Ask HN: Building a Meal Ministry App

I'm looking to build a meal ministry application for people within a church community to come together and support one another in times of need.

Since I'd rather not build out a database of contact information that could be breachable in the future, I'd like to use Diffie-Hellman to ensure that only the people who have agreed to share a meal will send their contact information to each other. The encryption method seems within my reach, but due to each client having to save their private key, is this possible all within the browser?

Is localstorage, cookies, etc. a good option for saving the private key?