How can i decode JWT using Keycloak - spring-boot

Question

I am using Keycloak with Spring-Boot in my application. My browser client request keycloak to generate JWT and then sends this JWT to my ZUUL server which validates JWT using keycloak-spring adapter and then i have written a pre-filter to decodes JWT payload and extract username.
I am using com.auth0.java-jwt library to decode JWT like in below snippet
DecodedJWT dJWT=JWT.decode(header);
String username=dJWT.getClaim("preferred_username").asString();
I was wondering if there is anyway i can do this without using external library. I want to use keycloak library to decode JWT explicitly. How can i achieve this?

Answer 1

You have to include keycloak's core library into your dependencies.
Gradle: compileOnly 'org.keycloak:keycloak-core:3.4.2.Final'
Then use the org.keycloak.TokenVerifier to parse the token.
Example:
try
{
// deprecated: AccessToken token = RSATokenVerifier.create(tokenString).getToken();
AccessToken token = TokenVerifier.create(tokenString, AccessToken.class).getToken();
System.out.printf("iss = %s%n", token.getIssuer());
System.out.printf("sub = %s%n", token.getSubject());
System.out.printf("typ = %s%n", token.getType());
}
catch (VerificationException e)
{
// some error handling
}
You can also activate various verifications on the RSATokenVerifier and in particular the signature validation by setting the public key:
RSATokenVerifier.create(tokenString).checkActive(true).publicKey(key).verify().getToken()

Answer 2

As i am using keycloak to authenticate jwt, it decodes jwt and puts details into SecurityContextHolder and I just pulled the details from there it self. here is the code.
Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
if (authentication != null) {
if (authentication.getPrincipal() instanceof KeycloakPrincipal) {
KeycloakPrincipal<KeycloakSecurityContext> kp = (KeycloakPrincipal<KeycloakSecurityContext>) authentication.getPrincipal();
// retrieving username here
String username = kp.getKeycloakSecurityContext().getToken().getPreferredUsername();
}
}
this solved it for me.