Make the payload encoder's -key argument usable

[arpitjain099]

Description

payload_encoder.py takes a -key flag, but passing one has never worked. argparse hands back a str, so key[i % len(key)] is a one character string and the XOR raises:

  File "app/utility/payload_encoder.py", line 31, in xor_bytes
    arr[i] = val ^ cur_key
TypeError: unsupported operand type(s) for ^: 'int' and 'str'

The default path is fine, because DEFAULT_KEY is a list of ints. Only a user supplied key breaks, so the flag is effectively dead.

The server already solves this: file_svc.py:69 does xor_bytes(contents, xor_key.encode()) for the xor_key header. This does the same thing at the argparse layer, so the two paths agree on what a key is and xor_bytes keeps taking anything that indexes to ints.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Added tests/utility/test_payload_encoder.py with three cases: xor_bytes with a byte string key, the CLI with the default key, and the CLI with -key. The third fails on master with the TypeError above and passes here. Ran the file under python 3.12 with flake8 clean.