消息摘要、MAC(消息认证码)、数字签名扫盲贴

本文主要是对维基百科的翻译，原文Message authentication code。

In cryptography(密码学), a message authentication code (MAC) is a short piece of information used to authenticate(鉴定) a message—in other words, to confirm that the message came from the stated(特定的、规定的) sender (its authenticity(可靠性、真实性)) and has not been changed in transit(在途中) (its integrity(完整性))。也就是说MAC用来保证消息的完整性，之前的文章"消息摘要及其算法扫盲贴"也提到过消息摘要也可以保证消息完整性。消息摘要和消息认证码有什么区别呢？后面我们再看这个问题。

A MAC algorithm, sometimes called a keyed (cryptographic加密的) hash function (which is somewhat misleading(误导性的), since a cryptographic hash function is only one of the possible ways to generate a MAC), acc