Firebase 3.0 + GAE

AppengineFirebaseAuth.java

public class AppengineFirebaseAuth {

    private static final String APP_ID = "YOUR_APP_ID";

    public static AppengineFirebaseToken verifyIdToken(String token) {
        Map<String, String> publicKeys = GooglePublicKeys.getKeys();

        for (String kid : publicKeys.keySet()) {
            String publicKey = publicKeys.get(kid);

            try {
                Jws<Claims> claimsJws = verifyIdTokey(token, kid, publicKey);
                return new AppengineFirebaseToken(claimsJws.getBody());
            } catch (SignatureException e) {
                continue; // try next key
            } catch (ExpiredJwtException e) {
                throw new InvalidFirebaseTokenException(e.getMessage());
            }
        }

        throw new InvalidFirebaseTokenException("No Google public keys for JWT Token");
    }

    private static Jws<Claims> verifyIdTokey(String token, String kid, String publicKey) {
        PublicKey pk = createPk(publicKey);
        Jws<Claims> claimsJws = Jwts.parser().setSigningKey(pk).parseClaimsJws(token);
        validate(claimsJws, kid);
        return claimsJws;
    }

    private static void validate(Jws<Claims> claimsJws, String kid) {
        if (claimsJws.getHeader().getAlgorithm().equals("RS256") &&
                claimsJws.getBody().getAudience().equals(APP_ID) &&
                claimsJws.getBody().getIssuer().equals("https://securetoken.google.com/" + APP_ID) &&
                claimsJws.getBody().getSubject() != null &&
                claimsJws.getHeader().getKeyId().equals(kid)) {
            return;
        }
        throw new InvalidFirebaseTokenException("Invalid Firebase Id Token");
    }

    private static PublicKey createPk(String publicKey) {
        try {
            CertificateFactory cf = CertificateFactory.getInstance("X.509");
            InputStream stream = new ByteArrayInputStream(publicKey.getBytes("UTF-8"));
            java.security.cert.Certificate cert = cf.generateCertificate(stream);
            return cert.getPublicKey();
        } catch (CertificateException | UnsupportedEncodingException e) {
            throw new RuntimeException(e);
        }
    }

}

AppengineFirebaseToken.java

public class AppengineFirebaseToken {

    private Claims claims;

    private String uid;

    private String issuer;

    private String name;

    private String picture;

    private String email;

    private String emailVerified;

    public AppengineFirebaseToken(Claims claims) {
        this.claims = claims;
    }

    public String getUid() {
        return (String) claims.get("user_id");
    }

    public String getIssuer() {
        return (String) claims.get("iss");
    }

    public String getName() {
        return (String) claims.get("name");
    }

    public String getPicture() {
        return (String) claims.get("picture");
    }

    public String getEmail() {
        return (String) claims.get("email");
    }

    public boolean isEmailVerified() {
        return Boolean.valueOf((String) claims.get("email_verified"));
    }

}

GooglePublicKeys.java

public class GooglePublicKeys {

    private static final String GOOGLE_PUBLIC_KEYS = "https://www.googleapis.com/robot/v1/metadata/x509/securetoken@system.gserviceaccount.com";

    private static final int ONE_DAY = 1000 * 60 * 60 * 24;

    private static GooglePublicKeys instance;

    private long timestamp;

    private Map<String, String> keys;

    private GooglePublicKeys() {
    }

    public static Map<String, String> getKeys() {
        if (instance == null || isOutdated()) {
            reload();
        }

        return instance.keys;
    }

    private static boolean isOutdated() {
        return instance.timestamp < System.currentTimeMillis() - ONE_DAY;
    }

    private synchronized static void reload() {
        if (instance != null && !isOutdated()) {
            return;
        }

        instance = new GooglePublicKeys();
        instance.keys = fetchGooglePublicKeys();
        instance.timestamp = System.currentTimeMillis();
    }

    private static Map<String, String> fetchGooglePublicKeys() {
        return parseJson(fetchKeysJson());
    }

    private static String fetchKeysJson() {
        try {
            URLFetchService urlFetch = URLFetchServiceFactory.getURLFetchService();

            HTTPResponse response = urlFetch.fetch(new URL(GOOGLE_PUBLIC_KEYS));
            return new String(response.getContent());
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    private static Map<String, String> parseJson(String json) {
        Gson gson = new Gson();
        Map<String, String> map = new HashMap<>();
        return gson.fromJson(json, map.getClass());
    }

}

InvalidFirebaseToken.java

public class InvalidFirebaseTokenException extends RuntimeException {
    public InvalidFirebaseTokenException(String message) {
        super(message);
    }
}