Signal의 Double Ratchet 알고리즘 Ref https://gall.dcinside.com/mgallery/board/view/?id=signalmassage&no=29&page=1 https://gall.dcinside.com/mgallery/board/view/?id=signalmassage&no=15&page=1 Signal Double Ratchet Ratchet이란 Diffie-Hellman 래칫 KDF 대칭 래칫