Canon's *.fir upgrade files seemed to be encrypted by a simple stream cypher. Analysis of B:\CAMERA.EXE decryption routines allows one to understand encryption/decryption algorithm.

Legal Disclaimer
While this information could be used to decrypt *.fir files provided by Canon, this would violate Canon's software license that you agree to when you download firmware update. However, you are not bound by any license when you buy camera in store, open the box, connect camera to you computer, and extract files this way. The eventual goal of this project is to create programs in *.fir format that is understood by the camera that modify some of the camera's settings. Such programs should be created using only the information gleaned from the camera itself, and not from Canon's licensed firmware updates.