This talk will illustrate how standard cryptographic techniques can be applied to real-life security products and services. This article presents in detail one of the examples given in the talk. It is intended to help the audience follow that part of our presentation. We chose as a characteristic example a little noticed yet ingenious Microsoft patent by Thomlinson and Walker. The Thomlinson-Walker system distributes encrypted patches to avoid reverse engineering by opponents (who would then be able to launch attacks on unpatched users). When the proportion of users who downloaded the encrypted patch becomes big enough, the decryption key is disclosed and all users install the patch.