Microsoft is testing a new feature for Skype: end-to-end encrypted chat conversations, secured using the industry standard Signal Protocol by Open Whisper Systems.

The company announced the experimental feature named “Private Conversations” in a blog post today. It’ll be available to anyone running the beta Skype Insider build (version 8.13.76.8) on Windows, Mac, Linux, Android, or iOS, although users will only be able to start private chats with other individuals with the same Insider build. The encryption will cover chat, files, and audio messages, but not audio or video calls.

Skype does already offer some degree of encryption that protects the communication channel itself. But end-to-end encryption is more secure. It means the contents of messages can only be read by the sender and recipient; they don’t just sit around on Microsoft’s servers. That doesn’t mean this data can’t be intercepted in other ways, of course, but it’s still more secure than most methods of online chat.

If Windows takes this feature into its main Skype build, it’ll be joining chat platforms like Facebooks’ Messenger, Apple’s iMessage, and WhatsApp in offering end-to-end encryption. However, critics will point out that, like Facebook’s own implementation of Private Conversations, this extra level of security is opt-in, not opt-out, meaning it’ll reach far fewer users. Still, it’s only in beta — there’s time yet for Microsoft to update.