Replace subject with [...] for outgoing mail

`authclean` cleanup server is used by
reinjecting smtpd running on localhost:10025 by default.
It runs after filtermail
and currently removes `Received` header
to avoid leaking IP address.
Can as well be used to replace `Subject` lines
with `Subject: [...]`.
If there are multiple `Subject` lines,
all of them should be replaced.

This allows us to avoid dealing with
localized subjects, including SecureJoin
messages `vc-request` and `vg-request`
which can have Subject lines like
Subject: =?utf-8?q?Nachricht_von_nrn178fi4=40nine=2Etestrun=2Eorg?=

CHANGELOG.md

@@ -17,6 +17,9 @@
 - remove MTA-STS daemon
   ([#488](https://github.com/deltachat/chatmail/pull/488))
 
+- replace `Subject` with `[...]` for all outgoing mails.
+  ([#481](https://github.com/deltachat/chatmail/pull/481))
+
 ## 1.5.0 2024-12-20
 
 - cmdeploy dns: always show recommended DNS records

chatmaild/src/chatmaild/common_encrypted_subjects.py

@@ -1,59 +0,0 @@
-"""Generated from deltachat, draft-ietf-lamps-header-protection, and
-encrypted_subject localizations in
-https://github.com/thunderbird/thunderbird-android/
-
-"""
-
-common_encrypted_subjects = {
-    "...",
-    "[...]",
-    "암호화된 메시지",
-    "Ĉifrita mesaĝo",
-    "Courriel chiffré",
-    "Dulrituð skilaboð",
-    "Encrypted Message",
-    "Fersifere berjocht",
-    "Kemennadenn enrineget",
-    "Krüptitud kiri",
-    "Krypterat meddelande",
-    "Krypteret besked",
-    "Kryptert melding",
-    "Mensagem criptografada",
-    "Mensagem encriptada",
-    "Mensaje cifrado",
-    "Mensaxe cifrada",
-    "Mesaj Criptat",
-    "Mesazh i Fshehtëzuar",
-    "Messaggio criptato",
-    "Messaghju cifratu",
-    "Missatge encriptat",
-    "Neges wedi'i Hamgryptio",
-    "Pesan terenkripsi",
-    "Salattu viesti",
-    "Şifreli İleti",
-    "Šifrēta ziņa",
-    "Šifrirana poruka",
-    "Šifrirano sporočilo",
-    "Šifruotas laiškas",
-    "Tin nhắn được mã hóa",
-    "Titkosított üzenet",
-    "Verschlüsselte Nachricht",
-    "Versleuteld bericht",
-    "Zašifrovaná zpráva",
-    "Zaszyfrowana wiadomość",
-    "Zifratu mezua",
-    "Κρυπτογραφημένο μήνυμα",
-    "Зашифроване повідомлення",
-    "Зашифрованное сообщение",
-    "Зашыфраваны ліст",
-    "Криптирано съобщение",
-    "Шифрована порука",
-    "დაშიფრული წერილი",
-    "הודעה מוצפנת",
-    "پیام رمزنگاری‌شده",
-    "رسالة مشفّرة",
-    "എൻക്രിപ്റ്റുചെയ്‌ത സന്ദേശം",
-    "加密邮件",
-    "已加密的訊息",
-    "暗号化されたメッセージ",
-}

chatmaild/src/chatmaild/filtermail.py

@@ -12,7 +12,6 @@ from smtplib import SMTP as SMTPClient
 
 from aiosmtpd.controller import Controller
 
-from .common_encrypted_subjects import common_encrypted_subjects
 from .config import read_config
 
 
@@ -128,8 +127,6 @@ def check_encrypted(message):
     """
     if not message.is_multipart():
         return False
-    if message.get("subject") not in common_encrypted_subjects:
-        return False
     if message.get_content_type() != "multipart/encrypted":
         return False
     parts_count = 0

chatmaild/src/chatmaild/tests/plugin.py

@@ -69,7 +69,7 @@ def maildata(request):
 
     assert datadir.exists(), datadir
 
-    def maildata(name, from_addr, to_addr, subject="..."):
+    def maildata(name, from_addr, to_addr, subject="[...]"):
         # Using `.read_bytes().decode()` instead of `.read_text()` to preserve newlines.
         data = datadir.joinpath(name).read_bytes().decode()
 

chatmaild/src/chatmaild/tests/test_filtermail.py

@@ -5,7 +5,6 @@ from chatmaild.filtermail import (
     SendRateLimiter,
     check_armored_payload,
     check_encrypted,
-    common_encrypted_subjects,
     is_securejoin,
 )
 
@@ -71,18 +70,13 @@ def test_filtermail_securejoin_detection(maildata):
 
 
 def test_filtermail_encryption_detection(maildata):
-    for subject in common_encrypted_subjects:
-        msg = maildata(
-            "encrypted.eml",
-            from_addr="1@example.org",
-            to_addr="2@example.org",
-            subject=subject,
-        )
-        assert check_encrypted(msg)
-
-    # if the subject is not a known encrypted subject value, it is not considered ac-encrypted
-    msg.replace_header("Subject", "Click this link")
-    assert not check_encrypted(msg)
+    msg = maildata(
+        "encrypted.eml",
+        from_addr="1@example.org",
+        to_addr="2@example.org",
+        subject="Subject does not matter, will be replaced anyway",
+    )
+    assert check_encrypted(msg)
 
 
 def test_filtermail_no_literal_packets(maildata):

cmdeploy/src/cmdeploy/postfix/submission_header_cleanup

@@ -2,3 +2,4 @@
 /^X-Originating-IP:/    IGNORE
 /^X-Mailer:/            IGNORE
 /^User-Agent:/          IGNORE
+/^Subject:/             REPLACE Subject: [...]

cmdeploy/src/cmdeploy/tests/online/test_1_basic.py

@@ -115,6 +115,27 @@ def test_reject_missing_dkim(cmsetup, maildata, from_addr):
             s.sendmail(from_addr=from_addr, to_addrs=recipient.addr, msg=msg)
 
 
+def test_rewrite_subject(cmsetup, maildata):
+    """Test that subject gets replaced with [...]."""
+    user1, user2 = cmsetup.gen_users(2)
+
+    sent_msg = maildata(
+        "encrypted.eml",
+        from_addr=user1.addr,
+        to_addr=user2.addr,
+        subject="Unencrypted subject",
+    ).as_string()
+    user1.smtp.sendmail(from_addr=user1.addr, to_addrs=[user2.addr], msg=sent_msg)
+
+    messages = user2.imap.fetch_all_messages()
+    assert len(messages) == 1
+    rcvd_msg = messages[0]
+    assert "Subject: [...]" not in sent_msg
+    assert "Subject: [...]" in rcvd_msg
+    assert "Subject: Unencrypted subject" in sent_msg
+    assert "Subject: Unencrypted subject" not in rcvd_msg
+
+
 @pytest.mark.slow
 def test_exceed_rate_limit(cmsetup, gencreds, maildata, chatmail_config):
     """Test that the per-account send-mail limit is exceeded."""