fix Cryptgeon protocol and add CA passphrase / external config resolution

Cryptgeon's real protocol (verified against upstream occulto/frontend/backend
source) never matched what create_note() sent: it used a SHA-256-derived key
instead of the raw one, a single-blob ciphertext instead of the delimited
AES-GCM--nonce--ciphertext format, an empty meta field instead of a JSON
string, and a hash-bang URL instead of the real /note/<id>#<key> route -
notes uploaded fine but were undecryptable in the browser.

Also adds CA_PASSPHRASE auto-detection/prompting ("" auto-detects an
encrypted CA key and prompts, "!empty"/"!ask" opt out of/force the prompt)
and an external .conf file (--config / CONFIG_PATH / <script>.conf) that can
override the SETTINGS block without editing the script.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_cryptgeon.py

@@ -1,6 +1,8 @@
+import base64
 import json
 import pytest
 from unittest.mock import patch, MagicMock
+from cryptography.hazmat.primitives.ciphers.aead import AESGCM
 from openvpncertupdate import create_note, CryptgeonError
 
 
@@ -13,11 +15,13 @@ def mock_response(body: bytes):
 
 
 @patch("openvpncertupdate.urllib.request.urlopen")
-def test_returns_url_with_id(mock_open):
+def test_url_uses_path_based_note_route_with_hex_key_fragment(mock_open):
     mock_open.return_value = mock_response(b'{"id":"abc-123"}')
     url = create_note("secret", "https://cg.example.com")
-    assert "abc-123" in url
-    assert url.startswith("https://cg.example.com/#/note/abc-123/")
+    assert url.startswith("https://cg.example.com/note/abc-123#")
+    fragment = url.split("#", 1)[1]
+    assert len(fragment) == 64
+    bytes.fromhex(fragment)  # must be valid hex, not base64url
 
 
 @patch("openvpncertupdate.urllib.request.urlopen")
@@ -26,11 +30,29 @@ def test_posts_to_correct_endpoint(mock_open):
     create_note("hello", "https://cg.example.com")
     req = mock_open.call_args[0][0]
     assert req.full_url == "https://cg.example.com/api/notes/"
+
+
+@patch("openvpncertupdate.urllib.request.urlopen")
+def test_meta_is_json_string_describing_text_note(mock_open):
+    mock_open.return_value = mock_response(b'{"id":"x"}')
+    create_note("hello", "https://cg.example.com")
+    req = mock_open.call_args[0][0]
     body = json.loads(req.data)
-    assert isinstance(body["contents"], str)
-    assert body["meta"] == ""
+    assert json.loads(body["meta"]) == {"type": "text"}
     assert body["views"] == 1
-    assert body["type"] == "text"
+
+
+@patch("openvpncertupdate.urllib.request.urlopen")
+def test_contents_is_three_part_dash_delimited_base64(mock_open):
+    mock_open.return_value = mock_response(b'{"id":"x"}')
+    create_note("hello", "https://cg.example.com")
+    req = mock_open.call_args[0][0]
+    body = json.loads(req.data)
+    parts = body["contents"].split("--")
+    assert len(parts) == 3
+    alg, nonce, ciphertext = (base64.b64decode(p) for p in parts)
+    assert alg == b"AES-GCM"
+    assert len(nonce) == 12
 
 
 @patch("openvpncertupdate.urllib.request.urlopen")
@@ -42,8 +64,28 @@ def test_raises_on_http_error(mock_open):
 
 
 @patch("openvpncertupdate.urllib.request.urlopen")
-def test_different_passwords_give_different_urls(mock_open):
+def test_different_contents_give_different_urls(mock_open):
     mock_open.return_value = mock_response(b'{"id":"same"}')
     url1 = create_note("pass1", "https://cg.example.com")
     url2 = create_note("pass2", "https://cg.example.com")
     assert url1 != url2
+
+
+@patch("openvpncertupdate.urllib.request.urlopen")
+def test_decrypts_like_the_browser_client(mock_open):
+    """Reproduces the note/[id] view: key from the URL fragment must be the
+    exact key used to encrypt (no extra hashing step), or GCM auth fails."""
+    mock_open.return_value = mock_response(b'{"id":"abc"}')
+    url = create_note("my secret password", "https://cg.example.com")
+    req = mock_open.call_args[0][0]
+    body = json.loads(req.data)
+
+    key_hex = url.split("#", 1)[1]
+    key = bytes.fromhex(key_hex)
+
+    alg_b64, nonce_b64, ciphertext_b64 = body["contents"].split("--")
+    nonce = base64.b64decode(nonce_b64)
+    ciphertext = base64.b64decode(ciphertext_b64)
+
+    plaintext = AESGCM(key).decrypt(nonce, ciphertext, None)
+    assert plaintext == b"my secret password"