Don't degrade encryption when MDN is received

deltachat-rpc-client/tests/test_something.py

@@ -140,12 +140,9 @@ def test_chat(acfactory) -> None:
     alice_chat_bob = alice_contact_bob.create_chat()
     alice_chat_bob.send_text("Hello!")
 
-    while True:
-        event = bob.wait_for_event()
-        if event.kind == EventType.INCOMING_MSG:
-            chat_id = event.chat_id
-            msg_id = event.msg_id
-            break
+    event = bob.wait_for_incoming_msg_event()
+    chat_id = event.chat_id
+    msg_id = event.msg_id
     message = bob.get_message_by_id(msg_id)
     snapshot = message.get_snapshot()
     assert snapshot.chat_id == chat_id
@@ -224,12 +221,9 @@ def test_message(acfactory) -> None:
     alice_chat_bob = alice_contact_bob.create_chat()
     alice_chat_bob.send_text("Hello!")
 
-    while True:
-        event = bob.wait_for_event()
-        if event.kind == EventType.INCOMING_MSG:
-            chat_id = event.chat_id
-            msg_id = event.msg_id
-            break
+    event = bob.wait_for_incoming_msg_event()
+    chat_id = event.chat_id
+    msg_id = event.msg_id
 
     message = bob.get_message_by_id(msg_id)
     snapshot = message.get_snapshot()
@@ -402,3 +396,46 @@ def test_provider_info(rpc) -> None:
     rpc.set_config(account_id, "socks5_enabled", "1")
     provider_info = rpc.get_provider_info(account_id, "github.com")
     assert provider_info is None
+
+
+def test_mdn_doesnt_break_autocrypt(acfactory) -> None:
+    alice, bob = acfactory.get_online_accounts(2)
+
+    bob_addr = bob.get_config("addr")
+
+    alice_contact_bob = alice.create_contact(bob_addr, "Bob")
+
+    # Bob creates chat manually so chat with Alice is accepted.
+    alice_chat_bob = alice_contact_bob.create_chat()
+
+    # Alice sends a message to Bob.
+    alice_chat_bob.send_text("Hello Bob!")
+    event = bob.wait_for_incoming_msg_event()
+    msg_id = event.msg_id
+    message = bob.get_message_by_id(msg_id)
+    snapshot = message.get_snapshot()
+
+    # Bob sends a message to Alice.
+    bob_chat_alice = snapshot.chat
+    bob_chat_alice.accept()
+    bob_chat_alice.send_text("Hello Alice!")
+    event = alice.wait_for_incoming_msg_event()
+    msg_id = event.msg_id
+    message = alice.get_message_by_id(msg_id)
+    snapshot = message.get_snapshot()
+    assert snapshot.show_padlock
+
+    # Alice reads Bob's message.
+    message.mark_seen()
+    while True:
+        event = bob.wait_for_event()
+        if event.kind == EventType.MSG_READ:
+            break
+
+    # Bob sends a message to Alice, it should also be encrypted.
+    bob_chat_alice.send_text("Hi Alice!")
+    event = alice.wait_for_incoming_msg_event()
+    msg_id = event.msg_id
+    message = alice.get_message_by_id(msg_id)
+    snapshot = message.get_snapshot()
+    assert snapshot.show_padlock

src/mimefactory.rs

@@ -768,6 +768,30 @@ impl<'a> MimeFactory<'a> {
                         .build(),
                 )
                 .header(("Subject".to_string(), "...".to_string()))
+        } else if matches!(self.loaded, Loaded::Mdn { .. }) {
+            // Never add outer multipart/mixed wrapper to MDN
+            // as multipart/report Content-Type is used to recognize MDNs
+            // by Delta Chat receiver and Chatmail servers
+            // allowing them to be unencrypted and not contain Autocrypt header
+            // without resetting Autocrypt encryption or triggering Chatmail filter
+            // that normally only allows encrypted mails.
+
+            // Hidden headers are dropped.
+
+            // Store protected headers in the outer message.
+            let message = headers
+                .protected
+                .iter()
+                .fold(message, |message, header| message.header(header.clone()));
+
+            let protected: HashSet<Header> = HashSet::from_iter(headers.protected.into_iter());
+            for h in headers.unprotected.split_off(0) {
+                if !protected.contains(&h) {
+                    headers.unprotected.push(h);
+                }
+            }
+
+            message
         } else {
             // Store hidden headers in the inner unencrypted message.
             let message = headers