# Authentication in TurboGears 2 applications

This document describes how `repoze.who` is integrated into TurboGears and how you make get started with it. For more information, you may want to check `repoze.who`‘s website.

`repoze.who` is a powerful and extensible `authentication` package for arbitrary WSGI applications. By default TurboGears2 configures it to log using a form and retrieving the user information through the user_name field of the User class. This is made possible by the `authenticator plugin` that TurboGears2 uses by default which asks `base_config.sa_auth.authmetadata` to `authenticate` the user against given login and password.

## How it works in TurboGears

The authentication layer it’s a WSGI middleware which is able to authenticate the user through the method you want (e.g., LDAP or HTTP authentication), “remember” the user in future requests and log the user out.

You can customize the interaction with the user through four kinds of plugins, sorted by the order in which they are run on each request:

- An `identifier plugin`, with no action required on the user’s side, is able to tell whether it’s possible to authenticate the user (e.g., if it finds HTTP Authentication headers in the HTTP request). If so, it will extract the data required for the authentication (e.g., username and password, or a session cookie). There may be many identifiers and `repoze.who` will run each of them until one finds the required data to authenticate the user.
- If at least one of the identifiers could find data necessary to authenticate the current user, then an `authenticator plugin` will try to use the extracted data to authenticate the user. There may be many authenticators and `repoze.who` will run each of them until one authenticates the user.
- When the user tries to access a protected area or the login page, a `challenger plugin` will come up to request an action from the user (e.g., enter a user name and password and then submit the form). The user’s response will start another request on the application, which should be caught by an identifier to extract the login data and then such data will be used by the authenticator.
- For authenticated users, the `IdentityApplicationWrapper` provides the ability to load related data (e.g., real name, email) so that it can be easily used in the application. Such a functionality is provided by so-called `ApplicationAuthMetadata` in your `app_cfg.py`.

When the `IdentityApplicationWrapper` retrieves the user identity and its metadata it makes them available inside request as `request.identity`.

For example, to check whether the user has been authenticated you may use:

    # ...
    from tg import request
    # ...
    if request.identity:
        flash('You are authenticated!')

`request.identity` will equal to `None` if the user has not been authenticated.

Also the whole `repoze.who` authentication information are available in WSGI environment with `repoze.who.identity` key, which can be accessed using the code below:

    from tg import request

    # The authenticated user's data kept by repoze.who:
    who_identity = request.environ.get('repoze.who.identity')

The username will be available in `identity['repoze.who.userid']` (or `request.identity['repoze.who.userid']`, depending on the method you select).

### The FastFormPlugin

By default, TurboGears 2.3.6 configures `repoze.who` to use `tg.configuration.auth.fastform.FastFormPlugin` as the first identifier and challenger – using `/login` as the relative URL that will display the login form, `/login_handler` as the relative URL where the form will be sent and `/logout_handler` as the relative URL where the user will be logged out. The so-called rememberer of such identifier will be an instance of `repoze.who.plugins.cookie.AuthTktCookiePlugin`.

All these settings can be customized through the `config.app_cfg.base_config.sa_auth` options in your project. Identifiers, Authenticators and Challengers can be overridden providing a different list for each of them as:

    base_config.sa_auth['identifiers'] = [('myidentifier', myidentifier)]

You don’t have to use `repoze.who` directly either, unless you decide not to use it the way TurboGears configures it.

## Customizing authentication and authorization

It’s very easy for you to customize authentication and identification settings in `repoze.who` from `{yourproject}.config.app_cfg.base_config.sa_auth`.

### Customizing how user information, groups and permissions are retrieved

TurboGears provides an easy shortcut to customize how your authorization data is retrieved without having to face the complexity of the underlying authentication layer. This is performed by the `TGAuthMetadata` object which is configured in your project `config.app_cfg.base_config`.

This object provides three methods which have to return respectively the user, its groups and its permissions. You can freely change them as you wish as they are part of your own application behavior.

### Advanced Customizations

For more advanced customizations or to use repoze plugins to implement different forms of authentication you can freely customize the whole authentication layer using through the `{yourproject}.config.app_cfg.base_config.sa_auth` options.

The available directives are all optional:

- `form_plugin`: This is a replacement for the FriendlyForm plugin and will be

    always used as a challenger. If `form_identifies` option is True it will also be appended to the list of identifiers.

- `ìdentifiers`: A custom list of `repoze.who` identifiers.

    By default it contains the `form_plugin` and the `AuthTktCookiePlugin`.

- `challengers`: A custom list of `repoze.who` challengers.

    The `form_plugin` is always appended to this list, so if you have only one challenger you will want to change the `form_plugin` instead of overridding this list.

- `authmetadata`: This is the object that TG will use to fetch authorization metadata.

    Changing the authmetadata object you will be able to change how TurboGears fetches your user data, groups and permissions.

- `mdproviders`: This is a list of `repoze.who` metadata providers.

    Those usually to the same work that `authmetadata` does and in case a `repoze.who` metadata provider already provided identity metadata it will be available inside `identity` in `authmetadata` and can be used.

### Customizing the model structure assumed by the quickstart

Your auth-related model doesn’t have to be like the default one, where the class for your users, groups and permissions are, respectively, `User`, `Group` and `Permission`, and your users’ user name is available in `User.user_name`. What if you prefer `Member` and `Team` instead of `User` and `Group`, respectively?

First of all we need to inform the authentication layer that our user is stored in a different class. This makes `repoze.who` know where to look for the user to check its password:

    # what is the class you want to use to search for users in the database
    base_config.sa_auth.user_class = model.Member

Then we have to tell out `authmetadata` how to retrieve the user, its groups and permissions:

    from tg.configuration.auth import TGAuthMetadata

    #This tells to TurboGears how to retrieve the data for your user
    class ApplicationAuthMetadata(TGAuthMetadata):
        def __init__(self, sa_auth):
            self.sa_auth = sa_auth

        def authenticate(self, environ, identity):
            user = self.sa_auth.dbsession.query(self.sa_auth.user_class).filter_by(user_name=identity['login']).first()
            if user and user.validate_password(identity['password']):
                return identity['login']

        def get_user(self, identity, userid):
            return self.sa_auth.user_class.query.get(user_name=userid)

        def get_groups(self, identity, userid):
            return [team.team_name for team in identity['user'].teams]

        def get_permissions(self, identity, userid):
            return [p.permission_name for p in identity['user'].permissions]

    base_config.sa_auth.authmetadata = ApplicationAuthMetadata(base_config.sa_auth)

Now our application is able to fetch the user from the `Member` table and its groups from the `Team` table. Using `TGAuthMetadata` makes also possible to introduce a caching layer to avoid performing too many queries to fetch the authentication data for each request.

### BasicAuth Example

The following is an example of an advanced authentication stack customization to use browser basic authentication instead of form based authentication.

#### Declaring a Custom Authentication Backend

First required step is to declare that we are going to use a custom authentication backend:

    base_config.auth_backend = 'htpasswd'

When this is valued to `ming` or `sqlalchemy` TurboGears will configure a default authentication stack based on users stored on the according database, if `auth_backend` is `None` the whole stack will be disabled.

Then we must remove all the simple authentication options, deleting all the `basic_config.sa_auth` from `app_cfg.py` is usually enough. Leaving unexpected options behind (options our authentication stack doesn’t use) might lead to a crash on application startup.

#### Using HTPasswd file for users

Next step is storing our users inside an `htpasswd` file, this can be achieved by using the `HTPasswdPlugin` authenticator:

    from repoze.who.plugins.htpasswd import HTPasswdPlugin, plain_check
    base_config.sa_auth.authenticators = [('htpasswd', HTPasswdPlugin('./htpasswd', plain_check))]

This will make TurboGears load users from an htpasswd file inside the directory we are starting the application from. The `plain_check` function is the one used to decode password stored inside the htpasswd file. In this case passwords are expected to be in plain text in the form:

    manager:managepass

#### Challenging and Identifying users with BasicAuth

Now that we are correctly able to authenticate users from an htpasswd file, we need to use BasicAuth for identifying returning users:

    from repoze.who.plugins.basicauth import BasicAuthPlugin

    base_auth = BasicAuthPlugin('MyTGApp')
    base_config.sa_auth.identifiers = [('basicauth', base_auth)]

This will correctly identify users that are already logged using BasicAuth, but we are still sending users to login form to perform the actual login.

As BasicAuth requires the login to be performed through the browser we must disable the login form and set the basic auth plugin as a challenger:

    # Disable the login form, it won't work anyway as the credentials
    # for basic auth must be provided through the browser itself
    base_config.sa_auth.form_identifies = False

    # Use BasicAuth plugin to ask user for credentials, this will replace
    # the whole login form.
    base_config.sa_auth.challengers = [('basicauth', base_auth)]

#### Providing User Data

The previous steps are focused on providing a working authentication layer, but we will need to also identify the authenticated user so that also `request.identity` and the authorization layer can work as expected.

This is achieved through the `authmetadata` option, which tells TurboGears how to retrieve the user and it’s informations. In this case as we don’t have a database of users we will just provide a simple user with only `display_name` and `user_name` so that most things can work. For `manager` user we will also provide the `managers` group so that user can access the TurboGears admin:

    from tg.configuration.auth import TGAuthMetadata

    class ApplicationAuthMetadata(TGAuthMetadata):
        def __init__(self, sa_auth):
            self.sa_auth = sa_auth

        def get_user(self, identity, userid):
            # As we use htpasswd for authentication
            # we cannot lookup the user in a database,
            # so just return a fake user object
            from tg.util import Bunch
            return Bunch(display_name=userid, user_name=userid)

        def get_groups(self, identity, userid):
            # If the user is manager we give him the
            # managers group, otherwise no groups
            if userid == 'manager':
                return ['managers']
            else:
                return []

        def get_permissions(self, identity, userid):
            return []

    base_config.sa_auth.authmetadata = ApplicationAuthMetadata(base_config.sa_auth)

#### Removing Login Form

As the whole authentication is now performed through BasicAuth the login form is now unused, so probably want to remove the login form related urls which are now unused:

> - /login
> - /post_login
> - /post_logout

## Disabling authentication and authorization

If you need more flexibility than that provided by the quickstart, or you are not going to use `repoze.who`, you should prevent TurboGears from dealing with authentication/authorization by removing (or commenting) the following line from `{yourproject}.config.app_cfg`:

    base_config.auth_backend = '{whatever you find here}'

Then you may also want to delete those settings like `base_config.sa_auth.*` – they’ll be ignored.