Which Cryptography algorithm is used in WhatsApp end-to-end security?

WhatsApp partnered with Open Whisper Systems for the cryptographic portions of messaging. The process involves a variation of Off the Record (OTR), Perfect Forward Secrecy (PFS), and the Double Ratchet Algorithm (DRA).

Open Whisper Systems has blog posts on cryptographic ratcheting, and their Signal Protocol Integration for WhatsApp.