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_easyrsa.py

@@ -2,6 +2,7 @@ import pytest
 from unittest.mock import patch, MagicMock
 from openvpncertupdate import (
     revoke_issued, build_client_full, gen_crl, copy_crl, EasyRSAError,
+    is_ca_key_encrypted, resolve_ca_passphrase,
 )
 
 
@@ -65,3 +66,75 @@ def test_copy_crl_copies_and_chmods(mock_chmod, mock_copy):
     copy_crl("/pki", "/etc/openvpn/crl.pem")
     mock_copy.assert_called_once_with("/pki/crl.pem", "/etc/openvpn/crl.pem")
     mock_chmod.assert_called_once_with("/etc/openvpn/crl.pem", 0o644)
+
+
+# ---------------------------------------------------------------------------
+# is_ca_key_encrypted
+# ---------------------------------------------------------------------------
+
+def _write_ca_key(pki_dir, content):
+    private = pki_dir / "private"
+    private.mkdir(parents=True, exist_ok=True)
+    (private / "ca.key").write_text(content)
+
+
+def test_is_ca_key_encrypted_true_for_pkcs8_encrypted_header(tmp_path):
+    _write_ca_key(tmp_path, "-----BEGIN ENCRYPTED PRIVATE KEY-----\nAAA\n-----END ENCRYPTED PRIVATE KEY-----\n")
+    assert is_ca_key_encrypted(str(tmp_path)) is True
+
+
+def test_is_ca_key_encrypted_true_for_legacy_proc_type_header(tmp_path):
+    _write_ca_key(tmp_path, "-----BEGIN RSA PRIVATE KEY-----\nProc-Type: 4,ENCRYPTED\nDEK-Info: AES-256-CBC,...\n\nAAA\n-----END RSA PRIVATE KEY-----\n")
+    assert is_ca_key_encrypted(str(tmp_path)) is True
+
+
+def test_is_ca_key_encrypted_false_for_plain_key(tmp_path):
+    _write_ca_key(tmp_path, "-----BEGIN PRIVATE KEY-----\nAAA\n-----END PRIVATE KEY-----\n")
+    assert is_ca_key_encrypted(str(tmp_path)) is False
+
+
+def test_is_ca_key_encrypted_false_when_key_missing(tmp_path):
+    assert is_ca_key_encrypted(str(tmp_path)) is False
+
+
+# ---------------------------------------------------------------------------
+# resolve_ca_passphrase
+# ---------------------------------------------------------------------------
+
+def test_resolve_empty_string_prompts_when_ca_is_encrypted(tmp_path):
+    _write_ca_key(tmp_path, "-----BEGIN ENCRYPTED PRIVATE KEY-----\nAAA\n-----END ENCRYPTED PRIVATE KEY-----\n")
+    prompt = MagicMock(return_value="typed-pass")
+    result = resolve_ca_passphrase("", str(tmp_path), prompt=prompt)
+    assert result == "typed-pass"
+    prompt.assert_called_once()
+
+
+def test_resolve_empty_string_skips_prompt_when_ca_not_encrypted(tmp_path):
+    _write_ca_key(tmp_path, "-----BEGIN PRIVATE KEY-----\nAAA\n-----END PRIVATE KEY-----\n")
+    prompt = MagicMock()
+    result = resolve_ca_passphrase("", str(tmp_path), prompt=prompt)
+    assert result == ""
+    prompt.assert_not_called()
+
+
+def test_resolve_empty_sentinel_never_checks_or_prompts(tmp_path):
+    _write_ca_key(tmp_path, "-----BEGIN ENCRYPTED PRIVATE KEY-----\nAAA\n-----END ENCRYPTED PRIVATE KEY-----\n")
+    prompt = MagicMock()
+    result = resolve_ca_passphrase("!empty", str(tmp_path), prompt=prompt)
+    assert result == ""
+    prompt.assert_not_called()
+
+
+def test_resolve_ask_sentinel_always_prompts_without_checking(tmp_path):
+    # no ca.key on disk at all -- must not be checked/read for "!ask"
+    prompt = MagicMock(return_value="typed-pass")
+    result = resolve_ca_passphrase("!ask", str(tmp_path), prompt=prompt)
+    assert result == "typed-pass"
+    prompt.assert_called_once()
+
+
+def test_resolve_literal_passphrase_passed_through_unchanged(tmp_path):
+    prompt = MagicMock()
+    result = resolve_ca_passphrase("mysecret", str(tmp_path), prompt=prompt)
+    assert result == "mysecret"
+    prompt.assert_not_called()