fix: More resilience when calling encrypt_for_log with missing key (#29878)

It's likely that someone will at some point enable encrypted logging but
forget to deploy the config change that sets the key; if this happens, we
should gracefully return a warning rather than raise an exception.

Along the same lines, make sure that safe-sessions won't raise an exception
if the setting is missing, and document the suggested use of getattr.

common/djangoapps/util/log_sensitive.py

@@ -10,7 +10,7 @@ Usage:
 
      logger.info(
          "Received invalid auth token %s in Authorization header",
-         encrypt_for_log(token, settings.<YOUR_DEBUG_PUBLIC_KEY>)
+         encrypt_for_log(token, getattr(settings, 'YOUR_DEBUG_PUBLIC_KEY', None))
      )
 
    This will log a message like::
@@ -69,7 +69,16 @@ def encrypt_for_log(message, reader_public_key_b64):
     Returns a string <sender public key> "|" <ciphertext> wrapped in
     some framing text "[encrypted: ...]"; the inner string can be
     decrypted with decrypt_log_message.
+
+    For ease of use, key may be None or empty; a warning message will be
+    returned instead of data, encrypted or otherwise.
     """
+    # If no key was configured, don't raise an error. This method is
+    # expected to be called from debugging code, so it shouldn't blow
+    # up on misconfiguration.
+    if not reader_public_key_b64:
+        return '[encryption failed, no key]'
+
     reader_public_key = PublicKey(b64decode(reader_public_key_b64))
 
     encrypted = Box(logger_private_key, reader_public_key).encrypt(message.encode())

common/djangoapps/util/tests/test_log_sensitive.py

@@ -3,9 +3,15 @@ Tests for util.logging
 """
 
 import re
+
 from common.djangoapps.util.log_sensitive import decrypt_log_message, encrypt_for_log, generate_reader_keys
 
 
+def test_encryption_no_key():
+    to_log = encrypt_for_log("Testing testing 1234", None)
+    assert to_log == '[encryption failed, no key]'
+
+
 def test_encryption_round_trip():
     reader_keys = generate_reader_keys()
     reader_public_64 = reader_keys['public']

openedx/core/djangoapps/safe_sessions/middleware.py

@@ -765,7 +765,7 @@ class SafeSessionMiddleware(SessionMiddleware, MiddlewareMixin):
         """
         # NOTE: request.headers seems to pick up initial values, but won't adjust as the request object is edited.
         #   For example, the session cookie will likely be the safe session version.
-        return encrypt_for_log(str(request.headers), settings.SAFE_SESSIONS_DEBUG_PUBLIC_KEY)
+        return encrypt_for_log(str(request.headers), getattr(settings, 'SAFE_SESSIONS_DEBUG_PUBLIC_KEY', None))
 
 
 def obscure_token(value: Union[str, None]) -> Union[str, None]: