Toughen vs Stiffen - What's the difference?

In lang=en terms the difference between toughen and stiffen

is that toughen is to become tough while stiffen is to become stiff.

is that toughen is to make tough while stiffen is to make stiff.

(en verb)

To make tough.

To become tough.

(en verb)

To make stiff.

To become stiff.

* English ergative verbs