Algorithm
Header
alg is set from the algorithm above, so the header can't disagree with the signature.
Payload
Token
How this one works
The header and payload are serialised as compact JSON, Base64URL-encoded and joined with a dot. That string is signed with your browser's WebCrypto and the signature is appended as the third part.
HS algorithms sign with a shared secret. For RS, PS, ES and EdDSA a key pair is made in this tab when you pick one: the private key signs, and the public key, shown as PEM or JWK, is what your server verifies with. You can paste your own private key instead, as PKCS#8, PKCS#1 or SEC1 PEM, or as a JWK.
The expiry buttons set iat to now and exp that far ahead, in seconds, which is what JWT libraries expect.
Questions
What should I use generated tokens for?
Local development and tests: fixtures for an API that expects a bearer token, reproducing a bug with specific claims, or checking how your code handles an expired token.
Which algorithm should I pick?
Match what your server expects. RS256 is what most identity providers issue. HS256 suits a single service that both signs and checks. ES256 and EdDSA give the same strength as RS256 with much shorter keys and tokens.
Is the key stored anywhere?
No. Secrets and key pairs live in this page's memory while the tab is open and are never written to storage or sent over the network. A new key pair is made each time you open the page.