Authenticating Users by their Username and Password

An authentication provider will attempt to authenticate a user based on the credentials they provided. Usually these are a username and a password. Most web applications store their user's username and a hash of the user's password combined with a randomly generated salt. This means that the average authentication would consist of fetching the salt and the hashed password from the user data storage, hash the password the user has just provided (e.g. using a login form) with the salt and compare both to determine if the given password is valid.

This functionality is offered by the DaoAuthenticationProvider. It fetches the user's data from a UserProviderInterface, uses a PasswordEncoderInterface to create a hash of the password and returns an authenticated token if the password was valid:

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use Symfony\Component\Security\Core\Authentication\Provider\DaoAuthenticationProvider;
use Symfony\Component\Security\Core\User\UserChecker;
use Symfony\Component\Security\Core\User\InMemoryUserProvider;
use Symfony\Component\Security\Core\Encoder\EncoderFactory;

$userProvider = new InMemoryUserProvider(
    [
        'admin' => [
            // password is "foo"
            'password' => '5FZ2Z8QIkA7UTZ4BYkoC+GsReLf569mSKDsfods6LYQ8t+a8EW9oaircfMpmaLbPBh4FOBiiFyLfuZmTSUwzZg==',
            'roles'    => ['ROLE_ADMIN'],
        ],
    ]
);

// for some extra checks: is account enabled, locked, expired, etc.
$userChecker = new UserChecker();

// an array of password encoders (see below)
$encoderFactory = new EncoderFactory(...);

$daoProvider = new DaoAuthenticationProvider(
    $userProvider,
    $userChecker,
    'secured_area',
    $encoderFactory
);

$daoProvider->authenticate($unauthenticatedToken);

The Password Encoder Factory

The DaoAuthenticationProvider uses an encoder factory to create a password encoder for a given type of user. This allows you to use different encoding strategies for different types of users. The default EncoderFactory receives an array of encoders:

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use Acme\Entity\LegacyUser;
use Symfony\Component\Security\Core\Encoder\EncoderFactory;
use Symfony\Component\Security\Core\Encoder\MessageDigestPasswordEncoder;
use Symfony\Component\Security\Core\User\User;

$defaultEncoder = new MessageDigestPasswordEncoder('sha512', true, 5000);
$weakEncoder = new MessageDigestPasswordEncoder('md5', true, 1);

$encoders = [
    User::class       => $defaultEncoder,
    LegacyUser::class => $weakEncoder,
    // ...
];
$encoderFactory = new EncoderFactory($encoders);

Each encoder should implement PasswordEncoderInterface or be an array with a class and an arguments key, which allows the encoder factory to construct the encoder only when it is needed.

Creating a custom Password Encoder

There are many built-in password encoders. But if you need to create your own, it needs to follow these rules:

1. The class must implement PasswordEncoderInterface;

2. The implementations of encodePassword() and isPasswordValid() must first of all make sure the password is not too long, i.e. the password length is no longer than 4096 characters. This is for security reasons (see CVE-2013-5750), and you can use the isPasswordTooLong() method for this check:

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   use Symfony\Component\Security\Core\Encoder\BasePasswordEncoder;
   use Symfony\Component\Security\Core\Exception\BadCredentialsException;
   
   class FoobarEncoder extends BasePasswordEncoder
   {
       public function encodePassword($raw, $salt)
       {
           if ($this->isPasswordTooLong($raw)) {
               throw new BadCredentialsException('Invalid password.');
           }
   
           // ...
       }
   
       public function isPasswordValid($encoded, $raw, $salt)
       {
           if ($this->isPasswordTooLong($raw)) {
               return false;
           }
   
           // ...
       }
   }

Using Password Encoders

When the getEncoder() method of the password encoder factory is called with the user object as its first argument, it will return an encoder of type PasswordEncoderInterface which should be used to encode this user's password:

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// a Acme\Entity\LegacyUser instance
$user = ...;

// the password that was submitted, e.g. when registering
$plainPassword = ...;

$encoder = $encoderFactory->getEncoder($user);

// returns $weakEncoder (see above)
$encodedPassword = $encoder->encodePassword($plainPassword, $user->getSalt());

$user->setPassword($encodedPassword);

// ... save the user

Now, when you want to check if the submitted password (e.g. when trying to log in) is correct, you can use:

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// fetch the Acme\Entity\LegacyUser
$user = ...;

// the submitted password, e.g. from the login form
$plainPassword = ...;

$validPassword = $encoder->isPasswordValid(
    $user->getPassword(), // the encoded password
    $plainPassword,       // the submitted password
    $user->getSalt()
);

Authentication Success and Failure Events

When a provider authenticates the user, a security.authentication.success event is dispatched. But beware - this event will fire, for example, on every request if you have session-based authentication. See security.interactive_login below if you need to do something when a user actually logs in.

When a provider attempts authentication but fails (i.e. throws an AuthenticationException), a security.authentication.failure event is dispatched. You could listen on the security.authentication.failure event, for example, in order to log failed login attempts.

Security Events

The security.interactive_login event is triggered after a user has actively logged into your website. It is important to distinguish this action from non-interactive authentication methods, such as:

• authentication based on your session.
• authentication using a HTTP basic header.

You could listen on the security.interactive_login event, for example, in order to give your user a welcome flash message every time they log in.

The security.switch_user event is triggered every time you activate the switch_user firewall listener.