Do not allow peerstate reset if DKIM check failed

The problem was that a message without Autocrypt key or with a wrong
signature resets peerstate regardless of what DKIM check says. I
inserted sleep(1.1) to make sure reset always happens and make the bug
reproducible, then fixed it by forbidding reset if DKIM check fails.

https://github.com/deltachat/deltachat-core-rust/pull/3731

CHANGELOG.md

@@ -40,6 +40,7 @@
 ### Fixes
 - `dc_search_msgs()` returns unaccepted requests #3694
 - emit "contacts changed" event when the contact is no longer "seen recently" #3703
+- do not allow peerstate reset if DKIM check failed #3731
 
 
 ## 1.98.0

src/authres.rs

@@ -357,6 +357,7 @@ mod tests {
 
     use super::*;
 
+    use crate::aheader::EncryptPreference;
     use crate::e2ee;
     use crate::mimeparser;
     use crate::peerstate::Peerstate;
@@ -665,6 +666,10 @@ Authentication-Results: box.hispanilandia.net; spf=pass smtp.mailfrom=adbenitez@
 
         tcm.section("An attacker, bob2, sends a from-forged email to Alice!");
 
+        // Sleep to make sure key reset is ignored because of DKIM failure
+        // and not because reordering is suspected.
+        tokio::time::sleep(std::time::Duration::from_millis(1100)).await;
+
         let bob2 = tcm.unconfigured().await;
         bob2.configure_addr("bob@example.net").await;
         e2ee::ensure_secret_key_exists(&bob2).await?;
@@ -686,6 +691,9 @@ Authentication-Results: box.hispanilandia.net; spf=pass smtp.mailfrom=adbenitez@
             .await?
             .unwrap();
 
+        // Encryption preference is still mutual.
+        assert_eq!(bob_state.prefer_encrypt, EncryptPreference::Mutual);
+
         // Also check that the keypair was not changed
         assert_eq!(
             bob_state.public_key.unwrap(),

src/mimeparser.rs

@@ -298,6 +298,7 @@ impl MimeMessage {
                     if let Some(peerstate) = &mut decryption_info.peerstate {
                         if message_time > peerstate.last_seen_autocrypt
                             && mail.ctype.mimetype != "multipart/report"
+                            && decryption_info.dkim_results.allow_keychange
                         {
                             peerstate.degrade_encryption(message_time);
                             peerstate.save_to_db(&context.sql, false).await?;