What’s the safest way to build an emulated SSH terminal for an ARG?

Another option is a normal SSH server configured with a dedicated account whose login shell is your custom program. That program can display menus and simulated files without exposing an actual shell. However, public SSH services attract constant scanning and attack attempts, so this still needs careful isolation, minimal privileges, rate limiting, logging, and ideally a disposable container or virtual machine. Never rely on obscurity or a password alone as the security boundary.

Be careful with the design of the client experience. SSH clients use real authentication, host-key checking, terminals, and sometimes local configuration files, so players should not be asked to modify their existing keys or trust settings unnecessarily. An embedded SSH implementation can provide the familiar connection process while keeping the game state entirely in memory or in a restricted data store. Do not expose an interactive ssh command inside the emulated environment unless it is another tightly controlled simulation.

If confidentiality is not important, Telnet may be simpler than SSH. It is essentially a TCP connection carrying terminal input and output, so a small Python or similar program can provide the fake interface without implementing SSH authentication or terminal handling. A crashed process should only close the connection, provided it is not running with access to a real shell or sensitive files. The downside is that usernames, passwords, and puzzle data are sent unencrypted, so don’t use this approach for anything genuinely secret.