fix: do not use Secure-Join-Group header

Alice already knows which auth token corresponds to which group.
There is no need to trust Bob on sending the correct group ID.

src/headerdef.rs

@@ -74,6 +74,11 @@ pub enum HeaderDef {
     Autocrypt,
     AutocryptSetupMessage,
     SecureJoin,
+
+    /// Deprecated header containing Group-ID in `vg-request-with-auth` message.
+    ///
+    /// It is not used by Alice as Alice knows the group corresponding to the AUTH token.
+    /// Bob still sends it for backwards compatibility.
     SecureJoinGroup,
     SecureJoinFingerprint,
     SecureJoinInvitenumber,

src/securejoin.rs

@@ -398,7 +398,7 @@ pub(crate) async fn handle_securejoin_handshake(
                 .await?;
                 return Ok(HandshakeMessage::Ignore);
             };
-            if !token::exists(context, token::Namespace::Auth, auth).await? {
+            let Some(group_chat_id) = token::auth_chat_id(context, auth).await? else {
                 could_not_establish_secure_connection(
                     context,
                     contact_id,
@@ -407,7 +407,8 @@ pub(crate) async fn handle_securejoin_handshake(
                 )
                 .await?;
                 return Ok(HandshakeMessage::Ignore);
-            }
+            };
+
             let contact_addr = Contact::get_by_id(context, contact_id)
                 .await?
                 .get_addr()
@@ -435,41 +436,8 @@ pub(crate) async fn handle_securejoin_handshake(
             info!(context, "Auth verified.",);
             context.emit_event(EventType::ContactsChanged(Some(contact_id)));
             inviter_progress(context, contact_id, 600);
-            if join_vg {
+            if group_chat_id.is_unset() {
-                // the vg-member-added message is special:
+                // Setup verified contact.
-                // this is a normal Chat-Group-Member-Added message
-                // with an additional Secure-Join header
-                let field_grpid = match mime_message.get_header(HeaderDef::SecureJoinGroup) {
-                    Some(s) => s.as_str(),
-                    None => {
-                        warn!(context, "Missing Secure-Join-Group header");
-                        return Ok(HandshakeMessage::Ignore);
-                    }
-                };
-                match chat::get_chat_id_by_grpid(context, field_grpid).await? {
-                    Some((group_chat_id, _, _)) => {
-                        secure_connection_established(
-                            context,
-                            contact_id,
-                            group_chat_id,
-                            mime_message.timestamp_sent,
-                        )
-                        .await?;
-                        chat::add_contact_to_chat_ex(
-                            context,
-                            Nosync,
-                            group_chat_id,
-                            contact_id,
-                            true,
-                        )
-                        .await?;
-                    }
-                    None => bail!("Chat {} not found", &field_grpid),
-                }
-                inviter_progress(context, contact_id, 800);
-                inviter_progress(context, contact_id, 1000);
-            } else {
-                // Alice -> Bob
                 secure_connection_established(
                     context,
                     contact_id,
@@ -482,6 +450,19 @@ pub(crate) async fn handle_securejoin_handshake(
                     .context("failed sending vc-contact-confirm message")?;
 
                 inviter_progress(context, contact_id, 1000);
+            } else {
+                // Join group.
+                secure_connection_established(
+                    context,
+                    contact_id,
+                    group_chat_id,
+                    mime_message.timestamp_sent,
+                )
+                .await?;
+                chat::add_contact_to_chat_ex(context, Nosync, group_chat_id, contact_id, true)
+                    .await?;
+                inviter_progress(context, contact_id, 800);
+                inviter_progress(context, contact_id, 1000);
             }
             Ok(HandshakeMessage::Ignore) // "Done" would delete the message and break multi-device (the key from Autocrypt-header is needed)
         }

src/token.rs

@@ -114,6 +114,25 @@ pub async fn exists(context: &Context, namespace: Namespace, token: &str) -> Res
     Ok(exists)
 }
 
+/// Looks up ChatId by auth token.
+///
+/// Returns None if auth token is not valid.
+/// Returns zero/unset ChatId if the token corresponds to "setup contact" rather than group join.
+pub async fn auth_chat_id(context: &Context, token: &str) -> Result<Option<ChatId>> {
+    let chat_id: Option<ChatId> = context
+        .sql
+        .query_row_optional(
+            "SELECT foreign_id FROM tokens WHERE namespc=? AND token=?",
+            (Namespace::Auth, token),
+            |row| {
+                let chat_id: ChatId = row.get(0)?;
+                Ok(chat_id)
+            },
+        )
+        .await?;
+    Ok(chat_id)
+}
+
 pub async fn delete(context: &Context, namespace: Namespace, token: &str) -> Result<()> {
     context
         .sql