Add body hash to LTI outcome message.

This is necessary to properly implement the LTI outcome passback
spec. It was not included previously because it was causing problems
with Canvas, but Blackboard will not accept outcomes unless they are
properly signed.

The requests_oauthlib doesn't support the body hash spec out of the
box, so BodyHashClient needed to be made. Fortunately, it's a pretty
simple spec:

https://oauth.googlecode.com/svn/spec/ext/body_hash/1.0/oauth-bodyhash.html

lms/djangoapps/lti_provider/outcomes.py

@@ -3,18 +3,39 @@ Helper functions for managing interactions with the LTI outcomes service defined
 in LTI v1.1.
 """
 
+from hashlib import sha1
+from base64 import b64encode
 import logging
+import uuid
+
 from lxml import etree
 from lxml.builder import ElementMaker
+from oauthlib.oauth1 import Client
+from oauthlib.common import to_unicode
 import requests
 import requests_oauthlib
-import uuid
 
 from lti_provider.models import GradedAssignment, OutcomeService
 
 log = logging.getLogger("edx.lti_provider")
 
 
+class BodyHashClient(Client):
+    """
+    OAuth1 Client that adds body hash support (required by LTI).
+
+    The default Client doesn't support body hashes, so we have to add it ourselves.
+    The spec:
+        https://oauth.googlecode.com/svn/spec/ext/body_hash/1.0/oauth-bodyhash.html
+    """
+    def get_oauth_params(self, request):
+        """Override get_oauth_params to add the body hash."""
+        params = super(BodyHashClient, self).get_oauth_params(request)
+        digest = b64encode(sha1(request.body.encode('UTF-8')).digest())
+        params.append((u'oauth_body_hash', to_unicode(digest)))
+        return params
+
+
 def store_outcome_parameters(request_params, user, lti_consumer):
     """
     Determine whether a set of LTI launch parameters contains information about
@@ -112,7 +133,13 @@ def sign_and_send_replace_result(assignment, xml):
     # message. Testing with Canvas throws an error when this field is included.
     # This code may need to be revisited once we test with other LMS platforms,
     # and confirm whether there's a bug in Canvas.
-    oauth = requests_oauthlib.OAuth1(consumer_key, consumer_secret)
+    oauth = requests_oauthlib.OAuth1(
+        consumer_key,
+        consumer_secret,
+        signature_method='HMAC-SHA1',
+        client_class=BodyHashClient,
+        force_include_body=True
+    )
 
     headers = {'content-type': 'application/xml'}
     response = requests.post(
@@ -121,6 +148,7 @@ def sign_and_send_replace_result(assignment, xml):
         auth=oauth,
         headers=headers
     )
+
     return response
 
 

lms/djangoapps/lti_provider/tests/test_outcomes.py

 """
 Tests for the LTI outcome service handlers, both in outcomes.py and in tasks.py
 """
+import unittest
 
 from django.test import TestCase
 from lxml import etree
 from mock import patch, MagicMock, ANY
+import requests_oauthlib
+import requests
+
+from opaque_keys.edx.locator import CourseLocator, BlockUsageLocator
 from student.tests.factories import UserFactory
 
 from lti_provider.models import GradedAssignment, LtiConsumer, OutcomeService
 import lti_provider.outcomes as outcomes
 import lti_provider.tasks as tasks
-from opaque_keys.edx.locator import CourseLocator, BlockUsageLocator
 
 
 class StoreOutcomeParametersTest(TestCase):
@@ -363,3 +367,44 @@ class XmlHandlingTest(TestCase):
             major_code='<imsx_codeMajor>failure</imsx_codeMajor>'
         )
         self.assertFalse(outcomes.check_replace_result_response(response))
+
+
+class TestBodyHashClient(unittest.TestCase):
+    """
+    Test our custom BodyHashClient
+
+    This Client should do everything a normal oauthlib.oauth1.Client would do,
+    except it also adds oauth_body_hash to the Authorization headers.
+    """
+    def test_simple_message(self):
+        oauth = requests_oauthlib.OAuth1(
+            '1000000000000000',  # fake consumer key
+            '2000000000000000',  # fake consumer secret
+            signature_method='HMAC-SHA1',
+            client_class=outcomes.BodyHashClient,
+            force_include_body=True
+        )
+        headers = {'content-type': 'application/xml'}
+        req = requests.Request(
+            'POST',
+            "http://example.edx.org/fake",
+            data="Hello world!",
+            auth=oauth,
+            headers=headers
+        )
+        prepped_req = req.prepare()
+
+        # Make sure that our body hash is now part of the test...
+        self.assertIn(
+            'oauth_body_hash="00hq6RNueFa8QiEjhep5cJRHWAI%3D"',
+            prepped_req.headers['Authorization']
+        )
+
+        # But make sure we haven't wiped out any of the other oauth values
+        # that we would expect to be in the Authorization header as well
+        expected_oauth_headers = [
+            "oauth_nonce", "oauth_timestamp", "oauth_version",
+            "oauth_signature_method", "oauth_consumer_key", "oauth_signature",
+        ]
+        for oauth_header in expected_oauth_headers:
+            self.assertIn(oauth_header, prepped_req.headers['Authorization'])