keyed_hashing.py

import argparse, hashlib, sys
from pathlib import Path

def store(path, data, key):
    data_path   = Path(path)
    hash_path   = data_path.with_suffix('.hash')
    hash_value  = hashlib.blake2b(data, key=key).hexdigest()

    with data_path.open(mode='x'), hash_path.open(mode='x'):
        data_path.write_bytes(data)
        hash_path.write_text(hash_value)

def is_modified(path, key):
    data_path = Path(path)
    hash_path = data_path.with_suffix('.hash')

    data = data_path.read_bytes()
    original_hash_value = hash_path.read_text()

    hash_value = hashlib.blake2b(data, key=key).hexdigest()

    return original_hash_value != hash_value

if __name__ == '__main__':
    parser = argparse.ArgumentParser(description='Demo keyed hashing.')
    parser.add_argument('--store', action='store_true', help='store hash (default: verify hash)')
    args = parser.parse_args()

    path = 'x'
    data = b'bla'
    key = b'123'

    if args.store:
        store(path, data, key)
        sys.exit(0)

    if is_modified(path, key):
        print("Mallory was here!")