Fix: don't accept unencrypted messages

client.go

@@ -185,53 +185,65 @@ func (c *Client) read_loop(ctx context.Context, pending []string) {
 }
 
 func (c *Client) handle_incoming_text(s string) {
-	if len(s) > 0 && s[0] == '/' {
-		// Ignore unknown commands after handshake for now.
-		return
-	}
+    if len(s) > 0 && s[0] == '/' {
+        return
+    }
 
-	var in incoming_frame
-	if err := json.Unmarshal([]byte(s), &in); err != nil {
-		return
-	}
+    var in incoming_frame
+    if err := json.Unmarshal([]byte(s), &in); err != nil {
+        return
+    }
 
-	c.mu.Lock()
-	if !c.ready {
-		c.mu.Unlock()
-		return
-	}
+    c.mu.Lock()
+    if !c.ready {
+        c.mu.Unlock()
+        return
+    }
 
-	shared_key := c.state.SharedKey
-	session_id_bytes := c.state.SessionIdBytes
-	c.mu.Unlock()
+    shared_key := c.state.SharedKey
+    session_id_bytes := c.state.SessionIdBytes
+    c.mu.Unlock()
 
-	content := in.Content
+    // Enforce encryption after handshake.
+    if !in.IsEncrypted {
+        c.emit(Event{
+            Kind: EventError,
+            Err:  fmt.Errorf("received unencrypted message after handshake"),
+        })
+        return
+    }
 
-	if in.IsEncrypted {
-		plain, err := DecryptAESGCM(shared_key, session_id_bytes, in.Content, in.Salt, in.SeqNum, in.IsResponse)
-		if err != nil {
-			c.emit(Event{Kind: EventError, Err: err})
-			return
-		}
-		content = string(plain)
-	}
+    plain, err := DecryptAESGCM(
+        shared_key,
+        session_id_bytes,
+        in.Content,
+        in.Salt,
+        in.SeqNum,
+        in.IsResponse,
+    )
+    if err != nil {
+        c.emit(Event{Kind: EventError, Err: err})
+        return
+    }
 
-	c.mu.Lock()
-	if in.SeqNum > c.state.SeqInMax {
-		c.state.SeqInMax = in.SeqNum
-	}
-	c.mu.Unlock()
+    content := string(plain)
 
-	c.emit(Event{
-		Kind: EventMessage,
-		Message: Message{
-			Content:    content,
-			Metadata:   in.Metadata,
-			SeqNum:     in.SeqNum,
-			IsResponse: in.IsResponse,
-			CreatedAt:  in.CreatedAt,
-		},
-	})
+    c.mu.Lock()
+    if in.SeqNum > c.state.SeqInMax {
+        c.state.SeqInMax = in.SeqNum
+    }
+    c.mu.Unlock()
+
+    c.emit(Event{
+        Kind: EventMessage,
+        Message: Message{
+            Content:    content,
+            Metadata:   in.Metadata,
+            SeqNum:     in.SeqNum,
+            IsResponse: in.IsResponse,
+            CreatedAt:  in.CreatedAt,
+        },
+    })
 }
 
 func (c *Client) emit(ev Event) {