Where does jwt.io get the public key from JWT token?

Question

I was decoding a JWT token via jwt.io (in the Debugger section) to see Headers, Payload. Surprisingly, it also verified, and I could see it (jwt.io debugger) is able to retrieve

Answer 1

The token contains the issuer (iss) of the token and the key id (kid), which identifies the public key that is needed to verify the signature With this information, jwt.io can find the public key in form of a JWK (JSON Web Key) on a JWKS endpoint (/.well-known/jwks.json), to verify the token. A JWKS (JSON Web Key Set) contains an array of JWKs, the link shows an example.

According to the cognito documentation, this mechanism is used, when you use the Amazon user pool to authenticate your users.

Providing keys via a jwks endpoint is a standard mechanism which is also used by other providers, e.g. Microsoft Azure.