How to get current user authentication inside UserDetailsService - java

In my application I am trying to unite ActiveDirectory authentication with OAuth2 refresh tokens.
I was able to successfully authenticate via ActiveDirectoryLdapAuthenticationProvider. I have also provided my custom implementation of LdapUserDetailsMapper that populates the UserDetails with some custom attributes taken from ActiveDirectory. Key thing here is that these attributes have a confidentialty flag set on them and are only available to the user itself (i.e. authenticated user could read the values of these attributes for himself but not for the others). These attributes are stored in Authentication object and are used by an application in a context of an authenticated user.
Things get tricky when I try to add refresh tokens to the picture. Refresh tokens require me to implement a UserDetailsService where I have to provide new UserDetails having just a user name. This is not feasible due to confidentialty flag. Even if I have some master account in my application with the ability to browse ActiveDirectory I will not be able to retrieve the confidential attributes.
So I would rather prefer to provide more atomic implementations like the function that checks if the user is still active or the function that provides a renewed set of user authorities. Unfortunately I did not find this level of atomicity in Spring Security. So it looks like for refresh tokens I have to provide an implementation of UserDetailsService.
If I have to provide new user details I would like to have an access to previous user Authentication object. In this case I will check the user and if it is still active I will copy all the confidential information from previous Authentication. The problem is that it does not seem to be available. At the moment when UserDetailsService::loadUserByUsername() is called SecurityContextHolder.getContext() does not contain the user authentication. Authentication is also not available from UserDetailsService API - I only get the user name. At the same time user's Authentication object is present just one stack frame up in UserDetailsByNameServiceWrapper class:
public UserDetails loadUserDetails(T authentication) throws UsernameNotFoundException {
return this.userDetailsService.loadUserByUsername(authentication.getName());
}
The least thing I want to do here is to implement some in-memory storage for all user confidential information to be used whenever I need to provide new UserDetails. I already have all the required information in user authentication managed by Spring and doing this on my end seems to be just surplus.
And here comes question list:
If you feel that I am doing something terribly wrong from the perspective of application security architecture, please tell me
Is there a way to tell Spring during refresh token procedure to use previous UserDetails object so that application could just answer the question if the user is still active and should be issued a new access token (and not provide the UserDetailsService at all)?
Is there a way to get previous user Authentication object during the call to UserDetailsService::loadUserByUsername() so that I could use it as a source of confidential info?
Is there some other approach that I do not see at the moment to add refresh tokens to my application?
Update:
Here I saw a comment that you could implement your own AuthenticationUserDetailsService to work around the problem. This I do not see how to do. It is hardcoded in AuthorizationServerEndpointsConfigurer that it always creates an instance of UserDetailsByNameServiceWrapper so to provide your own implementation you would have to interfere into AuthorizationServerEndpointsConfigurer initialization process.

OK, looks like the answer with Spring Security 4.0 is you can't.
So I had to apply the following hack which works, but I do not like it very much. Since it works I am posting it here. Since it does not solve the original problem, but works around it I will not mark it as accepted by the author.
Switch to JWT tokens
Use custom TokenEnhancer to inject all information that is required to recreate the user (user secret in my case) to the token directly. Of course, the value must be encrypted by the server with symmetrical crypto algorithm before adding it to the token.
Instruct authorization server to use custom AccessTokenConverter. This implementation of AccessTokenConverter would extract the secret value from the token, decrypt it and put it to ThreadLocal field.
Instruct custom UserDetailsService to retrieve the user secret from the ThreadLocal field set in step 3. This is the best way I found so far to deliver the current authorization context to UserDetailsService. And this is the part that I do not like most in my solution.
Use custom security filter to erase the value set in step 3 from ThreadLocal field.
P.S. I still do not see the possibility to implement custom AuthenticationUserDetailsService that was mentioned earlier. If such possibility exists it could have been another way to solve the problem.
Some useful links:
Extending Spring Security OAuth for Multi-Tenant
Detailed explanation of the problem in spring-security-oauth GitHub

I've got the response from Joe Grandja on spring-security-oauth github page.
Posting it here since it actually provides an answer to the original question.
Hi #masm22. To help with question 1 and 2, below is a custom configuration that will allow you to hook into the refresh_token grant and provide your own behaviour or delegate to super to proceed with current behaviour. It will also allow you to access the user Authentication so you can read your custom (confidential) attributes.
#Configuration
#EnableAuthorizationServer
public class AuthorizationServerConfig extends AuthorizationServerConfigurerAdapter {
..... // other config
#Autowired
private ClientDetailsService clientDetailsService;
#Override
public void configure(AuthorizationServerEndpointsConfigurer endpoints) throws Exception {
endpoints.tokenServices(this.customTokenServices());
}
private DefaultTokenServices customTokenServices() {
DefaultTokenServices tokenServices = new CustomTokenServices();
tokenServices.setTokenStore(new InMemoryTokenStore());
tokenServices.setSupportRefreshToken(true);
tokenServices.setReuseRefreshToken(true);
tokenServices.setClientDetailsService(this.clientDetailsService);
return tokenServices;
}
private static class CustomTokenServices extends DefaultTokenServices {
private TokenStore tokenStore;
#Override
public OAuth2AccessToken refreshAccessToken(String refreshTokenValue, TokenRequest tokenRequest) throws AuthenticationException {
OAuth2RefreshToken refreshToken = this.tokenStore.readRefreshToken(refreshTokenValue);
OAuth2Authentication authentication = this.tokenStore.readAuthenticationForRefreshToken(refreshToken);
// Check attributes in the authentication and
// decide whether to grant the refresh token
boolean allowRefresh = true;
if (!allowRefresh) {
// throw UnauthorizedClientException or something similar
}
return super.refreshAccessToken(refreshTokenValue, tokenRequest);
}
#Override
public void setTokenStore(TokenStore tokenStore) {
super.setTokenStore(tokenStore);
this.tokenStore = tokenStore;
}
}
}
The other thing I want to point out for your information is in DefaultTokenServices.refreshAccessToken(String refreshTokenValue, TokenRequest tokenRequest)
has the following code:
OAuth2Authentication authentication = tokenStore.readAuthenticationForRefreshToken(refreshToken);
if (this.authenticationManager != null && !authentication.isClientOnly()) {
// The client has already been authenticated, but the user authentication might be old now, so give it a
// chance to re-authenticate.
Authentication user = new PreAuthenticatedAuthenticationToken(authentication.getUserAuthentication(), "", authentication.getAuthorities());
user = authenticationManager.authenticate(user);
Object details = authentication.getDetails();
authentication = new OAuth2Authentication(authentication.getOAuth2Request(), user);
authentication.setDetails(details);
}
The user is being re-authenticated. Possibly something you may want to do in your custom implementation if need be.

Related

Spring boot Basic Authentication and OAuth2 in same project?

Is it possible to use OAuth2 for certain endpoints in my rest application and use basic authentication too for some other endpoints.
It should all work on spring security version 2.0.1.RELEASE. I hope someone can help me further.
Yes, it's possible to use a basic authentication as well as an OAuth2 authentication intertwined, but I doubt you'll be able to set it up easily as HttpSecurity's authenticated() method doesn't allow you to pick which of your authentication method (oauth2Login/formLogin) will work.
However, there's a way to easily bypass that:
You could add a custom authority, let's call it ROLE_BASICAUTH, when an user connects using basic auth, and ROLE_OAUTH2 when an user connects using OAuth2. That way, you can use
.antMatchers("/endpoint-that-requires-basic-auth").hasRole("BASICAUTH")
.antMatchers("/endpoint-that-requires-oauth2").hasRole("OAUTH2")
.anyRequest().authenticated()
When they reach an endpoint that you want basic authentication (and not OAuth2), you check their current authorities, and if it's not BASICAUTH, then you invalidate their session you display a login form without OAuth2 (to force them to use the basic authentication).
The downside to doing that is that you'd need to implement both a custom UserDetailsService as well as a custom OAuth2UserService...
But that's actually not that hard:
#Service
public class UserService extends DefaultOAuth2UserService implements UserDetailsService {
// ...
#Override
public OAuth2User loadUser(OAuth2UserRequest oAuth2UserRequest) throws OAuth2AuthenticationException {
OAuth2User user = super.loadUser(oAuth2UserRequest);
Map<String, Object> attributes = user.getAttributes();
Set<GrantedAuthority> authoritySet = new HashSet<>(user.getAuthorities());
String userNameAttributeName = oAuth2UserRequest.getClientRegistration().getProviderDetails()
.getUserInfoEndpoint().getUserNameAttributeName();
authoritySet.add(new SimpleGrantedAuthority("ROLE_OAUTH2"));
return new DefaultOAuth2User(authoritySet, attributes, userNameAttributeName);
}
#Override
public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
UserDetails user = getUserFromDatabase(username); // you'll need to provide that method (where are the username/password stored?)
if (user == null) { // UserDetailsService doesn't allow loadUserByUsername to return null, so throw exception
throw new UsernameNotFoundException("Couldn't find user with username '"+username+"'");
}
// add ROLE_BASICAUTH (you might need a custom UserDetails implementation here, because by defaut, UserDetails.getAuthorities() is immutable (I think, I might be a liar)
return user;
}
}
Note that this is a rough implementation, so you'll have to work it out a bit on your end as well.
You can also use this repository I made https://github.com/TwinProduction/spring-security-oauth2-client-example/tree/master/custom-userservice-sample as a guideline for the custom OAuth2UserService
Good luck.

spring security manual login best practice

I'm using spring security to implement a programmatic, manual user login. I have a scenario where I have positively established the user's identity, and wish to log them in. I don't know their password, and so can't use the regular login code path where you submit a form to a url, which spring intercepts via a servlet Filter, doing all of it's auth+session magic.
I've searched, and it seems most people create their own Authentication object, and then tell spring about via:
PreAuthenticatedAuthenticationToken authentication = new PreAuthenticatedAuthenticationToken(user, "", user.getAuthorities());
SecurityContextHolder.getContext().setAuthentication(authentication);
Indeed, this works. Spring even puts it into the session for me, making subsequent http requests maintain their auth status.
However, I feel like this is a dirty hack. I'll present some details that I hope will give concrete examples of the problems associated with using setAuthentication() inside a controller to achieve a manual login:
To give an idea, my config is:
httpSecurity
.authorizeRequests()
.antMatchers("/test/**").permitAll()
.antMatchers("/admin/**", "/api/admin/**").hasRole("USER_SUPER_ADMIN")
.and()
.formLogin()
.loginPage("/sign-in?sp")
.loginProcessingUrl("/api/auth/sign-in")
.successHandler(createLoginSuccessHandler())
.failureHandler(createLoginFailureHandler())
.permitAll()
.and()
.logout()
.logoutUrl("/api/auth/sign-out")
.logoutSuccessHandler(createLogoutSuccessHandler())
.and()
.sessionManagement()
.maximumSessions(1)
.maxSessionsPreventsLogin(true)
.sessionRegistry(sessionRegistry)
;
Key points in the above config:
I use custom success and failure handlers for the form login
I want to config behavior for max concurrent sessions per user
I want to maintain spring's default session fixation protection (changing session id upon login).
I want to use a session registry
... more of these session/login functionalities, had I chosen to config it.
I stepped through the code to see how spring processes a form login. As expected, Spring does all the session/login functionalities that my HttpSecurity config told it to do when I use the form login. But, when I do my own custom/manual login via SecurityContextHolder.getContext().setAuthentication(), it does NONE of those functionalities. This is because spring does all of it's session/login functionalities stuff inside of a servlet Filter, and my programmatic code can't really call a Filter. Now, I can attempt to add the missing functionalities myself, duplicating their code: I see that the spring Filter uses: ConcurrentSessionControlAuthenticationStrategy, ChangeSessionIdAuthenticationStrategy, and RegisterSessionAuthenticationStrategy. I can create these objects myself, configure them, and call them after my custom login. But, that's really lame to duplicate all that spring code. Furthermore, there's still other behaviors I'm missing - I noticed that when using the form login code path, that spring triggers some login events which don't get triggered when I do my custom login. And there's probably other stuff that I'm missing or don't understand. The whole process is pretty complicated, and I feel like there's a high chance of introducing bugs if this isn't done right, not to mention that library updates would be a pain if I started duplicating spring code.
So, I feel like I'm approaching this from the wrong way. Should I be using a different strategy, so that I'm not bypassing so much of the stuff that spring does for me? Maybe I should try to make my own AuthenticationProvider to accomplish this custom login?
*To clarify, my code more or less works. But, I feel like I accomplished it using a poor strategy because I had to write code duplicating a lot of stuff that spring does for me. Further, my code doesn't perfectly replicate what spring does, making me wonder what negative implications might result. There must be a better way to programatically achieve login.
I wanted to elaborate on how I implemented the advice of dur. In my scenario, I only used a custom AuthenticationProvider.
Instead of creating a custom servlet Filter, such as extending AbstractAuthenticationProcessingFilter, which seemed like a lot of work, I choose to instead use the following strategy:
At the point in my code where I was confident that I had identified the user, and wanted them to be "logged in", I stuck a flag in the user's session, marking that they should be logged in on the next request, along with any other identity/bookkeeping info I needed, such as their username.
Then, I told the browser client to make an http post to the loginProcessingUrl (the same one I configured spring security to use for form-based login), telling them to send the standard username and password form params, although they don't need to send real values - dummy values like foo are fine.
When the user makes that post request (eg to /login), spring will invoke my custom AuthenticationProvider, which will look in the user's session to check for the flag, and to gather the username. Then it will create and return an Authentication object, such as PreAuthenticatedAuthenticationToken, which identifies the user.
Spring will handle the rest. The user is now logged in.
By doing it this way, you stay within the "normal" way of doing logins, and so spring will still automatically:
Call any custom success and failure handlers you configured for the form login, which is nice if you use that place to do certain things on login, like query or update a db.
It will respect any max concurrent sessions per user settings that you may be using.
You get to keep spring's default session fixation attack protection (changing session id upon login).
If you set a custom session timeout, eg via server.session.timeout in a properties file, spring will use it. There's probably other session config attributes that are done at this time too.
If you enabled spring's "remember me" functionality, it will work.
It will fire a login event, which is used for other spring components, such as storing the user's session in a SessionRegistry. I think the events are also used by other parts of spring, such as the actuator, and for auditing.
When I first tried just doing the typically recommended SecurityContextHolder.getContext().setAuthentication(authentication) to login my user, instead of the custom AuthenticationProvider, none of the above bullets were done for me, which can utterly break your app... or cause subtle security bugs - neither are good.
Here's some code to help solidify what I said:
Custom AuthenticationProvider
#Component
public class AccountVerificationAuthenticationProvider implements AuthenticationProvider {
#Autowired
private AppAuthenticatedUserService appAuthenticatedUserService;
#Autowired
private AuthService authService;
#Override
public Authentication authenticate(Authentication authentication) throws AuthenticationException {
// This will look in the user's session to get their username, and to make sure the flag is set to allow login without password on this request.
UserAccount userAccount = authService.getUserAccountFromRecentAccountVerificationProcess();
if (userAccount == null) {
// Tell spring we can't process this AuthenticationProvider obj.
// Spring will continue, and try another AuthenticationProvider, if it can.
return null;
}
// A service to create a custom UserDetails object for this user.
UserDetails appAuthenticatedUser = appAuthenticatedUserService.create(userAccount.getEmail(), "", true);
PreAuthenticatedAuthenticationToken authenticationToken = new PreAuthenticatedAuthenticationToken(appAuthenticatedUser, "", appAuthenticatedUser.getAuthorities());
authenticationToken.setAuthenticated(true);
return authenticationToken;
}
#Override
public boolean supports(Class<?> authentication) {
return authentication.equals(UsernamePasswordAuthenticationToken.class);
}
}
Config spring security to use the provider
// In your WebSecurityConfigurerAdapter
#Configuration
#EnableWebSecurity
public class AppLoginConfig extends WebSecurityConfigurerAdapter {
#Autowired
private AccountVerificationAuthenticationProvider accountVerificationAuthenticationProvider;
#Autowired
private ActiveDirectoryLdapAuthenticationProvider activeDirectoryLdapAuthenticationProvider;
#Override
protected void configure(AuthenticationManagerBuilder authenticationManagerBuilder) throws Exception {
// Spring will try these auth providers in the order we register them.
// We do the accountVerificationAuthenticationProvider provider first, since it doesn't need to do any slow IO to check,
// so it's very fast. Only if this AuthenticationProvider rejects (which means this http request is not for programmatic login), will spring then try the next AuthenticationProvider in the list.
authenticationManagerBuilder
.authenticationProvider(accountVerificationAuthenticationProvider)
// I'm using ActiveDirectory / LDAP for when a user logs in via entering a user + password via the html form, but whatever you want to use here should work.
.authenticationProvider(activeDirectoryLdapAuthenticationProvider);
}
#Override
protected void configure(HttpSecurity httpSecurity) throws Exception {
...
}
}
For custom web authentication you should implement combination of a custom authentication filter (for example AbstractAuthenticationProcessingFilter or just GenericFilterBean), a custom authentication provider (AuthenticationProvider) or/and custom authentication token (AbstractAuthenticationToken).
For example, see source of Spring Security Kerberos.
See also:
The AuthenticationManager, ProviderManager and AuthenticationProvider

Dropwizard auth users

I was doing some research about drop-wizard security and authentication. Here is the link that I used http://howtodoinjava.com/dropwizard/dropwizard-basic-auth-security-example/.
My question is how to actually create new users, since VALID_USERS is a static final and can't be changed. I was thinking about creating a database, and that would consist of user object that contains the username and role ex. admin. (I don't need a password) But I am confused what I would return. In their example they returned Optional.of(new User(credentials.getUsername(), VALID_USERS.get(credentials.getUsername())));
Would I return a user object?
Essentially, I want to authenticate a user by the username and give them a role of authorization ex. admin, basic. But I guess I am confused how to generate a list of users and their roles. I was thinking of making a database, but I am not sure how exactly I would implement that.
public class AppBasicAuthenticator implements Authenticator<BasicCredentials, User>
{
private static final Map<String, Set<String>> VALID_USERS = ImmutableMap.of(
"guest", ImmutableSet.of(),
"user", ImmutableSet.of("USER"),
"admin", ImmutableSet.of("ADMIN", "USER")
);
#Override
public Optional<User> authenticate(BasicCredentials credentials) throws AuthenticationException
{
if (VALID_USERS.containsKey(credentials.getUsername()) && "password".equals(credentials.getPassword()))
{
return Optional.of(new User(credentials.getUsername(), VALID_USERS.get(credentials.getUsername())));
}
return Optional.empty();
}
}
in the latest version of DropWizard you can find it possible both to do Authentication and Authorization. The former, in a nutshell, instructs DropWizard to ask a user for credentials if you use basic authentication when she tries to access a resource or provide some other identity check. The latter allows one to grant a user access to various resources based on user's roles.
The are various possibilities how you can store user data and roles. Examples include a database which you mentioned, a LDAP server and a third-party identity management system.
If you are interested in Basic Authentication, you can take a look at my example here. A database is used to store user's credentials. Also, here is my little bit dated tutorial on DropWizard authentication. The code for the latest version is in the aforementioned example application.
To implement authentication only, you can skip adding roles and registering an Authorizer. To add authorization you can add a roles collection to your User entity and use annotations such as #RolesAllowed and #PermitAll along with Authorizer implementation to grant/deny access to your resources.
A link to DropWizard authentication docs is here.
Please feel free to ask more questions in comments and good luck.

User Authorization for Cloud Endpoints

I'm working on a cloud endpoints backend and want to restrict certain operations to admin users.
My current code works like this:
#ApiMethod(httpMethod = "PATCH", name = "item.update", path = "items")
public Item update(Item newObject, User user)
throws UnauthorizedException, OAuthRequestException {
OAuthService oAuthService = OAuthServiceFactory.getOAuthService();
if (!oAuthService.isUserAdmin()) {
throw new UnauthorizedException("Only admin users can modify content.");
}
...
}
I know app engine has a concept of user roles, but I'm curious if Endpoints
do. I've tried using the OAuthService.isUserAdmin() call but that doesn't
seem to be working out very well and the docs have a big old warning saying
Note: You should not confuse Endpoints auth with the auth for
non-Endpoints App Engine web apps described in the article on configuration settings
https://developers.google.com/appengine/articles/auth in the Admin
Console, where you also specify the user login requirement in your
web.xmlhttps://developers.google.com/appengine/docs/java/config/webxml#Security_and_Authentication
file. That approach is not used with Endpoints."
Do I have to create some sort of authorization myself that uses the User object that's passed into the update method? Any thoughts?
I had similar issues. Indeed OAuth user service has nothing to do with AppEngine user service. What I ended up doing was having a dedicated user type entity in my datastore where I store a specific flag (regular/admin) for each user. This flag is updated when I use AppEngine user service (i.e. so that the administrators I specified in the console get the proper admin flag).
In my endpoints API I get the current user authDomain and id, look up in my datastore to check whether it has the admin flag. The key of my user entity is composed of "authDomain:userId" and as I only support google user for now, it looks like (gmail.com:123456789)
This means that an administrator has to login once using the AppEngine UserService (i.e. a dedicated webpage in my case) so that the flag is properly updated
I needed to do the same thing and validate some endpoint to grant access only to admin members listed in the project console and used the same implementation presented above, but the oAuthService.isUserAdmin() accept one or more string parameters, this parameters are scopes that you specify and the Oauth uses to get user informations, in my case i just set this parameter and it works like the code bellow.
OAuthService authService = OAuthServiceFactory.getOAuthService();
User user;
try {
com.google.appengine.api.users.User currentUser =
authService.getCurrentUser(Constants.EMAIL_SCOPE);
if (currentUser != null && authService.isUserAdmin(Constants.EMAIL_SCOPE)) {
user = new User(currentUser.getEmail());
return user;
}
...
The EMAIL_SCOPE constant is defined by
public static final String EMAIL_SCOPE = "https://www.googleapis.com/auth/userinfo.email";
In my case i implemented an authenticator, to pass user information to endpoint only if it's admin user, you can read more about the authenticators if you want.
https://cloud.google.com/appengine/docs/java/endpoints/javadoc/com/google/api/server/spi/config/Authenticator

Spring security openId support and user deauthentication

I am trying to handle a situation when after an successful authentication with openId provider I discover that there is no account in my db associated with user openId identifier.
Can you tell me how should I handle the situation. Now, I am displaying register form and ask a user for creating an account. However, I have a problem with user authentication status, he is now being seen as authenticated by spring SecurityContext class.
How do I deauthenticate user in my controller action before redirecting to ''register new user page''? Is this approach a good one or should I do it in some other way?
Ok, so separating authentication from authorization as was mentioned in Samuel's post was really helpful. However there are still many gotchas and I found deauthentication still a must because there is no easy way in spring to add to user new roles. So the easiest way is to force user to login again and let spring handle role assignment during login.
In order to deauthenticate user in spring security you have to invoke:
SecurityContextHolder.clearContext();
as an alternative you can throw an exception in your UserDetailsService implementation (see below). It has the downside that you would deauthenticate user and lose user context data so it would be impossible to match new user accout with openid account during process of creating new local account. And you have to match those account after user login with traditional username and password. My solution was to deauthenticate user just after creating new account.
In order to grant user roles(privileges) you have to override UserDetailsService, in case someone find this useful here is my implementation:
public final class MyUserDetailsService implements UserDetailsService {
private final UsersDao usersDao;
#Autowired
public UserDetailsServiceImpl(final UsersDao usersDao) {
this.usersDao = usersDao;
}
#Override
public UserDetails loadUserByUsername(final String username) {
UserEntity user = usersDao.getUserByOpenIdIdentifier(username);
if (user == null) {
// there is no such user in our db, we could here throw
// an Exception instead then the user would also be deuthenticated
return new User(username, "", new ArrayList<GrantedAuthority>());
}
//here we are granting to users roles based on values from db
final Collection<GrantedAuthority> authorities = new ArrayList<GrantedAuthority>();
authorities.add(new SimpleGrantedAuthority(user.getUserType().toString()));
final UserDetails result = new User(username, "", authorities);
return result;
}
}
I think that you might be mixing two concepts: authentication and authorization. Authentication is knowing who the user is, authorization is the right to use access a resource of a feature.
In spring security, this two concepts are implemented by the authentication-manager and the access-decision-manager.
The fact that a user does not exist in your database is not a reason to deny him is identity: no deauthentication! But beeing authenticated can be a criterion in the access decision management. Example: the AuthenticatedVoter.
You should not touch at the authentication, but customize the access-decision-manager to apply the following rules:
A user who exists in your database has access to everything except account creation feature
A user who doesn't exist in your database has access only to the account creation feature.
This is all about access management, not authentication.
Read more at http://static.springsource.org/spring-security/site/docs/3.0.x/reference/ns-config.html#ns-access-manager
PS: The documentation is not exhaustive in spring security, but the source code is very readable. My advice is to check it out and look at the implementation of the elements you need to customize.

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