Java offline validation of JWT access token from Keycloak

I get a signed JWT token from Keycloak http://localhost:8080/auth/realms/MyRealm/protocol/openid-connect/token:

{
"access_token": "eyJhbGciOiJSUzI1NiIsInR5cCIgOiAiSldUIiwia2lkIiA6ICJqQThGdzdhRk1rTGhGc2......",
"expires_in": 300,
"refresh_expires_in": 1800,
"refresh_token": "eyJhbGciOiJIUzI1NiIsInR5cCIgOiAiSldUIiwia2lkIiA6ICIzMWRjZTBjNS01MGU0LTQxZjMtODAxNC1kMTcyMjdk....",
"token_type": "bearer",
"not-before-policy": 0,
"session_state": "bd1728eb-ceda-43cf-a6e6-d637ba0da5e3",
"scope": "email profile"
}

So the access token is:

eyJhbGciOiJSUzI1NiIsInR5cCIgOiAiSldUIiwia2lkIiA6ICJqQThGdzdhRk1rTGhGc2......

I query Keycloak for http://localhost:8080/auth/realms/Myrealm/:

{
"realm": "MyRealm",
"public_key": "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuKQF8ewcJ/pDzhgzbTfFCoS1FLjZyO5z7CbmeWl.......",
"token-service": "http://localhost:8080/auth/realms/Myrealm/protocol/openid-connect",
"account-service": "http://localhost:8080/auth/realms/Myrealm/account",
"tokens-not-before": 0
}

So the public key is:

MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuKQF8ewcJ/pDzhgzbTfFCoS1FLjZyO5z7CbmeWl.......

Now I'm trying to do an offline validation as follows:

import java.security.KeyFactory;
import java.security.spec.X509EncodedKeySpec;
import java.security.PublicKey;

String keyFromKeycloak = "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuKQF8ewcJ/pDzhgzbTfFCoS1FLjZyO5z7CbmeWl.......";
byte[] bytes = keyFromKeycloak.getBytes();
KeyFactory factory = KeyFactory.getInstance("RSA");
X509EncodedKeySpec encodedKeySpec = new X509EncodedKeySpec(bytes);
PublicKey pk = factory.generatePublic(encodedKeySpec);

But I get an exception for invalid key:

java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: invalid key format
    at java.base/sun.security.rsa.RSAKeyFactory.engineGeneratePublic(RSAKeyFactory.java:239)
    at java.base/java.security.KeyFactory.generatePublic(KeyFactory.java:352)
Caused by: java.security.InvalidKeyException: invalid key format
    at java.base/sun.security.x509.X509Key.decode(X509Key.java:386)
    at java.base/sun.security.x509.X509Key.decode(X509Key.java:401)
    at java.base/sun.security.rsa.RSAPublicKeyImpl.<init>(RSAPublicKeyImpl.java:122)
    at java.base/sun.security.rsa.RSAKeyFactory.generatePublic(RSAKeyFactory.java:330)
    at java.base/sun.security.rsa.RSAKeyFactory.engineGeneratePublic(RSAKeyFactory.java:235)
    ... 3 more

I need the PublicKey object in order to validate the access token as follows:

Algorithm algorithm = Algorithm.RSA256(
    (RSAPublicKey) pk,
    null);
JWTVerifier verifier = JWT.require(algorithm).build();
DecodedJWT jwt = verifier.verify(token);

What is wrong? I see a lot the examples with this code so I suspect the problem is with the key itself.

Your keyFromKeycloak String is a Base64 encoded DER SubjectPublicKeyInfo. You should first decode it and then pass it to the X509EncodedKeySpec constructor :

String keyFromKeycloak = "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuKQF8ewcJ/pDzhgzbTfFCoS1FLjZyO5z7CbmeWl.......";
byte[] keyBytes = Base64.getDecoder().decode(keyFromKeycloak);
X509EncodedKeySpec encodedKeySpec = new X509EncodedKeySpec(keyBytes);