Cryptography is tricky to get right. There are a ton of subtle ways things can go wrong! I think the first thing to realize is your problem has probably been so

Cryptography is tricky to get right. There are a ton of subtle ways things can go wrong! I think the first thing to realize is your problem has probably been solved by someone else, so you can use off-the-shelf solutions. TLS has a pre-shared symmetric key option: https://en.wikipedia.org/wiki/TLS-PSK https://en.wikipedia.org/wiki/TLS-PSK for example. Second, I think it's important to know how crypto fails in order to use it properly. https://www.cryptopals.com https://www.cryptopals.com has a bunch of challenges that walk you through breaking a crypto implementation.