A system for processing stereo matching of a video image sequence in a real-time mode is provided. Accordingly, the light scanning apparatus can reduce or eliminate the deviation in exposure between scan lines of the multi-beam scan method and the deviation in exposure between photosensitive bodies of the tandem type image forming apparatus. The image is decomposed into a hierarchy of images having a top level and one or more lower levels each having a higher level parent, each lower level image defining a smaller region of the corresponding higher level parent image, the top level image defining a region that covers the item of information. Embodiments are applicable to forward secret ciphers that are used to protect protocols with unreliable transport, to ciphers that are used in multicast or other group settings, and to protection of packets using the IPSec protocols.