Implement efficient counter in storage.py.

src/apps/common/storage.py

@@ -11,6 +11,8 @@ HOMESCREEN_MAXSIZE = 16384
 _STORAGE_VERSION = b"\x01"
 _FALSE_BYTE = b"\x00"
 _TRUE_BYTE = b"\x01"
+_COUNTER_HEAD_LEN = 4
+_COUNTER_TAIL_LEN = 8
 
 # fmt: off
 _APP                = const(0x01)  # app namespace
@@ -41,6 +43,46 @@ def _get_bool(app: int, key: int, public: bool = False) -> bool:
     return config.get(app, key, public) == _TRUE_BYTE
 
 
+def _set_counter(app: int, key: int, count: int, public: bool = False) -> None:
+    value = count.to_bytes(_COUNTER_HEAD_LEN, "big")
+    if public:
+        value += _COUNTER_TAIL_LEN * b"\xff"
+    config.set(app, key, value, public)
+    
+
+def _next_counter(app: int, key: int, public: bool = False) -> int:
+# If the counter value is public, then it is stored as a four byte integer in
+# big endian byte order, called the "head", followed an eight byte "tail". The
+# counter value is equal to the integer value of the head plus the number of
+# zero bits in the tail. The counter value 0 is stored as 00000000FFFFFFFFFFFFFFFF.
+# With each increment the tail is shifted to the right by one bit. Thus after
+# three increments the stored value is 000000001FFFFFFFFFFFFFFF. Once all the
+# bits in the tail are set to zero, the next counter value is stored as
+# 00000021FFFFFFFFFFFFFFFF.
+
+    value = config.get(app, key, public)
+    if value is None:
+        _set_counter(app, key, 0, public)
+        return 0
+
+    head = value[: _COUNTER_HEAD_LEN]
+    tail = value[_COUNTER_HEAD_LEN :]
+    i = tail.rfind(b"\x00") + 1
+    count = int.from_bytes(head, "big") + 1 + 8*i
+    if i == len(tail):
+        _set_counter(app, key, count, public)
+        return count
+
+    zero_count = 0
+    while (tail[i] << zero_count) < 128:
+        zero_count += 1
+    count += zero_count
+
+    tail = tail[:i] + bytes([tail[i] >> 1]) + tail[i+1:]
+    config.set(app, key, head + tail, public)
+    return count
+
+
 def _new_device_id() -> str:
     return hexlify(random.bytes(12)).decode().upper()
 
@@ -171,20 +213,11 @@ def set_autolock_delay_ms(delay_ms: int) -> None:
 
 
 def next_u2f_counter() -> int:
-    b = config.get(_APP, _U2F_COUNTER)
-    if not b:
-        b = 0
-    else:
-        b = int.from_bytes(b, "big") + 1
-    set_u2f_counter(b)
-    return b
+    return _next_counter(_APP, _U2F_COUNTER)
 
 
-def set_u2f_counter(cntr: int):
-    if cntr:
-        config.set(_APP, _U2F_COUNTER, cntr.to_bytes(4, "big"))
-    else:
-        config.set(_APP, _U2F_COUNTER, b"")
+def set_u2f_counter(cntr: int) -> None:
+    _set_counter(_APP, _U2F_COUNTER, cntr)
 
 
 def wipe():

tests/test_apps.common.storage.py

@@ -0,0 +1,21 @@
+from common import *
+from trezor.pin import pin_to_int
+from trezor import config
+from apps.common import storage
+
+
+class TestConfig(unittest.TestCase):
+
+    def test_counter(self):
+        config.init()
+        config.wipe()
+        self.assertEqual(config.unlock(pin_to_int('')), True)
+        for i in range(150):
+            self.assertEqual(storage.next_u2f_counter(), i)
+        storage.set_u2f_counter(350)
+        for i in range(351, 500):
+            self.assertEqual(storage.next_u2f_counter(), i)
+
+
+if __name__ == '__main__':
+    unittest.main()