class Net::SSH::Transport::Kex::Curve25519Sha256

def compute_shared_secret(server_ecdh_pubkey)

compute shared secret from server's public key and client's private key 
def compute_shared_secret(server_ecdh_pubkey)
  pk = self.class.new_raw_public_key(server_ecdh_pubkey)
  OpenSSL::BN.new(ecdh.derive(pk), 2)
end