diff --git a/src/allmydata/test/web/test_webish.py b/src/allmydata/test/web/test_webish.py
index 4a77d21ae..050f77d1c 100644
--- a/src/allmydata/test/web/test_webish.py
+++ b/src/allmydata/test/web/test_webish.py
@@ -202,6 +202,16 @@ class TahoeLAFSSiteTests(SyncTestCase):
             ),
         )
 
+    def test_private_key_censoring(self):
+        """
+        The log event for a request including a **private-key** query
+        argument has the private key value censored.
+        """
+        self._test_censoring(
+            b"/uri?uri=URI:CHK:aaa:bbb&private-key=AAAAaaaabbbb==",
+            b"/uri?uri=[CENSORED]&private-key=[CENSORED]",
+        )
+
     def test_uri_censoring(self):
         """
         The log event for a request for **/uri/<CAP>** has the capability value
diff --git a/src/allmydata/webish.py b/src/allmydata/webish.py
index 519b3e1f0..1b2b8192a 100644
--- a/src/allmydata/webish.py
+++ b/src/allmydata/webish.py
@@ -1,18 +1,12 @@
 """
-Ported to Python 3.
+General web server-related utilities.
 """
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-from __future__ import unicode_literals
-
-from future.utils import PY2
-if PY2:
-    from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
+from __future__ import annotations
 
 from six import ensure_str
 
 import re, time, tempfile
+from urllib.parse import parse_qsl, urlencode
 
 from cgi import (
     FieldStorage,
@@ -45,40 +39,37 @@ from .web.storage_plugins import (
 )
 
 
-if PY2:
-    FileUploadFieldStorage = FieldStorage
-else:
-    class FileUploadFieldStorage(FieldStorage):
-        """
-        Do terrible things to ensure files are still bytes.
+class FileUploadFieldStorage(FieldStorage):
+    """
+    Do terrible things to ensure files are still bytes.
 
-        On Python 2, uploaded files were always bytes.  On Python 3, there's a
-        heuristic: if the filename is set on a field, it's assumed to be a file
-        upload and therefore bytes.  If no filename is set, it's Unicode.
+    On Python 2, uploaded files were always bytes.  On Python 3, there's a
+    heuristic: if the filename is set on a field, it's assumed to be a file
+    upload and therefore bytes.  If no filename is set, it's Unicode.
 
-        Unfortunately, we always want it to be bytes, and Tahoe-LAFS also
-        enables setting the filename not via the MIME filename, but via a
-        separate field called "name".
+    Unfortunately, we always want it to be bytes, and Tahoe-LAFS also
+    enables setting the filename not via the MIME filename, but via a
+    separate field called "name".
 
-        Thus we need to do this ridiculous workaround.  Mypy doesn't like it
-        either, thus the ``# type: ignore`` below.
+    Thus we need to do this ridiculous workaround.  Mypy doesn't like it
+    either, thus the ``# type: ignore`` below.
 
-        Source for idea:
-        https://mail.python.org/pipermail/python-dev/2017-February/147402.html
-        """
-        @property  # type: ignore
-        def filename(self):
-            if self.name == "file" and not self._mime_filename:
-                # We use the file field to upload files, see directory.py's
-                # _POST_upload. Lack of _mime_filename means we need to trick
-                # FieldStorage into thinking there is a filename so it'll
-                # return bytes.
-                return "unknown-filename"
-            return self._mime_filename
+    Source for idea:
+    https://mail.python.org/pipermail/python-dev/2017-February/147402.html
+    """
+    @property  # type: ignore
+    def filename(self):
+        if self.name == "file" and not self._mime_filename:
+            # We use the file field to upload files, see directory.py's
+            # _POST_upload. Lack of _mime_filename means we need to trick
+            # FieldStorage into thinking there is a filename so it'll
+            # return bytes.
+            return "unknown-filename"
+        return self._mime_filename
 
-        @filename.setter
-        def filename(self, value):
-            self._mime_filename = value
+    @filename.setter
+    def filename(self, value):
+        self._mime_filename = value
 
 
 class TahoeLAFSRequest(Request, object):
@@ -180,12 +171,7 @@ def _logFormatter(logDateTime, request):
         queryargs = b""
     else:
         path, queryargs = x
-        # there is a form handler which redirects POST /uri?uri=FOO into
-        # GET /uri/FOO so folks can paste in non-HTTP-prefixed uris. Make
-        # sure we censor these too.
-        if queryargs.startswith(b"uri="):
-            queryargs = b"uri=[CENSORED]"
-        queryargs = b"?" + queryargs
+        queryargs = b"?" + censor(queryargs)
     if path.startswith(b"/uri/"):
         path = b"/uri/[CENSORED]"
     elif path.startswith(b"/file/"):
@@ -207,6 +193,30 @@ def _logFormatter(logDateTime, request):
     )
 
 
+def censor(queryargs: bytes) -> bytes:
+    """
+    Replace potentially sensitive values in query arguments with a
+    constant string.
+    """
+    args = parse_qsl(queryargs.decode("ascii"), keep_blank_values=True, encoding="utf8")
+    result = []
+    for k, v in args:
+        if k == "uri":
+            # there is a form handler which redirects POST /uri?uri=FOO into
+            # GET /uri/FOO so folks can paste in non-HTTP-prefixed uris. Make
+            # sure we censor these.
+            v = "[CENSORED]"
+        elif k == "private-key":
+            # Likewise, sometimes a private key is supplied with mutable
+            # creation.
+            v = "[CENSORED]"
+
+        result.append((k, v))
+
+    # Customize safe to try to leave our markers intact.
+    return urlencode(result, safe="[]").encode("ascii")
+
+
 class TahoeLAFSSite(Site, object):
     """
     The HTTP protocol factory used by Tahoe-LAFS.