WIP, untested: Sending side of transferring the secret in member-added message

src/chat.rs

@@ -4067,7 +4067,7 @@ pub(crate) async fn add_contact_to_chat_ex(
         let contact_addr = contact.get_addr().to_lowercase();
         let added_by = if from_handshake && chat.is_out_broadcast() {
             // The contact was added via a QR code rather than explicit user action,
-            // and there is added information in saying 'You added member Alice'
+            // and there is no useful information in saying 'You added member Alice'
             // if self is the only one who can add members.
             ContactId::UNDEFINED
         } else {
@@ -4079,6 +4079,12 @@ pub(crate) async fn add_contact_to_chat_ex(
         msg.param.set_int(Param::Arg2, from_handshake.into());
         msg.param
             .set_int(Param::ContactAddedRemoved, contact.id.to_u32() as i32);
+        if chat.is_out_broadcast() {
+            let secret = load_broadcast_shared_secret(context, chat_id)
+                .await?
+                .context("Failed to find broadcast shared secret")?;
+            msg.param.set(Param::Arg3, secret);
+        }
         send_msg(context, chat_id, &mut msg).await?;
 
         sync = Nosync;

src/headerdef.rs

@@ -94,6 +94,11 @@ pub enum HeaderDef {
     /// This message obsoletes the text of the message defined here by rfc724_mid.
     ChatEdit,
 
+    /// The secret shared amongst all recipients of this broadcast channel,
+    /// used to encrypt and decrypt messages.
+    /// This secret is sent to a new member in the member-addition message.
+    ChatBroadcastSecret,
+
     /// [Autocrypt](https://autocrypt.org/) header.
     Autocrypt,
     AutocryptGossip,

src/mimefactory.rs

@@ -834,7 +834,7 @@ impl MimeFactory {
             }
         }
 
-        if let Loaded::Message { chat, .. } = &self.loaded {
+        if let Loaded::Message { msg, chat } = &self.loaded {
             if chat.typ == Chattype::OutBroadcast || chat.typ == Chattype::InBroadcast {
                 headers.push((
                     "List-ID",
@@ -844,6 +844,15 @@ impl MimeFactory {
                     ))
                     .into(),
                 ));
+
+                if msg.param.get_cmd() == SystemMessage::MemberAddedToGroup {
+                    if let Some(secret) = msg.param.get(Param::Arg3) {
+                        headers.push((
+                            "Chat-Broadcast-Secret",
+                            mail_builder::headers::text::Text::new(secret.to_string()).into(),
+                        ));
+                    }
+                }
             }
         }
 
@@ -1024,6 +1033,15 @@ impl MimeFactory {
                 } else {
                     unprotected_headers.push(header.clone());
                 }
+            } else if header_name == "chat-broadcast-secret" {
+                if is_encrypted {
+                    protected_headers.push(header.clone());
+                } else {
+                    warn!(
+                        context,
+                        "Message is unnecrypted, not including broadcast secret"
+                    );
+                }
             } else if is_encrypted {
                 protected_headers.push(header.clone());
 

src/param.rs

@@ -106,12 +106,29 @@ pub enum Param {
     Arg = b'E',
 
     /// For Messages
+    ///
+    /// For `BobHandshakeMsg::Request`, this is the `Secure-Join-Invitenumber` header.
+    ///
+    /// For `BobHandshakeMsg::RequestWithAuth`, this is the `Secure-Join-Auth` header.
+    ///
+    /// For [`SystemMessage::MultiDeviceSync`], this contains the ids that are synced.
+    ///
+    /// For [`SystemMessage::MemberAddedToGroup`],
+    /// this is '1' if it was added because of a securejoin-handshake, and '0' otherwise.
     Arg2 = b'F',
 
-    /// `Secure-Join-Fingerprint` header for `{vc,vg}-request-with-auth` messages.
+    /// For Messages
+    ///
+    /// For `BobHandshakeMsg::RequestWithAuth`,
+    /// this contains the `Secure-Join-Fingerprint` header.
+    ///
+    /// For [`SystemMessage::MemberAddedToGroup`] that add to a broadcast channel,
+    /// this contains the broadcast channel's shared secret.
     Arg3 = b'G',
 
-    /// Deprecated `Secure-Join-Group` header for messages.
+    /// For Messages
+    ///
+    /// Deprecated `Secure-Join-Group` header for `BobHandshakeMsg::RequestWithAuth` messages.
     Arg4 = b'H',
 
     /// For Messages