Sometimes we want better-than-firewall security for things. For instance: An industrial control system for a municipal water-treatment plant should never have data come in or out Or, a variant of the industrial control system: it should only permit telemetry and monitoring data out, and nothing else in or out A system dedicated to keeping your GPG private keys secure should only have material to sign (or decrypt) come in, and signatures (or decrypted data) go out A system keeping your tax rec...
