Yes, if the file size is small then an attack is possible. But if it's big, it's really very difficult. I'm also working on a pre-mask that removes repeating bytes from the input and makes it impossible to get double characters. It's just an experiment, I'll also put an encrypted file with a prize for whoever reveals the original file! Apart from these problems that are there, how are you going to guess a sequence that can have 256! different combinations and can be encrypted with several different keys?