Android anyone holding an unlocked handset read every conversation.
Biometrics OR the device credential, deliberately - a phone with a PIN and no
fingerprint reader should still be able to use this, the same reason iOS asks
for deviceOwnerAuthentication rather than biometrics specifically. Offered only
when the device can actually authenticate: a switch that turns on and then
cannot show a prompt locks somebody out of their own messages with no way back.
Two traps worth naming. MainActivity had to become a FragmentActivity -
BiometricPrompt hosts a fragment, so on a plain ComponentActivity it silently
cannot show and the callback never fires, which looks like a broken lock rather
than a missing base class. And the gate wraps Root rather than drawing over it:
composed underneath, the message list still renders for a frame, and a frame is
all a screenshot or a shoulder needs.
The lock screen deliberately says nothing - no sender, no preview, no count. A
lock screen reporting "3 new messages from Dana" has given away the two facts
that usually matter most.
The setting states what it does NOT do. This stops someone who picks up your
unlocked phone; it adds no encryption, because the vault and ratchet keys are
already sealed by the keystore either way. Claiming more would be selling a
screen as if it were a key.
Android engine, client and app suites green.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
