This is a totally new kind of security requirement. It has nothing to do with hiding information. Rather, it is about whether the adversary can actually generate a particular piece of data (e.g., a MAC forgery).To express such a requirement in a security definition, we need a slightly different approach to defining the libraries. Consider a much simpler statement: “no adversary should be able to guess a uniformly chosen value.” We can formalize this idea with the following two libraries: