Crackability of a software

[chessgod101]

Your first statement is not exactly correct. For example, it is not possible to patch a rar archive to accept a faux serial. I have also seen programs the the author encrypted specific features of a program which could only be decrypted with a valid serial. To solve these problems, brute force is the only viable option. Depending on the length of the serial/password, it could take centuries to brute force a valid value. Hypothetically, once you get a valid serial(be it brute forced of leaked), you could nop the decryption routine and patch the correctly decrypted code into the program.

Sure. I've forgotten to say that my statement isn't referred to cases that requires a key to decrypt something (this last problem, imho, isn't a reversing problem but is a cryptography problem because we have a piece of code encrypted without a routine that decrypt that piece of code)