Introduce user identifier normalization for enhanced consistency

Added support for normalizing user identifiers in Symfony 7.3 to ensure consistent comparison and storage. Updated documentation with implementation examples, highlighting best practices like converting identifiers to lowercase and handling various formats. This improvement reduces the risk of duplicates and improves reliability in user authentication.

Author: Spomky

security/custom_authenticator.rst

@@ -209,13 +209,20 @@ requires a user and some sort of "credentials" (e.g. a password).
 Use the
 :class:`Symfony\\Component\\Security\\Http\\Authenticator\\Passport\\Badge\\UserBadge`
 to attach the user to the passport. The ``UserBadge`` requires a user
-identifier (e.g. the username or email), which is used to load the user
-using :ref:`the user provider <security-user-providers>`::
+identifier (e.g. the username or email)::
 
     use Symfony\Component\Security\Http\Authenticator\Passport\Badge\UserBadge;
 
     // ...
-    $passport = new Passport(new UserBadge($email), $credentials);
+    $passport = new Passport(new UserBadge($userIdentifier), $credentials);
+
+User Identifier
+~~~~~~~~~~~~~~~
+
+The user identifier is a unique string that identifies the user. It is used
+to load the user using :ref:`the user provider <security-user-providers>`.
+This identifier is often something like the user's email address or username,
+but it could be any unique value associated with the user.
 
 .. note::
 
@@ -255,6 +262,91 @@ using :ref:`the user provider <security-user-providers>`::
             }
         }
 
+It is a good practice to normalize the user identifier before using it.
+For example, this ensures that variations such as "john.doe", "John.Doe",
+or "JOHN.DOE" refer to the same user.
+Normalization can include converting the identifier to lowercase
+and trimming unnecessary spaces.
+You can optionally pass a user identifier normalizer as third argument to the
+``UserBadge``. This callable receives the ``$userIdentifier``
+and must return a normalized user identifier as a string.
+
+.. versionadded:: 7.3
+
+    The support of the user identifier normalizer was introduced in Symfony 7.3.
+
+For instance, the example below uses a normalizer that converts usernames to a normalized, ASCII-only, lowercase format,
+suitable for consistent comparison and storage.
+
+    // src/Security/NormalizedUserBadge.php
+    namespace App\Security;
+
+    use Symfony\Component\Security\Http\Authenticator\Passport\Badge\UserBadge;
+    use Symfony\Component\String\UnicodeString;
+    use function Symfony\Component\String\u;
+
+    final class NormalizedUserBadge extends UserBadge
+    {
+        public function __construct(string $identifier)
+        {
+            $callback = static fn (string $identifier) => u($identifier)->normalize(UnicodeString::NFKC)->ascii()->lower()->toString();
+
+            parent::__construct($identifier, null, $callback);
+        }
+    }
+
+    // src/Security/PasswordAuthenticator.php
+    namespace App\Security;
+
+    final class PasswordAuthenticator extends AbstractLoginFormAuthenticator
+    {
+        // Simplified for brievety
+        public function authenticate(Request $request): Passport
+        {
+            $username = (string) $request->request->get('username', '');
+            $password = (string) $request->request->get('password', '');
+
+            $request->getSession()
+                ->set(SecurityRequestAttributes::LAST_USERNAME, $username);
+
+            return new Passport(
+                new NormalizedUserBadge($username),
+                new PasswordCredentials($password),
+                [
+                    //All other useful badges
+                ]
+            );
+        }
+    }
+
+.. note::
+
+    For example, Google treats the following email addresses as equivalent:
+    ``john.doe@gmail.com``, ``j.hon.d.oe@gmail.com``, and ``johndoe@gmail.com``.
+    This is because Google applies normalization rules that remove dots
+    and convert the address to lowercase (though behavior varies across services).
+
+.. note::
+
+    In enterprise environments, a user may authenticate using different formats
+    of their identifier, such as:
+
+    - ``john.doe@acme.com``
+    - ``acme.com\jdoe``
+    - ``https://acme.com/+jdoe``
+    - ``acct:jdoe@acme.com``
+
+    Applying normalization (e.g., trimming, lowercasing, or format unification)
+    helps ensure consistent identity recognition across systems and prevents
+    duplicates caused by format variations.
+
+User Credential
+~~~~~~~~~~~~~~~
+
+The user credential is used to authenticate the user i.e. to verify
+the validity of the provided information (such as a password, an API token,
+or other custom credentials).
+
 The following credential classes are supported by default:
 
 :class:`Symfony\\Component\\Security\\Http\\Authenticator\\Passport\\Credentials\\PasswordCredentials`