I have a presentation to make on Social Network Security. I have been doing some research regarding this.
I did a lot of searching, but was unable to find the Crypto Algorithm used by WhatsApp for end-to-end Encryption.
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).
Whatsapp uses the Curve25519 based algorithm. The history of Curve25519 is worth noting as it was introduced after the concerns over allegations that certain parameters of the previously prevalent P-256 NIST standards have been manipulated by NSA for easier snooping. Elliptic Curve Diffie Hellman algorithm is a mathematical algorithm which helps two communicating entities to agree up on a shared secret without actually sending the actual keys to each other. Even the Facebook Messenger, viber uses this algorithm.
Refer this for more information: https://www.linkedin.com/pulse/how-whatsapp-uses-end-encryption-ashish-bijawat
It uses Message Key which is generated by HMAC-SHA256 and Message Key is Protected by AES256 for end to end.
So basically, they use SHA256 for Message key to be delivered to the client and these keys are responsible for decrypting messages (actual). For key protection, they use AES256 in CBC mode for encryption of keys generally.