🚚(backend) create a dedicated authentication package

Prepare adding advanced authentication features. Create a dedicated
authentication Python package within the core app.

This code organization will be more extensible.

src/backend/core/authentication/backends.py

@@ -0,0 +1,100 @@
+"""Authentication Backends for the Impress core app."""
+
+from django.core.exceptions import SuspiciousOperation
+from django.utils.translation import gettext_lazy as _
+
+import requests
+from mozilla_django_oidc.auth import (
+    OIDCAuthenticationBackend as MozillaOIDCAuthenticationBackend,
+)
+
+from core.models import User
+
+
+class OIDCAuthenticationBackend(MozillaOIDCAuthenticationBackend):
+    """Custom OpenID Connect (OIDC) Authentication Backend.
+
+    This class overrides the default OIDC Authentication Backend to accommodate differences
+    in the User and Identity models, and handles signed and/or encrypted UserInfo response.
+    """
+
+    def get_userinfo(self, access_token, id_token, payload):
+        """Return user details dictionary.
+
+        Parameters:
+        - access_token (str): The access token.
+        - id_token (str): The id token (unused).
+        - payload (dict): The token payload (unused).
+
+        Note: The id_token and payload parameters are unused in this implementation,
+        but were kept to preserve base method signature.
+
+        Note: It handles signed and/or encrypted UserInfo Response. It is required by
+        Agent Connect, which follows the OIDC standard. It forces us to override the
+        base method, which deal with 'application/json' response.
+
+        Returns:
+        - dict: User details dictionary obtained from the OpenID Connect user endpoint.
+        """
+
+        user_response = requests.get(
+            self.OIDC_OP_USER_ENDPOINT,
+            headers={"Authorization": f"Bearer {access_token}"},
+            verify=self.get_settings("OIDC_VERIFY_SSL", True),
+            timeout=self.get_settings("OIDC_TIMEOUT", None),
+            proxies=self.get_settings("OIDC_PROXY", None),
+        )
+        user_response.raise_for_status()
+        userinfo = self.verify_token(user_response.text)
+        return userinfo
+
+    def get_or_create_user(self, access_token, id_token, payload):
+        """Return a User based on userinfo. Get or create a new user if no user matches the Sub.
+
+        Parameters:
+        - access_token (str): The access token.
+        - id_token (str): The ID token.
+        - payload (dict): The user payload.
+
+        Returns:
+        - User: An existing or newly created User instance.
+
+        Raises:
+        - Exception: Raised when user creation is not allowed and no existing user is found.
+        """
+
+        user_info = self.get_userinfo(access_token, id_token, payload)
+        sub = user_info.get("sub")
+
+        if sub is None:
+            raise SuspiciousOperation(
+                _("User info contained no recognizable user identification")
+            )
+
+        try:
+            user = User.objects.get(sub=sub)
+        except User.DoesNotExist:
+            if self.get_settings("OIDC_CREATE_USER", True):
+                user = self.create_user(user_info)
+            else:
+                user = None
+
+        return user
+
+    def create_user(self, claims):
+        """Return a newly created User instance."""
+
+        sub = claims.get("sub")
+
+        if sub is None:
+            raise SuspiciousOperation(
+                _("Claims contained no recognizable user identification")
+            )
+
+        user = User.objects.create(
+            sub=sub,
+            email=claims.get("email"),
+            password="!",  # noqa: S106
+        )
+
+        return user