I used ebtables and marking packets to allow our in-house network servers to function seamlessly with one of two different internet connections, each with different public addresses and gateways. It's now stock functionality in VPN.Firewall appliances, but at the time it nearly melted my brain.

A second hardest was writing a cooperative multi-tasker for Turbo Pascal programs under MS-DOS.[1] It made a copy of the stack, and the first time it returned from spawn twice, I was really, really confused.... until it dawned on me that it was actually working as designed! There was a really weird requirement in an upload protocol I was using that required knowing how long the file was, before I was even done generating it, so I built a text piping engine to allow that to happen, and Tasker was part of that.

[1] https://web.archive.org/web/20230319104502/https://www.pcorn...

The problem I'm currently trying to solve, building a business around a novel data protection mechanism using ML based encoding while continuing to do research on it.

triple openvpn setup with a webserver in the middle handling cloudflare web requests outside the vpn (pre-AI)

Its classified, but the second hardest was simulating Dynamic nuclear polarization, ab initio.