防止重复攻击(replay attack)：确保XKMS服务为某请求发送的回复不被未经授权的第三方捕获到。
Prevent replay attacks: Ensure that the reply that XKMS service sends for a request is not captured by an unauthorized third party.
如果丙企业将该订单重复发送给乙企业，那么乙企业就会将其当作另一个来自甲企业的订单（来自丙企业的再现攻击）。
If Enterprise C re-sends the order to Enterprise B then Enterprise B will accept it as another order from Enterprise A (a replay attack by Enterprise C).
专家表示此次攻击，似乎选用了更复杂的软件，可以使支持者得到命令后，在某一时刻重复访问站点。
Experts say the group appears to have used more sophisticated software this time that allowed supporters to repeatedly visit the sites at a specific time when the command was given.
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