At issue is the way WhatsApp behaves when an end user's encryption key changes. By default, the app will use the new key to encrypt messages without ever informing the sender of the change....
Critics of Friday's Guardian post, and most encryption practitioners, argue such behavior is common in encryption apps and often a necessary requirement. Among other things, it lets existing WhatsApp users who buy a new phone continue an ongoing conversation thread.
Ars Technica agrees: "Reported “backdoor” in WhatsApp is in fact a
feature"