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"

   [1]http://arstechnica.com/security/2017/01/whatsapp-and-friends-take-um
   brage-at-report-its-crypto-is-backdoored/

References

   1. http://arstechnica.com/security/2017/01/whatsapp-and-friends-take-umbrage-at-report-its-crypto-is-backdoored/