cool python script for aes-abc

def aes_abc_decrypt(ct):
    blocks = [ct[i * BLOCK_SIZE : (i+1) * BLOCK_SIZE] for i in range(len(ct) / BLOCK_SIZE)]

    for i in range(len(blocks)-1, 0, -1):
        upper_blk = int(blocks[i].encode('hex'), 16)
        lower_blk = int(blocks[i-1].encode('hex'), 16)

        blk_diff = upper_blk - lower_blk
        if blk_diff < 0:
            blk_diff += UMAX

        blocks[i] = to_bytes(blk_diff)

    ct_ecb = "".join(blocks)

    return ct_ecb

if __name__="__main__":
    header, data = '', ''

    with open('body.enc.ppm', 'rb') as f:
        header, data = parse_header_ppm(f)

    ct_ecb = aes_abc_decrypt(data)

    with open('flag.ecb.ppm', 'wb') as fw:
        fw.write(header)
        fw.write(ct_ecb)