Defending a Perimeter

<<Leakage<< Up to Leakage >>Confinement>>

Alice gives Bob a power that she wishes Mallet not to have.  Bob and Mallet are not supposed to be in communication, and Bob does not wish to give the power to Mallet.  Can Mallet break in and take the power anyway?  Preventing this is the problem of Perimeter Security.
Operating Systems MMUs
Open Networks Cryptography
Languages Compilers
Hardware Tag bits
Bounds Checks

Impenetrable Shields 
Firewalls Applet Sandbox
Sifting Garbage 
Tempest Timing Attacks
Rubber Hose 
Smart cards & Microwaves