how to perform the watermarking with cryptography?

If op is uint8, then r must be uint8. If op is uint16, then r must be uint16. It does not let you mix integer types when doing operations on them together. In other words, if op is uint8 and r is uint16, it won't allow that. Your code is more than I want to take the time to read and understand, so it's up to you to figure out what integer type they are and what you need, and make them match. You can use the functions cast(), typecast(), uint8() and uint16() to do any necessary conversion(s).