def digest_secure_bootloader(args):
    """ Calculate the digest of a bootloader image, in the same way the hardware
    secure boot engine would do so. Can be used with a pre-loaded key to update a
    secure bootloader. """
    if args.iv is not None:
        print("WARNING: --iv argument is for TESTING PURPOSES ONLY")
        iv = args.iv.read(128)
    else:
        iv = os.urandom(128)
    plaintext_image = args.image.read()
    args.image.seek(0)

    # secure boot engine reads in 128 byte blocks (ie SHA512 block
    # size), but also doesn't look for any appended SHA-256 digest
    fw_image = esptool.ESP32FirmwareImage(args.image)
    if fw_image.append_digest:
        if len(plaintext_image) % 128 <= 32:
            # ROM bootloader will read to the end of the 128 byte block, but not
            # to the end of the SHA-256 digest at the end
            new_len = len(plaintext_image) - (len(plaintext_image) % 128)
            plaintext_image = plaintext_image[:new_len]

    # if image isn't 128 byte multiple then pad with 0xFF (ie unwritten flash)
    # as this is what the secure boot engine will see
    if len(plaintext_image) % 128 != 0:
        plaintext_image += b"\xFF" * (128 - (len(plaintext_image) % 128))

    plaintext = iv + plaintext_image

    # Secure Boot digest algorithm in hardware uses AES256 ECB to
    # produce a ciphertext, then feeds output through SHA-512 to
    # produce the digest. Each block in/out of ECB is reordered
    # (due to hardware quirks not for security.)

    key = _load_hardware_key(args.keyfile)
    aes = pyaes.AESModeOfOperationECB(key)
    digest = hashlib.sha512()

    for block in get_chunks(plaintext, 16):
        block = block[::-1]  # reverse each input block

        cipher_block = aes.encrypt(block)
        # reverse and then byte swap each word in the output block
        cipher_block = cipher_block[::-1]
        for block in get_chunks(cipher_block, 4):
            # Python hashlib can build each SHA block internally
            digest.update(block[::-1])

    if args.output is None:
        args.output = os.path.splitext(args.image.name)[0] + "-digest-0x0000.bin"
    with open(args.output, "wb") as f:
        f.write(iv)
        digest = digest.digest()
        for word in get_chunks(digest, 4):
            f.write(word[::-1])  # swap word order in the result
        f.write(b'\xFF' * (0x1000 - f.tell()))  # pad to 0x1000
        f.write(plaintext_image)
    print("digest+image written to %s" % args.output)