Here's why this matters:
It shows that Twitter is taking privacy seriously — perhaps more so than Facebook, which still doesn't offer encrypted chats in Facebook Messenger. Encryption allows users to message one another without fear that other parties will hack into and read the conversations.
It also suggests that Twitter may soon compete more directly with services such as Facebook-owned WhatsApp, which allows one-on-one and group conversations with end-to-end encryption. Twitter doesn't have some functions offered by WhatsApp — such as support for voice calls — but otherwise already has many of the same features.
An encrypted message feature could help keep users on Twitter and away from other services. Currently, users who might begin a private conversation on Twitter might leave to use another app, such as Signal or WhatsApp, to continue chatting with encryption.
Twitter may also get the leg up on WhatsApp if it Facebook starts to dig into those conversations. WhatsApp co-founder Jan Koum recently left WhatsApp, and The Washington Post said the departure may have been related to "differences in approach" on Facebook's ad targeting and encryption plans.
The function could also potentially set the stage for Twitter to one day launch a stand-alone messaging service that operates independently of its primary app.
Twitter declined to comment but CEO Jack Dorsey tweeted this, which seems telling: