feat(pin): exactly 4 digits, entered on the 4th digit

The keypad has no OK key; the 4th digit submits, so a tap that also
arrives as START can no longer submit a partial PIN (the likely cause
of "Wrong PIN" before the code opened). PINs set by earlier versions
still unlock and are then replaced by a 4-digit one. Opening an entry
ignores a second request from the same tap. A Require PIN sent from
the phone while the watch is locked now applies at the next PIN
instead of waiting for Confirm all. The linter skips .claude.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 12 insertions, 4 deletions

diff --git a/tools/gen_fixture_wrap.py b/tools/gen_fixture_wrap.py
index c389fea..6eb207a 100644
--- a/tools/gen_fixture_wrap.py
+++ b/tools/gen_fixture_wrap.py
@@ -1,16 +1,19 @@
 #!/usr/bin/env python3
 # Author:  cirakg & claude ai
-# Summary: Compute the UI-test fixture's PIN wrap (fake PIN 123456) so the watch needn't run PBKDF2 at start-up.
+# Summary: Compute the UI-test fixture's PIN wrap (fake PIN 1234) so the watch needn't run PBKDF2 at start-up.
 """Print the fake PIN wrap that source/app/Fixtures.mc stores in the UI-test build.
 
 The wrap follows docs/SECURITY.md ("PIN wrap, version 1") with fixed, fake
-inputs: PIN 123456, salt 10..1f, IV 20..2f, master key 40..5f, 2000
+inputs: PIN 1234, salt 10..1f, IV 20..2f, master key 40..5f, 2000
 PBKDF2 iterations. Deriving the PIN key on the watch in one event trips the
 watchdog, and the UI tests need the store ready before the first screen, so
 the fixture stores this precomputed value instead. The app's own unlock code
 then checks it like any other wrap, which also cross-checks the format.
+The fixture also stores the 4-digit PIN marker (KeyManager.PIN_LENGTH_KEY =
+PIN_DIGITS), so the PIN isn't treated as a legacy 4-to-10-digit one.
 
-Usage: gen_fixture_wrap.py   (prints FIXTURE_WRAP and FIXTURE_MASTER_KEY in hex)
+Usage: gen_fixture_wrap.py   (prints FIXTURE_PIN, FIXTURE_WRAP and FIXTURE_MASTER_KEY in hex,
+                              and the marker the fixture stores)
 """
 
 from __future__ import annotations
@@ -20,7 +23,9 @@ import hmac
 import struct
 import subprocess
 
-PIN = b"123456"
+PIN = b"1234"
+PIN_DIGITS = 4  # source/storage/KeyManager.mc: every new PIN, and the marker value
+PIN_LENGTH_KEY = "wl"  # source/storage/KeyManager.mc
 ITERATIONS = 2000
 SALT = bytes(range(0x10, 0x20))
 IV = bytes(range(0x20, 0x30))
@@ -65,8 +70,11 @@ def fixture_wrap() -> bytes:
 
 
 def main() -> None:
+    assert len(PIN) == PIN_DIGITS
+    print("FIXTURE_PIN =", PIN.decode())
     print("FIXTURE_WRAP =", fixture_wrap().hex())
     print("FIXTURE_MASTER_KEY =", MASTER_KEY.hex())
+    print(f"marker: {PIN_LENGTH_KEY} = {PIN_DIGITS}")
 
 
 if __name__ == "__main__":