At a hackathon, my team and I built an application that used RSA encryption to encrypt text messages. The app would store your public and private key, and if you wanted to give/receive encrypted messages from your friends, you would have to share your public key. Using that key the person can encrypt their message, send it to you, and you could use your private key to decrypt it. In the future, it would be better to find a way to avoid the overhead of sharing the public key with your friends, but maybe have it be accessible by having the app connect to your contacts.