Extending the default Eloquent User Provider (Laravel 5.4+)

auth.php

<?php // config/auth.php

return [
    /* ... */

   /*
    |--------------------------------------------------------------------------
    | User Providers
    |--------------------------------------------------------------------------
    |
    | All authentication drivers have a user provider. This defines how the
    | users are actually retrieved out of your database or other storage
    | mechanisms used by this application to persist your user's data.
    |
    | If you have multiple user tables or models you may configure multiple
    | sources which represent each model / table. These sources may then
    | be assigned to any extra authentication guards you have defined.
    |
    | Supported: "database", "eloquent"
    |
    */
    'providers' => [
        'users' => [
            'driver' => 'custom',
            'model' => App\User::class,
        ],
    ],

    /* ... */
];

AuthServiceProvider.php

<?php

namespace App\Providers;

use App\Auth\UserProvider;
use Illuminate\Foundation\Support\Providers\AuthServiceProvider as ServiceProvider;

class AuthServiceProvider extends ServiceProvider
{
    /**
     * The policy mappings for the application.
     *
     * @var array
     */
    protected $policies = [
    ];

  /**
     * Register any authentication / authorization services.
     *
     * @return void
     */
    public function boot()
    {
        $this->registerPolicies();

        $this->app->auth->provider('custom', function ($app, array $config) {
            return new UserProvider($app['hash'], $config['model']);
        });
    }
}

LoginController.php

<?php // app/Http/Controllers/Auth/LoginController.php

namespace App\Http\Controllers\Auth;

use App\Http\Controllers\Controller;
use Illuminate\Foundation\Auth\AuthenticatesUsers;

class LoginController extends Controller
{
    /*
    |--------------------------------------------------------------------------
    | Login Controller
    |--------------------------------------------------------------------------
    |
    | This controller handles authenticating users for the application and
    | redirecting them to your home screen. The controller uses a trait
    | to conveniently provide its functionality to your applications.
    |
    */
    use AuthenticatesUsers;

    /**
     * Where to redirect users after login.
     *
     * @var string
     */
    protected $redirectTo = '/home';

    /**
     * Create a new controller instance.
     *
     * @return void
     */
    public function __construct()
    {
        $this->middleware('guest')->except('logout');
    }

    /**
     * Get the needed authorization credentials from the request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return array
     */
    protected function credentials(Request $request)
    {
        // Make changes
        return $request->only($this->username(), 'password');
    } 
}

UserProvider.php

<?php // app/Auth/UserProvider.php

namespace App\Auth;

use Illuminate\Support\Str;
use Illuminate\Auth\EloquentUserProvider;
use Illuminate\Contracts\Auth\UserProvider as UserProviderContract;

class UserProvider extends EloquentUserProvider implements UserProviderContract
{

    /**
     * Retrieve a user by the given credentials.
     *
     * @param  array  $credentials
     * @return \Illuminate\Contracts\Auth\Authenticatable|null
     */
    public function retrieveByCredentials(array $credentials)
    {
        // Make changes
        if (empty($credentials)) {
            return;
        }

        // First we will add each credential element to the query as a where clause.
        // Then we can execute the query and, if we found a user, return it in a
        // Eloquent User "model" that will be utilized by the Guard instances.
        $query = $this->createModel()->newQuery();

        foreach ($credentials as $key => $value) {
            if (! Str::contains($key, 'password')) {
                $query->where($key, $value);
            }
        }

        return $query->first();
    }
}