fix: Use keyring with all private keys when decrypting a message (#5046)

Before a keyring with the only default key was used, i.e. the key used for signing and encrypting to
self.

src/imex.rs

@@ -808,6 +808,7 @@ mod tests {
     use tokio::task;
 
     use super::*;
+    use crate::key;
     use crate::pgp::{split_armored_data, HEADER_AUTOCRYPT, HEADER_SETUPCODE};
     use crate::stock_str::StockMessage;
     use crate::test_utils::{alice_keypair, TestContext};
@@ -920,6 +921,37 @@ mod tests {
         }
     }
 
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_import_second_key() -> Result<()> {
+        let alice = &TestContext::new_alice().await;
+        let chat = alice.create_chat(alice).await;
+        let sent = alice.send_text(chat.id, "Encrypted with old key").await;
+        let export_dir = tempfile::tempdir().unwrap();
+
+        let alice = &TestContext::new().await;
+        alice.configure_addr("alice@example.org").await;
+        imex(alice, ImexMode::ExportSelfKeys, export_dir.path(), None).await?;
+
+        let alice = &TestContext::new_alice().await;
+        let old_key = key::load_self_secret_key(alice).await?;
+
+        imex(alice, ImexMode::ImportSelfKeys, export_dir.path(), None).await?;
+
+        let new_key = key::load_self_secret_key(alice).await?;
+        assert_ne!(new_key, old_key);
+        assert_eq!(
+            key::load_self_secret_keyring(alice).await?,
+            vec![new_key, old_key]
+        );
+
+        let msg = alice.recv_msg(&sent).await;
+        assert!(msg.get_showpadlock());
+        assert_eq!(msg.chat_id, alice.get_self_chat().await.id);
+        assert_eq!(msg.get_text(), "Encrypted with old key");
+
+        Ok(())
+    }
+
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
     async fn test_export_and_import_backup() -> Result<()> {
         for set_verified_oneonone_chats in [true, false] {