Merge pull request #151 from aleksihakli/master

Add signed file system storage

AUTHORS

@@ -11,5 +11,6 @@ Original code by `Novapost <http://www.novapost.fr>`_ team:
 * Gregory Tappero <https://github.com/coulix>
 * Rémy Hubscher <remy.hubscher@novapost.fr>
 * Benoît Bryon <benoit@marmelune.net>
+* Aleksi Häkli <https://github.com/aleksihakli>
 
 Developers: https://github.com/benoitbryon/django-downloadview/graphs/contributors

INSTALL

@@ -12,7 +12,7 @@ Install
 Requirements
 ************
 
-`django-downloadview` has been tested with `Python`_ 2.7, 3.3 and 3.4. Other
+`django-downloadview` has been tested with `Python`_ 3.6, 3.7 and 3.8. Other
 versions may work, but they are not part of the test suite at the moment.
 
 Installing `django-downloadview` will automatically trigger the installation of

django_downloadview/decorators.py

@@ -5,6 +5,12 @@ See also decorators provided by server-specific modules, such as
 
 """
 
+from functools import wraps
+
+from django.conf import settings
+from django.core.exceptions import PermissionDenied
+from django.core.signing import BadSignature, SignatureExpired, TimestampSigner
+
 
 class DownloadDecorator(object):
     """View decorator factory to apply middleware to ``view_func``'s response.
@@ -32,3 +38,39 @@ class DownloadDecorator(object):
             return middleware.process_response(request, response)
 
         return decorated
+
+
+def _signature_is_valid(request):
+    """
+    Validator that raises a PermissionDenied error on invalid and
+    mismatching signatures.
+    """
+
+    signer = TimestampSigner()
+    signature = request.GET.get("X-Signature")
+    expiration = getattr(settings, "DOWNLOADVIEW_URL_EXPIRATION", None)
+
+    try:
+        signature_path = signer.unsign(signature, max_age=expiration)
+    except SignatureExpired as e:
+        raise PermissionDenied("Signature expired") from e
+    except BadSignature as e:
+        raise PermissionDenied("Signature invalid") from e
+    except Exception as e:
+        raise PermissionDenied("Signature error") from e
+
+    if request.path != signature_path:
+        raise PermissionDenied("Signature mismatch")
+
+
+def signature_required(function):
+    """
+    Decorator that checks for X-Signature query parameter to authorize access to views.
+    """
+
+    @wraps(function)
+    def decorator(request, *args, **kwargs):
+        _signature_is_valid()
+        return function(request, *args, **kwargs)
+
+    return decorator

django_downloadview/storage.py

@@ -0,0 +1,22 @@
+from django.core.files.storage import FileSystemStorage
+from django.core.signing import TimestampSigner
+
+
+class SignedURLMixin:
+    """
+    Mixin for generating signed file URLs with compatible storage backends.
+
+    Adds X-Signature query parameters to the normal URLs generated by the storage class.
+    """
+
+    def url(self, name):
+        path = super(SignedURLMixin, self).url(name)
+        signer = TimestampSigner()
+        signature = signer.sign(path)
+        return "{}?X-Signature={}".format(path, signature)
+
+
+class SignedFileSystemStorage(SignedURLMixin, FileSystemStorage):
+    """
+    Specialized filesystem storage that signs file URLs for clients.
+    """

docs/settings.txt

@@ -29,6 +29,48 @@ Example:
    :end-before: END middlewares
 
 
+********************
+DEFAULT_FILE_STORAGE
+********************
+
+django-downloadview offers a built-in signed file storage, which cryptographically
+signs requested file URLs with the Django's built-in TimeStampSigner.
+
+To utilize the signed storage views you can configure
+
+.. code:: python
+
+   DEFAULT_FILE_STORAGE='django_downloadview.storage.SignedStorage'
+
+The signed file storage system inserts a ``X-Signature`` header to the requested file
+URLs, and they can then be verified with the supplied ``signature_required`` wrapper function:
+
+.. code:: python
+
+   from django.conf.urls import url, url_patterns
+
+   from django_downloadview import ObjectDownloadView
+   from django_downloadview.decorators import signature_required
+
+   from demoproject.download.models import Document  # A model with a FileField
+
+   # ObjectDownloadView inherits from django.views.generic.BaseDetailView.
+   download = ObjectDownloadView.as_view(model=Document, file_field='file')
+
+    urlpatterns = [
+        path('download/<str:slug>/', signature_required(download),
+    ]
+
+Make sure to test the desired functionality after configuration.
+
+***************************
+DOWNLOADVIEW_URL_EXPIRATION
+***************************
+
+Number of seconds signed download URLs are valid before expiring.
+
+Default value for this flag is None and URLs never expire.
+
 ********************
 DOWNLOADVIEW_BACKEND
 ********************
@@ -78,4 +120,4 @@ See :doc:`/optimizations/index` for details about builtin backends
 
 When ``django_downloadview.SmartDownloadMiddleware`` is in your
 ``MIDDLEWARE_CLASSES``, this setting must be explicitely configured (no default
-value). Else, you can ignore this setting.
+value). Else, you can ignore this setting.

setup.py

@@ -16,6 +16,7 @@ class Tox(TestCommand):
 
     def run_tests(self):
         import tox  # import here, cause outside the eggs aren't loaded.
+
         errno = tox.cmdline(self.test_args)
         sys.exit(errno)
 

tests/signature.py

@@ -0,0 +1,61 @@
+"""
+Test signature generation and validation.
+"""
+
+import unittest
+
+from django.core.exceptions import PermissionDenied
+from django.core.signing import TimestampSigner
+
+from django_downloadview.decorators import _signature_is_valid
+from django_downloadview.storage import SignedURLMixin
+
+
+class TestStorage:
+    def url(self, name):
+        return "https://example.com/{name}".format(name=name)
+
+
+class SignedTestStorage(SignedURLMixin, TestStorage):
+    pass
+
+
+class SignatureGeneratorTestCase(unittest.TestCase):
+    def test_signed_storage(self):
+        """
+        django_downloadview.storage.SignedURLMixin adds X-Signature to URLs.
+        """
+
+        storage = SignedTestStorage()
+        url = storage.url("test")
+        self.assertIn("https://example.com/test?X-Signature=", url)
+
+
+class SignatureValidatorTestCase(unittest.TestCase):
+    def test_verify_signature(self):
+        """
+        django_downloadview.decorators._signature_is_valid returns True on
+        valid signatures.
+        """
+
+        signer = TimestampSigner()
+        request = unittest.mock.MagicMock()
+
+        request.path = "test"
+        request.GET = {"X-Signature": signer.sign("test")}
+
+        self.assertIsNone(_signature_is_valid(request))
+
+    def test_verify_signature_invalid(self):
+        """
+        django_downloadview.decorators._signature_is_valid raises PermissionDenied
+        on invalid signatures.
+        """
+
+        request = unittest.mock.MagicMock()
+
+        request.path = "test"
+        request.GET = {"X-Signature": "not-valid"}
+
+        with self.assertRaises(PermissionDenied):
+            _signature_is_valid(request)