Thanks to Web Cryptography API, you can use private key crypto right in your browser.
This set of pages lets you exchange private information with your friend using encrypted messages using only public channels (like, agreeing on meeting in some place on IRC). Codewise, this is mostly a compilation of few first examples from webcrypto examples repo.
Open encrypt.html
Paste the public key into green area
Enter your message into yellow box
Click “Encrypt” button
Open generate.html
Click “Generate” button
Save private key (red area) for step 6 (or, just keep this page open)
Open decrypt.html
Paste private key from step 3 into red area
Paste encrypted message into blue area