[LoginLink] Add technical details

Author: alexandre-daubois

security/login_link.rst

@@ -111,9 +111,9 @@ intercept requests to this route:
                 throw new \LogicException('This code should never be reached');
             }
         }
-        
+
     .. code-block:: php-attributes
-    
+
         // src/Controller/SecurityController.php
         namespace App\Controller;
 
@@ -428,6 +428,12 @@ The signed URL contains 3 parameters:
     properties. Whenever these change, the hash changes and previous login
     links are invalidated.
 
+For a user that returns ``user@example.com`` on ``$user->getUserIdentifier()``
+call, the generated login link looks like this:
+
+.. code-block:: text
+    http://example.com/login_check?user=user@example.com&expires=1675707377&hash=f0Jbda56Y...A5sUCI~TQF701fwJ...7m2n4A~
+
 You can add more properties to the ``hash`` by using the
 ``signature_properties`` option:
 
@@ -673,6 +679,23 @@ user create this POST request (e.g. by clicking a button)::
         </form>
     {% endblock %}
 
+Hashing Strategy
+~~~~~~~~~~~~~~~~
+
+Internally, the :class:`Symfony\\Component\\Security\\Http\\LoginLink\\LoginLinkHandler`
+implementation uses the
+:class:`Symfony\\Component\\Security\\Core\\Signature\\SignatureHasher` to create the
+hash contained in the login link.
+
+This hasher creates a first hash with the expiration
+date of the link, the values of the configured signature properties and the
+user identifier. The used hashing algorithm is SHA-256.
+
+Once this first hash is processed and encoded in Base64, a new one is created
+from the first hash value and the ``kernel.secret`` container parameter. This
+allows Symfony to sign this final hash, which is contained in the login URL.
+The final hash is also a Base64 encoded SHA-256 hash.
+
 Customizing the Success Handler
 -------------------------------