src/main/java/com/github/scribejava/apis/GoogleTokenVerifier.java


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package com.github.scribejava.apis;

import java.net.MalformedURLException;
import java.net.URL;
import java.text.ParseException;
import java.util.Collections;
import java.util.Map;
import java.util.Optional;

import com.nimbusds.jose.JOSEException;
import com.nimbusds.jose.JWSAlgorithm;
import com.nimbusds.jose.jwk.source.JWKSource;
import com.nimbusds.jose.jwk.source.RemoteJWKSet;
import com.nimbusds.jose.proc.BadJOSEException;
import com.nimbusds.jose.proc.JWSKeySelector;
import com.nimbusds.jose.proc.JWSVerificationKeySelector;
import com.nimbusds.jose.proc.SecurityContext;
import com.nimbusds.jwt.proc.ConfigurableJWTProcessor;
import com.nimbusds.jwt.proc.DefaultJWTClaimsVerifier;
import com.nimbusds.jwt.proc.DefaultJWTProcessor;

public class GoogleTokenVerifier {

    public static Optional<String> validateToken(String clientId, String idToken) {

        // Create a JWT processor for the access tokens
                ConfigurableJWTProcessor<SecurityContext> jwtProcessor = new DefaultJWTProcessor<>();
        
        // The public RSA keys to validate the signatures will be sourced from the
        // OAuth 2.0 server's JWK set, published at a well-known URL. The RemoteJWKSet
        // object caches the retrieved keys to speed up subsequent look-ups and can
        // also handle key-rollover
                JWKSource<SecurityContext> keySource =
                        null;
                try {
                    keySource = new RemoteJWKSet<>(new URL("https://www.googleapis.com/oauth2/v3/certs"));
                } catch (MalformedURLException e) {
                    return Optional.empty();
                }
        
        // The expected JWS algorithm of the access tokens (agreed out-of-band)
                JWSAlgorithm expectedJWSAlg = JWSAlgorithm.RS256;
        
        // Configure the JWT processor with a key selector to feed matching public
        // RSA keys sourced from the JWK set URL
                JWSKeySelector<SecurityContext> keySelector =
                        new JWSVerificationKeySelector<>(expectedJWSAlg, keySource);
        
                jwtProcessor.setJWSKeySelector(keySelector);
        
        // Set the required JWT claims for access tokens issued by the server
                jwtProcessor.setJWTClaimsSetVerifier(new DefaultJWTClaimsVerifier<>(Collections.singleton(clientId), null, null, null));
        
        // Process the token
                Map<String, Object> claimsSet;
                try {
                    claimsSet = jwtProcessor.process(idToken, null).toJSONObject();
                } catch (BadJOSEException | JOSEException | ParseException e) {
                    return Optional.empty();
                }
        
                String email = (String)claimsSet.get("email");
                boolean verified = claimsSet.get("email_verified").equals(true);
                return verified ? Optional.of(email) : Optional.empty();
            }    
}