Ignore reordered autocrypt headers

This commit fixes condition which ignores reordered autocrypt messages.
If a plaintext message resetting peerstate has been received, autocrypt
header should only be applied if it has higher timestamp.

Previously, timestamp of the last received autocrypt header was used,
which may be lower than last_seen timestamp.

# Conflicts:
#	src/peerstate.rs

src/peerstate.rs

@@ -290,7 +290,7 @@ impl<'a> Peerstate<'a> {
             return;
         }
 
-        if message_time > self.last_seen_autocrypt {
+        if message_time > self.last_seen {
             self.last_seen = message_time;
             self.last_seen_autocrypt = message_time;
             self.to_save = Some(ToSave::Timestamps);
@@ -634,4 +634,45 @@ mod tests {
         assert_eq!(peerstate.gossip_key_fingerprint, None);
         assert_eq!(peerstate.verified_key_fingerprint, None);
     }
+
+    #[async_std::test]
+    async fn test_peerstate_degrade_reordering() {
+        let context = crate::test_utils::TestContext::new().await.ctx;
+        let addr = "example@example.org";
+        let pub_key = alice_keypair().public;
+        let header = Aheader::new(addr.to_string(), pub_key, EncryptPreference::Mutual);
+
+        let mut peerstate = Peerstate {
+            context: &context,
+            addr: addr.to_string(),
+            last_seen: 0,
+            last_seen_autocrypt: 0,
+            prefer_encrypt: EncryptPreference::NoPreference,
+            public_key: None,
+            public_key_fingerprint: None,
+            gossip_key: None,
+            gossip_timestamp: 0,
+            gossip_key_fingerprint: None,
+            verified_key: None,
+            verified_key_fingerprint: None,
+            to_save: None,
+            fingerprint_changed: false,
+        };
+        assert_eq!(peerstate.prefer_encrypt, EncryptPreference::NoPreference);
+
+        peerstate.apply_header(&header, 100);
+        assert_eq!(peerstate.prefer_encrypt, EncryptPreference::Mutual);
+
+        peerstate.degrade_encryption(300);
+        assert_eq!(peerstate.prefer_encrypt, EncryptPreference::Reset);
+
+        // This has message time 200, while encryption was degraded at timestamp 300.
+        // Because of reordering, header should not be applied.
+        peerstate.apply_header(&header, 200);
+        assert_eq!(peerstate.prefer_encrypt, EncryptPreference::Reset);
+
+        // Same header will be applied in the future.
+        peerstate.apply_header(&header, 400);
+        assert_eq!(peerstate.prefer_encrypt, EncryptPreference::Mutual);
+    }
 }