Hello

A few months ago my normally healthy cactus got a large white, dried out patch on its side. The outside of the plant is rather hard in this area, and it appears to be getting larger.

The cactus began shrinking soon after this, and the bottom part it become soft. From what I've read, this means it is probably rotting, and the best way to save it would be the cut off the healthy part and wait for it to re-root.
Does anyone know what this white patch may be, if it is related to the rot in anyway, or if the cactus can still be saved?