. "Below, we present an implementation of a parasitic computer . using the checksum function. In order for this to occur, . one needs to design a special message that coerces a target server . into performing the desired computation." This is the same principle that underlies denial of service attacks -- the irreducible residual vulnerability of a system to denial of service is proportional to the amount of work (or time) that system must do (or consume) before it can conclude its initial authorization decision. Ironically, the more precise and complex that authorization decision process, the greater the amount of work that the active (initiating) side of the connection can call on the passive side to perform. This critically bears on protocol and application security design. --dan