"I want to set up some sort of secure connection between the cell phone and the web site running on the Pi."
This should be doable via a vpn client/server. A quick google search on "raspberry pi cell phone vpn" returned this: "If you're going to be connecting to Pi VPN on a mobile device, I recommend OpenVPN Connect, the official client. It's completely free and integrates really well with iOS and Android. The first step is to open the App Store or Play Store, depending on your device. In either case, search for OpenVPN Connect" You should be able to easily find step-by-step instructions to do get this setup and working. That'll solve the secure connection between the cell ph and the Rpi hosted website, but that doesn't your "main concern is an attacker connecting to the web site and igniting the rocket while the user is connecting thew wires to the igniter." I'm going to suggest the probability of this happening is your best security. However, I'm not the adult that's responsible for children's safety. The big question here is whether the Rpi hosted website is accessible from the internet? My suspicion is that it would have a non-internet routable private not pubic ip addr in the following ip addr ranges: 10.0. 0.0 to 10.255. 255.255. 172.16. 0.0 to 172.31. 255.255. 192.168. 0.0 to 192.168. 255.255. If the Rpi website is accessible via the public internet than there's 2 other other options. 1. Learn about securing/hardening a Rpi. https://www.chrisapproved.com/blog/raspberry-pi-hardening.html 2. Change all the passwords and codes on launch day. I hope that's somewhat helpful.
