Make sure my_public_key length is exactly 128 bytes

Fixes 'Client public key size is invalid' error from KWallet.

Fixes 48.

Author: mitya57

secretstorage/dhcrypto.py

@@ -27,8 +27,10 @@
 )
 
 
-def int_to_bytes(number: int) -> bytes:
-    return number.to_bytes(math.ceil(number.bit_length() / 8), 'big')
+def int_to_bytes(number: int, length: int) -> bytes:
+    result = number.to_bytes(math.ceil(number.bit_length() / 8), 'big')
+    # Pad with zero bytes if needed.
+    return b'\x00' * (length - len(result)) + result
 
 
 DH_PRIME_1024 = int.from_bytes(DH_PRIME_1024_BYTES, 'big')
@@ -46,9 +48,7 @@ def __init__(self) -> None:
     def set_server_public_key(self, server_public_key: int) -> None:
         common_secret_int = pow(server_public_key, self.my_private_key,
                                 DH_PRIME_1024)
-        common_secret = int_to_bytes(common_secret_int)
-        # Prepend NULL bytes if needed
-        common_secret = b'\x00' * (0x80 - len(common_secret)) + common_secret
+        common_secret = int_to_bytes(common_secret_int, 128)
         # HKDF with null salt, empty info and SHA-256 hash
         salt = b'\x00' * 0x20
         pseudo_random_key = hmac.new(salt, common_secret, sha256).digest()

secretstorage/util.py

@@ -102,7 +102,7 @@ def open_session(connection: DBusConnection) -> Session:
         output, result = service.call(
             'OpenSession', 'sv',
             ALGORITHM_DH,
-            ('ay', int_to_bytes(session.my_public_key)))
+            ('ay', int_to_bytes(session.my_public_key, 128)))
     except DBusErrorResponse as resp:
         if resp.name != DBUS_NOT_SUPPORTED:
             raise

tests/test_dhcrypto.py

@@ -14,6 +14,7 @@ class ConversionTest(unittest.TestCase):
     between bytes and long."""
 
     def test_int_to_bytes(self) -> None:
-        self.assertEqual(int_to_bytes(1), b'\x01')
-        self.assertEqual(int_to_bytes(258), b'\x01\x02')
-        self.assertEqual(int_to_bytes(1 << 64), b'\x01' + b'\x00' * 8)
+        self.assertEqual(int_to_bytes(1, 1), b'\x01')
+        self.assertEqual(int_to_bytes(1, 2), b'\x00\x01')
+        self.assertEqual(int_to_bytes(258, 2), b'\x01\x02')
+        self.assertEqual(int_to_bytes(1 << 64, 9), b'\x01' + b'\x00' * 8)