Retrieve and verify AccessToken with Keycloak Client.

KeycloakClientAuthExample.java

package de.tdlabs.keycloak.client;

import com.fasterxml.jackson.databind.ObjectMapper;
import org.keycloak.OAuth2Constants;
import org.keycloak.RSATokenVerifier;
import org.keycloak.admin.client.Keycloak;
import org.keycloak.admin.client.KeycloakBuilder;
import org.keycloak.common.VerificationException;
import org.keycloak.jose.jws.JWSHeader;
import org.keycloak.representations.AccessToken;
import org.keycloak.representations.AccessTokenResponse;
import org.keycloak.representations.idm.KeysMetadataRepresentation.KeyMetadataRepresentation;

import java.math.BigInteger;
import java.net.URL;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.RSAPublicKeySpec;
import java.security.spec.X509EncodedKeySpec;
import java.util.Base64;
import java.util.Base64.Decoder;
import java.util.List;
import java.util.Map;

public class KeycloakClientAuthExample {

  public static void main(String[] args) {

    KeycloakClientFacade facade = new KeycloakClientFacade( //
      "http://localhost:8081/auth" //
      , "admin-client-demo" //
      , "demo-client-1" //
      , "da6947c2-6559-4c37-b219-d37bb72ec2fa" //
    );

    // Get raw AccessToken string for client credentials grant
    System.out.println(facade.getAccessTokenString());

    // Get decoded AccessToken for client credentials grant
//    System.out.println(facade.getAccessToken());

    // Get decoded AccessToken for password credentials grant
    AccessToken accessToken = facade.getAccessToken("tester", "test");
    System.out.println(accessToken.getSubject());
  }

  static class KeycloakClientFacade {

    private final String serverUrl;

    private final String realmId;

    private final String clientId;

    private final String clientSecret;

    public KeycloakClientFacade(String serverUrl, String realmId, String clientId, String clientSecret) {
      this.serverUrl = serverUrl;
      this.realmId = realmId;
      this.clientId = clientId;
      this.clientSecret = clientSecret;
    }

    public AccessToken getAccessToken() {
      return getAccessToken(newKeycloakBuilderWithClientCredentials().build());
    }

    public String getAccessTokenString() {
      return getAccessTokenString(newKeycloakBuilderWithClientCredentials().build());
    }

    public AccessToken getAccessToken(String username, String password) {
      return getAccessToken(newKeycloakBuilderWithPasswordCredentials(username, password).build());
    }

    public String getAccessTokenString(String username, String password) {
      return getAccessTokenString(newKeycloakBuilderWithPasswordCredentials(username, password).build());
    }

    private AccessToken getAccessToken(Keycloak keycloak) {
      return extractAccessTokenFrom(keycloak, getAccessTokenString(keycloak));
    }

    private String getAccessTokenString(Keycloak keycloak) {
      AccessTokenResponse tokenResponse = getAccessTokenResponse(keycloak);
      return tokenResponse == null ? null : tokenResponse.getToken();
    }

    private AccessToken extractAccessTokenFrom(Keycloak keycloak, String token) {

      if (token == null) {
        return null;
      }

      try {
        RSATokenVerifier verifier = RSATokenVerifier.create(token);
        PublicKey publicKey = getRealmPublicKey(keycloak, verifier.getHeader());
        return verifier.realmUrl(getRealmUrl()) //
          .publicKey(publicKey) //
          .verify() //
          .getToken();
      } catch (VerificationException e) {
        return null;
      }
    }

    private KeycloakBuilder newKeycloakBuilderWithPasswordCredentials(String username, String password) {
      return newKeycloakBuilderWithClientCredentials() //
        .username(username) //
        .password(password) //
        .grantType(OAuth2Constants.PASSWORD);
    }

    private KeycloakBuilder newKeycloakBuilderWithClientCredentials() {
      return KeycloakBuilder.builder() //
        .realm(realmId) //
        .serverUrl(serverUrl)//
        .clientId(clientId) //
        .clientSecret(clientSecret) //
        .grantType(OAuth2Constants.CLIENT_CREDENTIALS);
    }

    private AccessTokenResponse getAccessTokenResponse(Keycloak keycloak) {
      try {
        return keycloak.tokenManager().getAccessToken();
      } catch (Exception ex) {
        return null;
      }
    }

    public String getRealmUrl() {
      return serverUrl + "/realms/" + realmId;
    }

    public String getRealmCertsUrl() {
      return getRealmUrl() + "/protocol/openid-connect/certs";
    }

    private PublicKey getRealmPublicKey(Keycloak keycloak, JWSHeader jwsHeader) {

// Variant 1: use openid-connect /certs endpoint
      return retrievePublicKeyFromCertsEndpoint(jwsHeader);

// Variant 2: use the Public Key referenced by the "kid" in the JWSHeader
// in order to access realm public key we need at least realm role... e.g. view-realm
//      return retrieveActivePublicKeyFromKeysEndpoint(keycloak, jwsHeader);

// Variant 3: use the active RSA Public Key exported by the PublicRealmResource representation
//      return retrieveActivePublicKeyFromPublicRealmEndpoint();
    }

    private PublicKey retrievePublicKeyFromCertsEndpoint(JWSHeader jwsHeader) {
      try {
        ObjectMapper om = new ObjectMapper();
        @SuppressWarnings("unchecked")
        Map<String, Object> certInfos = om.readValue(new URL(getRealmCertsUrl()).openStream(), Map.class);

        List<Map<String, Object>> keys = (List<Map<String, Object>>) certInfos.get("keys");

        Map<String, Object> keyInfo = null;
        for (Map<String, Object> key : keys) {
          String kid = (String) key.get("kid");

          if (jwsHeader.getKeyId().equals(kid)) {
            keyInfo = key;
            break;
          }
        }

        if (keyInfo == null) {
          return null;
        }

        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        String modulusBase64 = (String) keyInfo.get("n");
        String exponentBase64 = (String) keyInfo.get("e");

        // see org.keycloak.jose.jwk.JWKBuilder#rs256
        Decoder urlDecoder = Base64.getUrlDecoder();
        BigInteger modulus = new BigInteger(1, urlDecoder.decode(modulusBase64));
        BigInteger publicExponent = new BigInteger(1, urlDecoder.decode(exponentBase64));

        return keyFactory.generatePublic(new RSAPublicKeySpec(modulus, publicExponent));

      } catch (Exception e) {
        e.printStackTrace();
      }
      return null;
    }

    private PublicKey retrieveActivePublicKeyFromPublicRealmEndpoint() {

      try {
        ObjectMapper om = new ObjectMapper();
        @SuppressWarnings("unchecked")
        Map<String, Object> realmInfo = om.readValue(new URL(getRealmUrl()).openStream(), Map.class);
        return toPublicKey((String) realmInfo.get("public_key"));
      } catch (Exception e) {
        e.printStackTrace();
      }

      return null;
    }

    private PublicKey retrieveActivePublicKeyFromKeysEndpoint(Keycloak keycloak, JWSHeader jwsHeader) {

      List<KeyMetadataRepresentation> keys = keycloak.realm(realmId).keys().getKeyMetadata().getKeys();

      String publicKeyString = null;
      for (KeyMetadataRepresentation key : keys) {
        if (key.getKid().equals(jwsHeader.getKeyId())) {
          publicKeyString = key.getPublicKey();
          break;
        }
      }

      return toPublicKey(publicKeyString);
    }

    public PublicKey toPublicKey(String publicKeyString) {
      try {
        byte[] publicBytes = Base64.getDecoder().decode(publicKeyString);
        X509EncodedKeySpec keySpec = new X509EncodedKeySpec(publicBytes);
        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        return keyFactory.generatePublic(keySpec);
      } catch (NoSuchAlgorithmException | InvalidKeySpecException e) {
        return null;
      }
    }
  }
}

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>de.tdlabs.training</groupId>
	<artifactId>idm-keycloak-client-example</artifactId>
	<version>0.0.1-SNAPSHOT</version>

	<properties>
		<keycloak.version>2.5.1.Final</keycloak.version>
		<resteasy.version>3.0.14.Final</resteasy.version>

		<maven.compiler.source>1.8</maven.compiler.source>
		<maven.compiler.target>1.8</maven.compiler.target>
	</properties>

	<dependencies>
		<dependency>
			<groupId>org.keycloak</groupId>
			<artifactId>keycloak-admin-client</artifactId>
			<version>${keycloak.version}</version>
		</dependency>

		<dependency>
			<groupId>org.jboss.resteasy</groupId>
			<artifactId>resteasy-client</artifactId>
			<version>${resteasy.version}</version>
		</dependency>

		<dependency>
			<groupId>org.jboss.resteasy</groupId>
			<artifactId>resteasy-jackson2-provider</artifactId>
			<version>${resteasy.version}</version>
		</dependency>
	</dependencies>
</project>

Does retrievePublicKeyFromCertsEndpoint(JWSHeader jwsHeader) support java 1.8? returning null
Could you pls give compatible code for java 1.8 for this method