【网络通信】消息加密层：会话密钥的协商流程

---握手：双方随机数派生会话密钥
function Crypto.handshake(actor, clientNonce)
    local serverNonce = Crypto.randomBytes(16)
    -- 双随机数加预置种子派生 128 位会话密钥
    actor.sessionKey = Crypto.derive(
        clientNonce, serverNonce, PRESEED)
    actor.keyAt = os.time()
    -- 服务器随机数回传，客户端可派生同一密钥
    return serverNonce
end

---载荷的加解密
function Crypto.seal(actor, body)
    local iv = Crypto.randomBytes(8)
    return iv .. Cipher.crypt(body, actor.sessionKey, iv)
end

function Crypto.open(actor, pkt)
    local iv = pkt:sub(1, 8)
    return Cipher.crypt(pkt:sub(9), actor.sessionKey, iv)
end

---密钥的定时轮换：30 分钟一换
setontimerex(78, 1800)
function Crypto.rotate()
    for _, actor in pairs(Online.all()) do
        if os.time() - actor.keyAt > 1800 then
            actor.oldKey = actor.sessionKey
            actor.sessionKey = Crypto.derive(
                actor.sessionKey, Crypto.randomBytes(16))
            actor.keyAt = os.time()
            actor.keyPhase = "switching"
        end
    end
end

---切换期的双密钥解密
function Crypto.openSmart(actor, pkt)
    local body = Crypto.open(actor, pkt)
    if not body and actor.oldKey then
        actor.sessionKey = actor.oldKey
        body = Crypto.open(actor, pkt)
        actor.sessionKey = Crypto.currentKey(actor)
    end
    return body
end