fix: do not add key-contacts to unencrypted groups

Encrypted message may create unencrypted groups
if the message does not have a Chat-Group-ID.
This can happen if v1 client sends an encrypted
message to opportunistically encrypted ad hoc group.
In this case `from_id` corresponds to the key-contact,
but we should add address-contact of the sender
to the member list.

src/receive_imf.rs

@@ -630,8 +630,7 @@ pub(crate) async fn receive_imf_inner(
         return Ok(None);
     };
 
-    let prevent_rename = (mime_parser.is_mailinglist_message() && !mime_parser.was_encrypted())
-        || mime_parser.get_header(HeaderDef::Sender).is_some();
+    let prevent_rename = should_prevent_rename(&mime_parser);
 
     // get From: (it can be an address list!) and check if it is known (for known From:'s we add
     // the other To:/Cc: in the 3rd pass)
@@ -1397,7 +1396,6 @@ async fn do_chat_assignment(
                     context,
                     mime_parser,
                     to_ids,
-                    from_id,
                     allow_creation || test_normal_chat.is_some(),
                     create_blocked,
                     is_partial_download.is_some(),
@@ -1567,7 +1565,6 @@ async fn do_chat_assignment(
                     context,
                     mime_parser,
                     to_ids,
-                    from_id,
                     allow_creation,
                     Blocked::Not,
                     is_partial_download.is_some(),
@@ -2464,7 +2461,6 @@ async fn lookup_or_create_adhoc_group(
     context: &Context,
     mime_parser: &MimeMessage,
     to_ids: &[Option<ContactId>],
-    from_id: ContactId,
     allow_creation: bool,
     create_blocked: Blocked,
     is_partial_download: bool,
@@ -2487,6 +2483,20 @@ async fn lookup_or_create_adhoc_group(
         return Ok(None);
     }
 
+    // Lookup address-contact by the From address.
+    let fingerprint = None;
+    let find_key_contact_by_addr = false;
+    let prevent_rename = should_prevent_rename(mime_parser);
+    let (from_id, _from_id_blocked, _incoming_origin) = from_field_to_contact_id(
+        context,
+        &mime_parser.from,
+        fingerprint,
+        prevent_rename,
+        find_key_contact_by_addr,
+    )
+    .await?
+    .context("Cannot lookup addres-contact by the From field")?;
+
     let grpname = mime_parser
         .get_subject()
         .map(|s| remove_subject_prefix(&s))
@@ -3963,5 +3973,12 @@ async fn lookup_key_contacts_by_address_list(
     Ok(contact_ids)
 }
 
+/// Returns true if the message should not result in renaming
+/// of the sender contact.
+fn should_prevent_rename(mime_parser: &MimeMessage) -> bool {
+    (mime_parser.is_mailinglist_message() && !mime_parser.was_encrypted())
+        || mime_parser.get_header(HeaderDef::Sender).is_some()
+}
+
 #[cfg(test)]
 mod receive_imf_tests;

src/receive_imf/receive_imf_tests.rs

@@ -5352,3 +5352,57 @@ async fn test_group_introduction_no_gossip() -> Result<()> {
 
     Ok(())
 }
+
+/// Tests reception of an encrypted group message
+/// without Chat-Group-ID.
+///
+/// The message should be displayed as
+/// encrypted and have key-contact `from_id`,
+/// but all group members should be address-contacts.
+///
+/// Due to a bug in v2.10.0 this resulted
+/// in creation of an ad hoc group with a key-contact.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_encrypted_adhoc_group_message() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let alice = &tcm.alice().await;
+    let bob = &tcm.bob().await;
+
+    // Bob receives encrypted message from Alice
+    // sent to multiple recipients,
+    // but without a group ID.
+    let received = receive_imf(
+        bob,
+        include_bytes!("../../test-data/message/encrypted-group-without-id.eml"),
+        false,
+    )
+    .await?
+    .unwrap();
+    let msg = Message::load_from_db(bob, *received.msg_ids.last().unwrap())
+        .await
+        .unwrap();
+
+    let chat = Chat::load_from_db(bob, msg.chat_id).await.unwrap();
+    assert_eq!(chat.typ, Chattype::Group);
+    assert_eq!(chat.is_encrypted(bob).await?, false);
+
+    let contact_ids = get_chat_contacts(bob, chat.id).await?;
+    assert_eq!(contact_ids.len(), 3);
+    assert!(chat.is_self_in_chat(bob).await?);
+
+    // Since the group is unencrypted, all contacts have
+    // to be address-contacts.
+    for contact_id in contact_ids {
+        let contact = Contact::get_by_id(bob, contact_id).await?;
+        if contact_id != ContactId::SELF {
+            assert_eq!(contact.is_key_contact(), false);
+        }
+    }
+
+    // `from_id` of the message corresponds to key-contact of Alice
+    // even though the message is assigned to unencrypted chat.
+    let alice_contact_id = bob.add_or_lookup_contact_id(alice).await;
+    assert_eq!(msg.from_id, alice_contact_id);
+
+    Ok(())
+}

test-data/message/encrypted-group-without-id.eml

@@ -0,0 +1,69 @@
+Content-Type: multipart/encrypted; protocol="application/pgp-encrypted"; 
+	boundary="18566fe03178296e_f40510c3b59ace3_ecf843ae3ccd2b17"
+MIME-Version: 1.0
+From: <alice@example.org>
+To: <bob@example.net>, <charlie@example.net>
+Subject: [...]
+Date: Mon, 28 Jul 2025 14:15:14 +0000
+Message-ID: <48b9e9cc-2bae-4d41-89b4-a409e2c60c28@localhost>
+References: <48b9e9cc-2bae-4d41-89b4-a409e2c60c28@localhost>
+Chat-Version: 1.0
+Autocrypt: addr=alice@example.org; prefer-encrypt=mutual; keydata=mDMEXlh13RYJKwYBBAHaRw8BAQdAzfVIAleCXMJrq8VeLlEVof6ITCviMktKjmcBKAu4m5
+	 C0GUFsaWNlIDxhbGljZUBleGFtcGxlLm9yZz7CkgQQFggAOgUCaIeF8RYhBC5vossjtTLXKGNLWGSw
+	 j2Gp7ZRDAhsDAh4BBQsJCAcCBhUKCQgLAgQWAgMBAScCGQEACgkQZLCPYantlEM66gD/b9qi1/H1Cr
+	 UwwlW2akVX86Q0gX6isyKfuNu/CdTdzaQBAIHRxvwlBNZr56qMGL7CyVy6LmBslLlbQwAdclM9t9UE
+	 uDgEXlh13RIKKwYBBAGXVQEFAQEHQAbtyNbLZIUBTwqeW2W5tVbrusWLJ+nTUmtF7perLbYdAwEIB8
+	 J4BBgWCAAgBQJoh4XxAhsMFiEELm+iyyO1MtcoY0tYZLCPYantlEMACgkQZLCPYantlEPG2QD8DthL
+	 48j1wnjw+Kby7CmAm/M+Me82izk8dGNPn442jJ4A/2r+YmqfUPK2XDXPRwvVBAIz5bL44fe7gNkUUu
+	 XMnzkP
+
+
+--18566fe03178296e_f40510c3b59ace3_ecf843ae3ccd2b17
+Content-Type: application/pgp-encrypted; charset="utf-8"
+Content-Description: PGP/MIME version identification
+Content-Transfer-Encoding: 7bit
+
+Version: 1
+
+--18566fe03178296e_f40510c3b59ace3_ecf843ae3ccd2b17
+Content-Type: application/octet-stream; name="encrypted.asc"; 
+	charset="utf-8"
+Content-Description: OpenPGP encrypted message
+Content-Disposition: inline; filename="encrypted.asc";
+Content-Transfer-Encoding: 7bit
+
+-----BEGIN PGP MESSAGE-----
+
+wV4D5tq63hTeebASAQdA5TP3lUjawDdYRzMW0HUyxu0kFUgWtYo+O6pyNE0FJRQw
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+WEp750aqWjdiu0uveoY7hoAxL+A/IshTQWhXMQ==
+=ESpw
+-----END PGP MESSAGE-----
+
+
+--18566fe03178296e_f40510c3b59ace3_ecf843ae3ccd2b17--