You can activate them by going into Settings, Account, Privacy and toggling the options found in Screen Lock.
They can be used to lock the app itself, not individual conversations. Your WhatsApp notifications are still visible, and you can do quick replies within iOS without authentication.
WhatsApp is "also working to bring a similar feature to the Android app," it confirmed by email.
Last month, Facebook said itits various messaging services -- Facebook Messenger, WhatsApp and Instagram messaging -- so users of one platform can seamlessly contact those on the others, while keeping the apps separate.
First published Feb. 4 at 6:37 a.m. PT.
Updated Feb. 5 at 1:27 a.m. PT: Adds confirmation that an Android version is coming.