crypto: Simplify the group session pair creation.

matrix_sdk_crypto/src/machine.rs

@@ -1058,26 +1058,13 @@ impl OlmMachine {
     /// This also creates a matching inbound group session and saves that one in
     /// the store.
     async fn create_outbound_group_session(&mut self, room_id: &RoomId) -> OlmResult<()> {
-        let session = OutboundGroupSession::new(room_id);
+        let (outbound, inbound) = self.account.create_group_session_pair(room_id).await;
-        let identity_keys = self.account.identity_keys();
 
-        let sender_key = identity_keys.curve25519();
+        let _ = self.store.save_inbound_group_session(inbound).await?;
-        let signing_key = identity_keys.ed25519();
-
-        let inbound_session = InboundGroupSession::new(
-            sender_key,
-            signing_key,
-            &room_id,
-            session.session_key().await,
-        )?;
-        let _ = self
-            .store
-            .save_inbound_group_session(inbound_session)
-            .await?;
 
         let _ = self
             .outbound_group_sessions
-            .insert(room_id.to_owned(), session);
+            .insert(room_id.to_owned(), outbound);
         Ok(())
     }
 

matrix_sdk_crypto/src/olm.rs

@@ -234,6 +234,38 @@ impl Account {
             last_use_time: Arc::new(now),
         })
     }
+
+    /// Create a group session pair.
+    ///
+    /// This session pair can be used to encrypt and decrypt messages meant for
+    /// a large group of participants.
+    ///
+    /// The outbound session is used to encrypt messages while the inbound one
+    /// is used to decrypt messages encrypted by the outbound one.
+    ///
+    /// # Arguments
+    ///
+    /// * `room_id` - The ID of the room where the group session will be used.
+    pub async fn create_group_session_pair(
+        &self,
+        room_id: &RoomId,
+    ) -> (OutboundGroupSession, InboundGroupSession) {
+        let outbound = OutboundGroupSession::new(room_id);
+        let identity_keys = self.identity_keys();
+
+        let sender_key = identity_keys.curve25519();
+        let signing_key = identity_keys.ed25519();
+
+        let inbound = InboundGroupSession::new(
+            sender_key,
+            signing_key,
+            &room_id,
+            outbound.session_key().await,
+        )
+        .expect("Can't create inbound group session from a newly created outbound group session");
+
+        (outbound, inbound)
+    }
 }
 
 impl PartialEq for Account {