Computer viruses can be used by their authors to harness the resources of in- fected machines for the author’s computation. By doing so without the permission or knowledge of the machine owners, viruses can be used to perform covert distributed proc- essing. We outline the class of problems for which covert distributed processing can be used. A bruteforce attack on cryptosystems is one such problem, and we give estimates of the time required to complete such an attack covertly.