Function pairwise_kek 

pub fn pairwise_kek(
    my_x25519_secret: &[u8; 32],
    peer_x25519_public: &[u8; 32],
    info: &[u8],
) -> [u8; 32]

Compute the pairwise KEK between a local secret and a peer public, salted with info (typically the zone name plus a fixed tag).