[Python] Knowing the hierarchy when removing fields from protobuf

I am trying to remove the top-level field 53 from a protobuf file(captured from YouTube android client) without a .proto file, but I found that the inner field 53 has also been removed.

protobuf file: https://www.mediafire.com/file/31srltie0btmrjh/input.bin/file

Code:

from typing import Tuple

class ProtobufFieldRemover:
def __init__(self):
self.output = bytearray()

# Read the varint encoded integer and return (value, new position)
def _read_varint(self, data: bytes, pos: int) -> Tuple[int, int]:
value = 0
shift = 0
while True:
if pos >= len(data):
raise ValueError("Invalid varint")
byte = data[pos]
value |= (byte & 0x7F) << shift
pos += 1
if not (byte & 0x80):
return value, pos
shift += 7

# Encode integers as varint
def _write_varint(self, value: int) -> bytes:
result = bytearray()
while True:
byte = value & 0x7F
value >>= 7
if value:
result.append(byte | 0x80)
else:
result.append(byte)
break
return result

def remove_field_53(self, input_data: bytes) -> bytes:
removed = False
self.output = bytearray()
pos = 0 # pos records the position index in the binary data being processed
length = len(input_data)
while pos < length:
tag, pos = self._read_varint(input_data, pos)
field_number = tag >> 3
wire_type = tag & 0x7
if field_number in {53}: # found field 53, which is at the top level
print(f"Found top-level field 53.")
if wire_type == 0: # Varint
_, pos = self._read_varint(input_data, pos)
elif wire_type == 1: # 64-bit
pos += 8
elif wire_type == 2: # Length-delimited
length1, pos = self._read_varint(input_data, pos)
pos += length1
elif wire_type == 5: # 32-bit
pos += 4
removed = True
else:
# Fields that need to be preserved
self.output.extend(self._write_varint(tag))
if wire_type == 0: # Varint
value, pos = self._read_varint(input_data, pos)
self.output.extend(self._write_varint(value))
elif wire_type == 1: # 64-bit
self.output.extend(input_data[posos+8])
pos += 8
elif wire_type == 2: # Length-delimited
length1, pos = self._read_varint(input_data, pos)
self.output.extend(self._write_varint(length1))
self.output.extend(input_data[posos+length1])
pos += length1
elif wire_type == 5: # 32-bit
self.output.extend(input_data[posos+4])
pos += 4
if removed:
return bytes(self.output)
return None

if __name__ == "__main__":
remover = ProtobufFieldRemover()

with open("input.bin", "rb") as f:
pb_data = f.read()

# Remove field 53
result = remover.remove_field_53(pb_data)
if result is not None:
print("Successfully removed field 53.")
with open("output.bin", "wb") as f:
f.write(result)
else:
print("53 field not found.")


After execution, print as follows:

Found top-level field 53.
Found top-level field 53.
Found top-level field 53.
Successfully removed field 53.

Continue reading...