diff --git a/core/tests/test_protobuf.py b/core/tests/test_protobuf.py
index 524b28069..3089469cb 100644
--- a/core/tests/test_protobuf.py
+++ b/core/tests/test_protobuf.py
@@ -1,7 +1,7 @@
 from common import *
 
 import protobuf
-from trezor.utils import BufferIO
+from trezor.utils import BufferReader, BufferWriter
 
 
 class Message(protobuf.MessageType):
@@ -18,12 +18,12 @@ class Message(protobuf.MessageType):
 
 
 def load_uvarint(data: bytes) -> int:
-    reader = BufferIO(data)
+    reader = BufferReader(data)
     return protobuf.load_uvarint(reader)
 
 
 def dump_uvarint(value: int) -> bytearray:
-    writer = BufferIO(bytearray(16))
+    writer = BufferWriter(bytearray(16))
     protobuf.dump_uvarint(writer, value)
     return memoryview(writer.buffer)[:writer.offset]
 
@@ -65,22 +65,23 @@ class TestProtobuf(unittest.TestCase):
         # ok message:
         msg = Message(-42, 5)
         length = protobuf.count_message(msg)
-        buffer_io = BufferIO(bytearray(length))
-        protobuf.dump_message(buffer_io, msg)
-        buffer_io.seek(0)
-        nmsg = protobuf.load_message(buffer_io, Message)
+        buffer_writer = BufferWriter(bytearray(length))
+        protobuf.dump_message(buffer_writer, msg)
+
+        buffer_reader = BufferReader(buffer_writer.buffer)
+        nmsg = protobuf.load_message(buffer_reader, Message)
 
         self.assertEqual(msg.sint_field, nmsg.sint_field)
         self.assertEqual(msg.enum_field, nmsg.enum_field)
 
         # bad enum value:
-        buffer_io.seek(0)
+        buffer_writer.seek(0)
         msg = Message(-42, 42)
         # XXX this assumes the message will have equal size
-        protobuf.dump_message(buffer_io, msg)
-        buffer_io.seek(0)
+        protobuf.dump_message(buffer_writer, msg)
+        buffer_reader.seek(0)
         with self.assertRaises(TypeError):
-            protobuf.load_message(buffer_io, Message)
+            protobuf.load_message(buffer_reader, Message)
 
 
 if __name__ == "__main__":
diff --git a/core/tests/test_trezor.wire.codec_v1.py b/core/tests/test_trezor.wire.codec_v1.py
index 5ff5c15b0..3e8cab290 100644
--- a/core/tests/test_trezor.wire.codec_v1.py
+++ b/core/tests/test_trezor.wire.codec_v1.py
@@ -4,7 +4,7 @@ import ustruct
 
 from trezor import io
 from trezor.loop import wait
-from trezor.utils import chunks, BufferIO
+from trezor.utils import chunks
 from trezor.wire import codec_v1
 
 
@@ -179,17 +179,20 @@ class TestWireCodecV1(unittest.TestCase):
         self.assertEqual(result.data.buffer, message)
 
     def test_read_huge_packet(self):
-        # length such that it fits into 1 000 000 USB packets:
-        header = make_header(
-            mtype=MESSAGE_TYPE, length=999_999 * 63 + HEADER_PAYLOAD_LENGTH
-        )
+        PACKET_COUNT = 100_000
+        # message that takes up 100 000 USB packets
+        message_size = (PACKET_COUNT - 1) * 63 + HEADER_PAYLOAD_LENGTH
+        # ensure that a message this big won't fit into memory
+        self.assertRaises(MemoryError, bytearray, message_size)
+
+        header = make_header(mtype=MESSAGE_TYPE, length=message_size)
         packet = header + b"\x00" * HEADER_PAYLOAD_LENGTH
 
         buffer = bytearray(65536)
         gen = codec_v1.read_message(self.interface, buffer)
 
         query = gen.send(None)
-        for _ in range(999_999):
+        for _ in range(PACKET_COUNT - 1):
             self.assertObjectEqual(query, self.interface.wait_object(io.POLL_READ))
             query = gen.send(packet)