What about in the case of a piece of software without a serial algo. For example one that submits a serial code for a server, which returns a value (1 or 0 for example) as to whether it's valid. In this case the entire algo is on the server and there is no possible way to determine what it is, unless you can get access to the server, which is no longer Reverse Engineering and becomes hacking/cracking, and therefore the only possible way to Reverse the program is to simply patch the server check.