I think using the same symbol to begin and end encryption will make users uncomfortable, because there is always the risk (real or imagined, it doesn't matter so far as user fears are concerned) that
I will turn off encryption when I intend to turn it on. It certainly makes me stop and think about whether I've got it right.
I suggest using ## to begin encryption and $$ to end encryption. They are next to each other on the keyboard, straight after @ with ## coming before $$