Gracefully handle credit provider keys with unicode type

openedx/core/djangoapps/credit/signature.py

@@ -16,17 +16,30 @@ we receive from the credit provider.
 
 """
 
+import logging
 import hashlib
 import hmac
 
 from django.conf import settings
 
 
+log = logging.getLogger(__name__)
+
+
 def get_shared_secret_key(provider_id):
     """
     Retrieve the shared secret key for a particular credit provider.
     """
-    return getattr(settings, "CREDIT_PROVIDER_SECRET_KEYS", {}).get(provider_id)
+    secret = getattr(settings, "CREDIT_PROVIDER_SECRET_KEYS", {}).get(provider_id)
+
+    if isinstance(secret, unicode):
+        try:
+            secret = str(secret)
+        except UnicodeEncodeError:
+            secret = None
+            log.error(u'Shared secret key for credit provider "%s" contains non-ASCII unicode.', provider_id)
+
+    return secret
 
 
 def signature(params, shared_secret):
@@ -35,6 +48,7 @@ def signature(params, shared_secret):
 
     Arguments:
         params (dict): Parameters to sign.  Ignores the "signature" key if present.
+        shared_secret (str): The shared secret string.
 
     Returns:
         str: The 32-character signature.

openedx/core/djangoapps/credit/tests/test_signature.py

+"""
+Tests for digital signatures used to validate messages to/from credit providers.
+"""
+
+from django.test import TestCase
+from django.test.utils import override_settings
+
+
+from openedx.core.djangoapps.credit import signature
+
+
+class SignatureTest(TestCase):
+    """
+    Tests for digital signatures.
+    """
+
+    @override_settings(CREDIT_PROVIDER_SECRET_KEYS={
+        "asu": u'abcd1234'
+    })
+    def test_unicode_secret_key(self):
+        # Test a key that has type `unicode` but consists of ASCII characters
+        # (This can happen, for example, when loading the key from a JSON configuration file)
+        # When retrieving the shared secret, the type should be converted to `str`
+        key = signature.get_shared_secret_key("asu")
+        sig = signature.signature({}, key)
+        self.assertEqual(sig, "7d70a26b834d9881cc14466eceac8d39188fc5ef5ffad9ab281a8327c2c0d093")
+
+    @override_settings(CREDIT_PROVIDER_SECRET_KEYS={
+        "asu": u'\u4567'
+    })
+    def test_non_ascii_unicode_secret_key(self):
+        # Test a key that contains non-ASCII unicode characters
+        # This should return `None` and log an error; the caller
+        # is then responsible for logging the appropriate errors
+        # so we can fix the misconfiguration.
+        key = signature.get_shared_secret_key("asu")
+        self.assertIs(key, None)