core: properly limit passphrase to 50 bytes

core/ChangeLog

@@ -4,6 +4,7 @@ Version 2.x.x [not yet released]
 
 Version 2.3.0 [not yet released]
 * Passphrase redesign
+* Properly limit passphrase to 50 bytes and not 50 characters
 
 Version 2.2.0 [Jan 2020]
 * Remove unused ButtonRequest.data field

core/src/apps/common/passphrase.py

@@ -33,8 +33,8 @@ async def _request_from_user(ctx: wire.Context) -> str:
         passphrase = await _request_on_device(ctx)
     else:
         passphrase = await _request_on_host(ctx)
-    if len(passphrase) > _MAX_PASSPHRASE_LEN:
-        raise wire.DataError("Maximum passphrase length is %d" % _MAX_PASSPHRASE_LEN)
+    if len(passphrase.encode()) > _MAX_PASSPHRASE_LEN:
+        raise wire.DataError("Maximum passphrase length is %d bytes" % _MAX_PASSPHRASE_LEN)
 
     return passphrase
 

tests/device_tests/test_session_id_and_passphrase.py

@@ -230,6 +230,30 @@ def test_passphrase_missing(client):
     assert response.code == FailureType.DataError
 
 
+@pytest.mark.skip_ui
+@pytest.mark.setup_client(passphrase=True)
+def test_passphrase_length(client):
+    def call(passphrase: str, expected_result: bool):
+        _init_session(client)
+        response = client.call_raw(XPUB_REQUEST)
+        assert isinstance(response, messages.PassphraseRequest)
+        response = client.call_raw(messages.PassphraseAck(passphrase))
+        if expected_result:
+            assert isinstance(response, messages.PublicKey)
+        else:
+            assert isinstance(response, messages.Failure)
+            assert response.code == FailureType.DataError
+
+    # 50 is ok
+    call(passphrase="A" * 50, expected_result=True)
+    # 51 is not
+    call(passphrase="A" * 51, expected_result=False)
+    # "š" has two bytes - 48x A and "š" should be fine (50 bytes)
+    call(passphrase="A" * 48 + "š", expected_result=True)
+    # "š" has two bytes - 49x A and "š" should not (51 bytes)
+    call(passphrase="A" * 49 + "š", expected_result=False)
+
+
 def _get_xpub_cardano(client, passphrase):
     msg = messages.CardanoGetPublicKey(address_n=parse_path("44'/1815'/0'/0/0"))
     response = client.call_raw(msg)