Facebook cookie authentication

FacebookCookie.java

package models;

import com.google.gson.Gson;
import org.apache.commons.codec.binary.Base64;

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.io.UnsupportedEncodingException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.util.regex.Pattern;

/**
 * @author huljas
 */
public class FacebookCookie {

    private String value;
    private boolean valid = false;
    private Data data;

    public FacebookCookie(String value) {
        this.value = value;
    }

    /**
     * Call before anything else!
     */
    public boolean validate(String secret) {
        String[] splits = value.split(Pattern.quote("."));
        String signature = splits[0];
        String encoded = splits[1];
        String jsonString = decodeBase64(encoded);
        String signature2 = digest(encoded, secret);
        if (!signature2.equals(signature)) {
            return false;
        }
        Gson gson = new Gson();
        this.data = gson.fromJson(jsonString, Data.class);
        return true;
    }

    public static String digest(String encoded, String secret) {
        try {
            Mac hmacSha256 = Mac.getInstance("HmacSHA256");
            SecretKeySpec key = new SecretKeySpec(secret.getBytes("UTF-8"), "HmacSHA256");
            hmacSha256.init(key);
            byte[] mac = hmacSha256.doFinal(encoded.getBytes("UTF-8"));
            byte[] base64 = Base64.encodeBase64(mac);
            String s = new String(base64, "UTF-8");
            return s.replace("/", "_").replace("+", "-").replace("=", "");
        } catch (NoSuchAlgorithmException e) {
            throw new Error("Never thrown", e);
        } catch (UnsupportedEncodingException e) {
            throw new Error("Never thrown", e);
        } catch (InvalidKeyException e) {
            throw new RuntimeException("Invalid key", e);
        }
    }

    public static String decodeBase64(String encoded) {
        return new String(Base64.decodeBase64(encoded));
    }

    public Data data() {
        return data;
    }

    public class Data {
        public String user_id;
        public String code;
    }
}

index.html

<div id="fb-root"></div>
<script>
  window.fbAsyncInit = function() {
    FB.init({appId : 'APP_ID', status : true, cookie : true, oauth : true, xfbml : true});
  };
  // Load the SDK Asynchronously
  (function(d){
     var js, id = 'facebook-jssdk'; if (d.getElementById(id)) {return;}
     js = d.createElement('script'); js.id = id; js.async = true;
     js.src = "//connect.facebook.net/en_US/all.js";
     d.getElementsByTagName('head')[0].appendChild(js);
   }(document));
</script>
<div class="fb-login-button">Login with Facebook</div>

Thank you for your work! After about 2 hours of searching in Google found your job and this just works! Helped a lot!