Stop leaking share material to /tmp and to other users

Two findings from a full read of src/, both reproduced before fixing.

Extracted share archives were left on disk whenever anything went wrong. The
rmtree sat at the end of the loop body, so it was skipped both by the `raise`
on an invalid share and by the `continue` on an unreadable one. A single failed
decrypt left /tmp/fractum_share_* holding share_N.txt and, when the archive had
been built with --bundle-encrypted, the ciphertext beside it. Accumulate enough
of those and the leftovers alone are enough to reconstruct. Cleanup moves into
a finally.

Nothing set permissions on any output, so the process umask decided them: the
share archives, the .enc file and the reconstructed plaintext all landed as
-rw-r--r--, readable by every user on the machine. They are now created 0600.

Verified: a share with corrupt JSON now leaves zero temp directories, the three
outputs are -rw-------, the encrypt/decrypt round trip through the official
image with --network=none returns an identical SHA-256, and the suite is still
107 tests, 8/8 files.

Author: WhiteStarSupreme

src/cli/commands.py

@@ -580,10 +580,18 @@ def decrypt(
                 if verbose:
                     click.echo(f"Error processing {share_file}: {str(e)}")
                 continue
-
-            # Clean up temporary directory if it was an archive
-            if is_zip and temp_dir:
-                shutil.rmtree(temp_dir, ignore_errors=True)
+            finally:
+                # Quoi qu'il arrive, l'archive extraite ne reste pas sur le
+                # disque : elle contient la valeur de la part, et le fichier
+                # chiffre lui-meme si --bundle-encrypted a ete utilise.
+                if is_zip and temp_dir:
+                    shutil.rmtree(temp_dir, ignore_errors=True)
+
+            # Nettoyage deplace dans le finally ci-dessous : place ici, il etait
+            # saute par le `raise` d une part invalide comme par le `continue`
+            # d une part illisible, et le dossier restait sur le disque avec la
+            # valeur de la part extraite, et le fichier chiffre si l archive
+            # avait ete creee avec --bundle-encrypted.
 
         if verbose:
             if shares_by_set_id:

src/crypto/encryption.py

@@ -1,4 +1,5 @@
 import json
+import os
 import time
 from typing import Any, Dict, Optional, Union
 
@@ -85,6 +86,7 @@ def encrypt_file(
             ciphertext, tag = cipher.encrypt_and_digest(data)
 
             with open(output_path, "wb") as f:
+                os.chmod(output_path, 0o600)
                 f.write(len(metadata_bytes).to_bytes(4, "big"))
                 f.write(metadata_bytes)
                 f.write(cipher.nonce)
@@ -140,6 +142,9 @@ def decrypt_file(self, input_path: str, output_path: str) -> None:
                 raise ValueError("Decrypted data is empty")
 
             with open(output_path, "wb") as f:
+                # Le fichier reconstitue est le secret lui-meme : il ne doit pas
+                # heriter d un umask permissif.
+                os.chmod(output_path, 0o600)
                 f.write(data)
 
         except (ValueError, IOError) as e:

src/shares/archiver.py

@@ -116,6 +116,9 @@ def create_share_archive(
                 counter += 1
 
             with zipfile.ZipFile(archive_path, "w", zipfile.ZIP_DEFLATED) as zipf:
+                # L archive porte une part du secret : droits restreints des sa
+                # creation, avant meme d y ecrire quoi que ce soit.
+                os.chmod(archive_path, 0o600)
                 for root, _, files in os.walk(temp_dir):
                     for file in files:
                         file_path = Path(root) / file