JSON Web Tokens

jwt.io/introduction is a good introduction to JWTs. Tool-wise authgear.com/tools/jwt-jwe-debugger is probably a good choice, as it runs completely in the web browser. Authgear claims: Your data security is our top priority. All encoding, decoding, encryption and decryption happen in this browser. This tool does not store or send your JWT and JWE outside of the browser.