For digital rights management (DRM) of e.g. digitally delivered music or video, a technique to make the decryption keys more secure. The technique fragments a message (song or video or other) into a number of portions, and uses a different decryption key for each portion. Each of the various keys is a function of the preceding key, in one version. In another version, each key is a function of a seed value and of the particular portion of the material with which the key is associated.