Compare node API keys in constant time on /upload

/upload compared the per-node API key with a plain !=, which short-circuits on
the first differing byte and so leaks a little information about how much of a
guess was correct.

The reason to fix it is less the timing channel itself -- an HTTP round trip is
noisy -- than the inconsistency: enrollment.py and dynsec.py both went out of
their way to use secrets.compare_digest for the same class of credential, so the
codebase contradicted itself on whether this mattered. Now it does not.

Also coalesces a missing api_key field to "" so compare_digest is never handed
None, which would raise TypeError and turn a malformed node_keys document into a
500 instead of a 401.

Closes logan/server-26#12

drb-c2-core/app/routers/upload.py

@@ -1,3 +1,4 @@
+import secrets
 from typing import Optional
 from datetime import datetime, timezone
 from fastapi import APIRouter, BackgroundTasks, UploadFile, File, Form, HTTPException, Security
@@ -36,7 +37,11 @@ async def upload_call_audio(
     if not key_doc:
         logger.warning(f"Upload 401: no key_doc in Firestore for node_id={node_id!r}")
         raise HTTPException(401, "Invalid node API key")
-    if key_doc.get("api_key") != credentials.credentials:
+    # compare_digest, not !=, so the comparison cost does not depend on how many
+    # leading characters matched. enrollment.py and dynsec.py were explicit about
+    # this for the same class of credential; this route was the odd one out.
+    stored_key = key_doc.get("api_key") or ""
+    if not secrets.compare_digest(stored_key, credentials.credentials):
         logger.warning(
             f"Upload 401: key mismatch for node_id={node_id!r} "
             f"(received prefix: {credentials.credentials[:8]}...)"