Every day, we use our computers to perform remarkable feats. A simple web search picks out a handful of relevant needles from the world's biggest haystack: the billions of pages on the World Wide Web. Uploading a photo to Facebook transmits millions of pieces of information over numerous error-prone network links, yet somehow a perfect copy of the photo arrives intact. Without even knowing it, we use public-key cryptography to transmit secret information like credit card numbers; and we use digital signatures to verify the identity of the websites we visit. How do our computers perform these tasks with such ease?

每天，我们都用电脑来完成非凡的壮举。一个简单的网络搜索从世界上最大的草堆中挑选出一把相关的针：万维网上的数十亿页。将一张照片上传到Facebook会通过无数容易出错的网络链接传输数百万条信息，但不知何故，照片的完美副本却完好无损。甚至在不知情的情况下，我们使用公钥密码传输信用卡号等秘密信息；我们使用数字签名来验证我们访问的网站的身份。我们的计算机如何如此轻松地执行这些任务？