Decode one grpc message

parseGrpc.py

# * provide a grpc encoded file as parameter: `python parseGrpc.py $FILE`
# * grpc compression must not be enabled
# * output: $FILE.dec is the first grpc message

# based on mitmproxy grpc parser:
# https://github.com/mitmproxy/mitmproxy/blob/a26013908a95a270291de4c462d4074477b9a8d2/mitmproxy/contentviews/grpc.py#L866-L888

import struct
import sys

assert len(sys.argv) == 2

with open(sys.argv[1], "rb") as file:
    data = file.read()

msg_is_compressed, length = struct.unpack("!?i", data[:5])

assert msg_is_compressed == False
decoded_message = struct.unpack("!%is" % length, data[5 : 5 + length])[0]

with open(sys.argv[1] + ".dec", "wb") as out_file:
    out_file.write(decoded_message)