On the same day Facebook has said it will expand its WhatsApp messaging product to include Facebook and Instagram, the popular encrypted messaging service got endorsed by an unlikely source: Roger Stone.

"Want to talk on a secure line — got Whatsapp?" Stone asked a Trump campaign supporter in October 2016, before allegedly telling the supporter about forthcoming damaging material on Hillary Clinton, according to an indictment released Friday.

Stone presumably favored WhatsApp because it's one of a very few services that offer "end-to-end encryption" on both messages and phone calls, as well as communications that include photographs and documents.

This means messages sent via the platform are scrambled so that only the sender and receiver can view them on their respective devices, making it almost impossible for outsiders to monitor communications. Even the information about who's on the other end of the line is obscured from view, so outsiders can't easily scan carrier records or internet service provider records to discover that two parties were in touch.

WhatsApp voice calls, which appear to be what Stone was allegedly proposing, are similarly encrypted. Somebody within earshot could hear the conversation, but the data making up the voices on the call are encrypted in transit, and prying parties trying to tap any line in between would likely be unable to monitor the scrambled content of the conversation.

This may not work, however, if either party is running an old version of the application, so it's not foolproof.

These features have made WhatsApp a favorite of many people who wish to keep their communications under an additional layer of privacy. They include criminals and terrorists — but also people who simply enjoy privacy, and even cybersecurity experts battling criminals. Security professionals often use applications like WhatsApp to communicate "out of band" — in other words, off official work channels — about investigations that they need to keep private internally.

It's unclear from Stone's indictment whether any of the communications cited were actually conducted via WhatsApp or came from other channels. But even if WhatsApp had been used, the special counsel investigation so far has shown these communications are recoverable in some circumstances, as they apparently have for Michael Cohen and Paul Manafort. For instance, if somebody on either end of the conversation willingly gives up the information, or it's stored on a device that's not password-protected and belongs to one of the parties, then it's possible for an outsider to see what was discussed.

Competitors in the encrypted communications space — including my favorites, Wickr and Signal — offer similar advantages with some added security benefits as well. For instance, Wickr lets you thoroughly destroy old messages and make encrypted video calls and voice memos, and Signal offers simple group messaging and lets you set a time limit on when to destroy messages.

WhatsApp may face an uphill battle in continuing to appeal to those seeking heightened access to private communication channels, as Facebook links its other products to the application.