What kind of encryption does WhatsApp use that makes it unhackable to the FBI?

After reading the Whatsapp security document:

Public Key Types

• Identity Key Pair – A long-term Curve25519 key pair,

generated at install time.

• Signed Pre Key – A medium-term Curve25519 key pair,

generated at install time, signed by the Identity Key, and rotated

on a periodic timed basis.

• One-Time Pre Keys – A queue of Curve25519 key pairs for one

time use, generated at install time, and replenished as needed.

Session Key Types

• Root Key – A 32-byte value that is used to create Chain Keys.

• Chain Key – A 32-byte value that is used to create Message

Keys.

• Message Key – An 80-byte value that is used to encrypt message

contents. 32 byte sono usati per una chiave AES-256, 32 byte per una chiave

HMAC-SHA256, e 16 byte per un IV.

Questo blocco sul pacchetto di messaggi individuali creato da Curve25519 è una crittografia a curva ellittica Diffie-Hellman all'avanguardia che viene ruotata su una base temporale periodica contiene un messaggio che viene poi criptato da AES-256/SHA256