GPG File Exchange & iptables Firewall Lab

Three separate Linux users, created and administered, so the exchange happens between actual accounts with actual permissions rather than three directories pretending to be people.

An RSA key pair per user, public keys exported armoured and imported by the others, then messages encrypted so that only the intended recipient can open them — including deleting a secret key and confirming exactly what that does and does not make unreadable.

Digital signatures in both forms, embedded and detached, each transmitted and then verified from the receiving account — which is the half that proves who sent a file, as opposed to who is able to read it.

The firewall starts from default-deny: INPUT and FORWARD policies set to DROP and OUTPUT to ACCEPT, so every allowance afterwards is explicit and the failure mode of a missing rule is a blocked packet rather than an open one.

Rules written at the granularity the scenario actually needs — multiport matching for HTTP and HTTPS together, `iprange` to admit one team’s address block to a single port, and FORWARD rules to control traffic passing through the host rather than landing on it.

DROP against REJECT tested rather than asserted: the same `scp` attempted under each policy, timing out under one and being refused immediately under the other. Both labs are evidenced step by step, so the attempts that failed are on the page next to the ones that worked.