I agree that obscurity has value in security designs --- security is in large part about imposing asymmetric costs on attackers.
I do not believe that port knocking and single-packet authentication are good examples of this; both are sort of textbook examples of cosmetic security mechanisms that are made irrelevant by SSH public key authentication.
I do not believe that port knocking and single-packet authentication are good examples of this; both are sort of textbook examples of cosmetic security mechanisms that are made irrelevant by SSH public key authentication.