The presented paper demonstrates a method to embed a unique signature into a coating material used in a smart card or in the covering material of some other secure hardware device. The method bases on the impossibility of exactly reproducing a specific piece of plastic or other material used to cover the secure hardware. By using a very inhomogeneous materials or mixtures of conductors and insulators such a cover is made unique by the method of production. This inhomogeneous piece and the non-reproducible and random properties are incorporated into an electronic signature which is checked whenever needed. Assuming that the surface is covered totally with an "active coating material" it is impossible to partially penetrate or destroy the coating without destroying the signature. Unpenetrable hardware is an inevitable element in nearly all secure designs and with the promotion of digital signatures such unpenetrable hardware becomes even more important. The result gained with the presented work is the possibility to make a hardware unique depending on randomness, and to assure that penetration is not only detected but also features logical destruction of the secure hardware [PAT96]. Implementing such penetration sensors with memory enhances the security to a large extent, and since the destruction upon perceived penetration is logical there is no possible false alarm.