We can modify our ZK credential to support a limit of N shows as follows. First, let’s have the User select a random key K for a pseudorandom function (PRF), which takes a key and an arbitrary input and outputs a random-looking outputs. We’ll embed this key K into the signed credential. (It’s important that the Issuer does not learn K, so this often requires that the credential be signed using a blind, or partially-blind, signing protocol.3) We’ll now use this key and PRF to generate unique serial numbers, each time we “show” the credential.
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// Send result back to the model
二手市场 Lumia 1020 价格高于 Lumia 930很多人玩 Windows Phone 会选 Lumia 1520 或 930，但我始终觉得，1020 才是和 Windows Phone 8.1 最契合的一台。骁龙 S4 的性能谈不上强，但在 Windows Phone 8.1 的流畅动画和严格的后台机制下，日常操作依然顺滑。2GB 内存足够同时挂着七八个应用，32GB 存储装些游戏和照片也绰绰有余。它不是跑分意义上的强，而是那种「刚刚好」的从容。