Pack .crx

derkey = Popen(['openssl', 'rsa', '-pubout', '-outform', 'DER', '-inform',
                'PEM', '-in', path_to_pem_key], stdout=PIPE).stdout.read()
signature = Popen(['openssl', 'sha1', '-sha1', '-sign', path_to_pem_key,
                   zip_file.filename], stdout=PIPE).stdout.read()
with open('extension.crx', 'w') as crx:
    crx.writelines(['Cr24',
                    struct.pack("<I", 2),  # crx version
                    struct.pack("<I", len(derkey)),
                    struct.pack("<I", len(signature)),
                    derkey,
                    signature,
                    open(zip_file.filename, 'r').read()])