What's the hardest problem you've ever solved?

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

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.

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

I used ebtables and packet labels/connection tracking to allow our in-house network VPN/Web servers to function seamlessly in our dual homed (but not BGP supported) internet connections, each with different public addresses and gateways. It's now stock functionality in VPN/Firewall appliances, we ended up with SonicWall, 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...