Using the box (4)

Only the owner of red key
Can unlock the big (outer) box
Contents delivered securely to Red-Owner

When Red-Owner opens box
Small box found inside
Indicator is blue
Blue key used to open small box
Must have been locked by purple key

So, contents delivered securely
and origin of contents known for sure

Terminology
Yellow (and Blue) Key
Public Key
Given to anyone at all
Red (and Purple) Key
Private Key
Kept secret to owner