#!/usr/bin/env python3
#
# Copyright (C) 2016 The University of Sheffield, UK
# 
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
#

import argparse
import binascii

from Crypto.PublicKey import RSA
from Crypto.Hash import SHA
from Crypto.Signature import PKCS1_v1_5

import zipfile
import io


class CrxFile:
    def __init__(self, filename, magic, version, pk_len, sig_len, pk, sig,
                 header_len, data):
        self.file = filename
        self.magic = magic
        self.version = version
        self.pk_len = pk_len
        self.sig_len = sig_len
        self.pk = pk
        self.sig = sig
        self.header_len = header_len
        self.data = data


def is_valid_magic(magic):
    return (b'Cr24' == magic)


def is_crxfile(filename):
    "Check magic number: crx files should start with \"Cr24\"."
    file = open(filename, 'rb')
    magic = file.read(4)
    file.close()
    return is_valid_magic(magic)


def check_signature(pk, sig, data):
    key = RSA.importKey(pk)
    hash = SHA.new(data)
    return PKCS1_v1_5.new(key).verify(hash, sig)


def read_crx(filename):
    "Read header of an crx file (https://developer.chrome.com/extensions/crx)."
    file = open(filename, 'rb')
    magic = file.read(4)
    version = int.from_bytes(file.read(4), byteorder='little')
    pk_len = int.from_bytes(file.read(4), byteorder='little')
    sig_len = int.from_bytes(file.read(4), byteorder='little')
    pk = file.read(pk_len)
    sig = file.read(sig_len)
    header_len = 16 + pk_len + sig_len
    data = file.read()
    file.close()
    return CrxFile(filename, magic, version, pk_len, sig_len, pk, sig,
                   header_len, data)


def print_crx_info(verbose, crx):
    if is_valid_magic(crx.magic):
        magic = "valid"
    else:
        magic = "invalid"
    if check_signature(crx.pk, crx.sig, crx.data):
        sig = "valid"
    else:
        sig = "invalid"
    print("Filename:    " + crx.file)
    print("Header size: " + str(crx.header_len))
    print("Size:        " + str(crx.header_len + len(crx.data)))
    print("Magic byte:  " + str(crx.magic.decode("utf-8")) + " (" + magic +
          ")")
    print("Version:     " + str(crx.version))
    print("Signature:   " + sig)
    print("Public Key [" + str(crx.pk_len) + "]:")
    key = RSA.importKey(crx.pk)
    print(key.exportKey().decode("utf-8"))
    if verbose:
        print("Signature [" + str(crx.sig_len) + "]: " + str(
            binascii.hexlify(crx.sig)))
    out = f = io.BytesIO(crx.data)
    zf = zipfile.ZipFile(out, 'r')
    print("Zip content:")
    for info in zf.infolist():
        print('{:8d} {:8d}'.format(info.file_size, info.compress_size),
              info.filename)


def verify_crxfile(verbose, filename):
    if is_crxfile(filename):
        if verbose:
            print("Found correct magic bytes.")
        print_crx_info(verbose, read_crx(filename))
        return 0
    else:
        if verbose:
            print("No valid magic bytes found")
        return -1


def extract_crxfile(verbose, force, filename, destdir):
    crx = read_crx(filename)
    if is_valid_magic(crx.magic) | force:
        if ("" == destdir) | (destdir is None):
            destdir = "."
        if filename.endswith(".crx"):
            dirname = filename[0:len(filename) - 4]
        else:
            dirname = filename
        out = f = io.BytesIO(crx.data)
        zf = zipfile.ZipFile(out, 'r')
        zf.extractall(destdir + "/" + dirname)
        print("Content extracted into: " + destdir + "/" + dirname)
    else:
        print("Input file not valid.")


def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("file", help="chrome extension archive (*.crx)")
    parser.add_argument('targetdir', nargs='?', default="")
    parser.add_argument(
        "-c",
        "--check",
        help="verify format and signature of <file>",
        action="store_true")
    parser.add_argument(
        "-e", "--extract", help="extract <file>", action="store_true")
    parser.add_argument(
        "-f",
        "--force",
        help="apply action also to (potential) invalid files",
        action="store_true")
    parser.add_argument(
        "-v", "--verbose", help="increase verbosity", action="store_true")
    args = parser.parse_args()

    if args.extract:
        retval = extract_crxfile(args.verbose, args.force, args.file,
                                 args.targetdir)
    else:
        retval = verify_crxfile(args.verbose, args.file)

    exit(retval)


if __name__ == "__main__":
    main()