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

src/chat.rs

@@ -42,9 +42,9 @@ use crate::smtp::send_msg_to_smtp;
 use crate::stock_str;
 use crate::sync::{self, Sync::*, SyncData};
 use crate::tools::{
-    IsNoneOrEmpty, SystemTime, buf_compress, create_id, create_outgoing_rfc724_mid,
-    create_smeared_timestamp, create_smeared_timestamps, get_abs_path, gm2local_offset,
-    smeared_time, time, truncate_msg_text,
+    IsNoneOrEmpty, SystemTime, buf_compress, create_broadcast_shared_secret, create_id,
+    create_outgoing_rfc724_mid, create_smeared_timestamp, create_smeared_timestamps, get_abs_path,
+    gm2local_offset, smeared_time, time, truncate_msg_text,
 };
 use crate::webxdc::StatusUpdateSerial;
 use crate::{chatlist_events, imap};
@@ -3710,14 +3710,18 @@ pub(crate) async fn create_broadcast_ex(
                     },
                 )?);
             }
+            let mut param = Params::new();
+            // param.set(Param::Unpromoted, 1); // TODO broadcasts will just never be unpromoted for now
+            param.set(Param::SymmetricKey, create_broadcast_shared_secret());
             t.execute(
                 "INSERT INTO chats \
                 (type, name, grpid, param, created_timestamp) \
-                VALUES(?, ?, ?, \'U=1\', ?);",
+                VALUES(?, ?, ?, ?, ?);",
                 (
                     Chattype::OutBroadcast,
                     &chat_name,
                     &grpid,
+                    param.to_string(),
                     create_smeared_timestamp(context),
                 ),
             )?;
@@ -3918,7 +3922,7 @@ pub(crate) async fn add_contact_to_chat_ex(
         }
         add_to_chat_contacts_table(context, time(), chat_id, &[contact_id]).await?;
     }
-    if chat.typ == Chattype::Group && chat.is_promoted() {
+    if chat.is_promoted() {
         msg.viewtype = Viewtype::Text;
 
         let contact_addr = contact.get_addr().to_lowercase();

src/headerdef.rs

@@ -93,6 +93,10 @@ 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.
+    /// This secret.
+    ChatBroadcastSecret,
+
     /// [Autocrypt](https://autocrypt.org/) header.
     Autocrypt,
     AutocryptGossip,

src/mimefactory.rs

@@ -789,7 +789,7 @@ 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",
@@ -799,6 +799,15 @@ impl MimeFactory {
                     ))
                     .into(),
                 ));
+
+                if msg.param.get_cmd() == SystemMessage::MemberAddedToGroup {
+                    if let Some(secret) = chat.param.get(Param::SymmetricKey) {
+                        headers.push((
+                            "Chat-Broadcast-Secret",
+                            mail_builder::headers::text::Text::new(secret.to_string()).into(),
+                        ));
+                    }
+                }
             }
         }
 
@@ -979,6 +988,15 @@ 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

@@ -169,7 +169,9 @@ pub enum Param {
     /// post something to the mailing list.
     ListPost = b'p',
 
-    /// For Chats of type [`Chattype::OutBroadcast`] and [`Chattype::InBroadcast`]: // TODO (or just OutBroadcast)
+    /// For Chats and Messages:
+    /// For chats of type [`Chattype::OutBroadcast`] and [`Chattype::InBroadcast`] // TODO (or just OutBroadcast)
+    /// and for messages adding members to such a chat.
     /// The symmetric key shared among all chat participants,
     /// used to encrypt and decrypt messages.
     SymmetricKey = b'z',

src/tools.rs

@@ -300,6 +300,25 @@ pub(crate) fn create_id() -> String {
     base64::engine::general_purpose::URL_SAFE.encode(arr)
 }
 
+/// Generate a shared secret for a broadcast channel, consisting of 64 characters..
+///
+/// The string generated by this function has 384 bits of entropy
+/// and is returned as 64 Base64 characters, each containing 6 bits of entropy.
+/// 384 is chosen because it is sufficiently secure
+/// (larger than AES-128 keys used for message encryption)
+/// and divides both by 8 (byte size) and 6 (number of bits in a single Base64 character).
+// TODO ask someone what a good size would be here
+pub(crate) fn create_broadcast_shared_secret() -> String {
+    // ThreadRng implements CryptoRng trait and is supposed to be cryptographically secure.
+    let mut rng = thread_rng();
+
+    // Generate 384 random bits.
+    let mut arr = [0u8; 48];
+    rng.fill(&mut arr[..]);
+
+    base64::engine::general_purpose::URL_SAFE.encode(arr)
+}
+
 /// Returns true if given string is a valid ID.
 ///
 /// All IDs generated with `create_id()` should be considered valid.