blind_replace_otacerts.py

#!/usr/bin/env python3

import argparse
import io
import mmap
import zipfile


def build_empty_zip():
    stream = io.BytesIO()

    with zipfile.ZipFile(stream, 'w'):
        pass

    return stream.getvalue()


def parse_args():
    parser = argparse.ArgumentParser()
    parser.add_argument('system', help='system.img to modify')
    parser.add_argument('otacerts', help='Recovery\'s otacerts.zip file')

    return parser.parse_args()


def main():
    args = parse_args()

    empty_zip_data = build_empty_zip()

    with open(args.otacerts, 'rb') as f:
        otacerts_data = f.read()
        assert len(empty_zip_data) < len(otacerts_data)

    with open(args.system, 'r+b') as f:
        system_data = mmap.mmap(f.fileno(), 0)

        offset = system_data.find(otacerts_data)
        assert offset >= 0

        # zero-pad in front of zip
        padding_size = len(otacerts_data) - len(empty_zip_data)
        system_data[offset:offset + padding_size] = b'\0' * padding_size

        system_data[offset + padding_size:offset + len(otacerts_data)] = \
            empty_zip_data


if __name__ == '__main__':
    main()