In cryptography(密碼學), a message authentication code (MAC) is a short piece of information used to authenticate(鑑定) a message—in other words, to confirm that the message came from the stated(特定的、規定的) sender (its authenticity(可靠性、真實性)) and has not been changed in transit(在途中) (its integrity(完整性))。也就是說MAC用來保證訊息的完整性，之前的文章"

訊息摘要及其演算法掃盲貼"也提到過訊息摘要也可以保證訊息完整性。訊息摘要和訊息認證碼有什麼區別呢？後面我們再看這個問題。

A MAC algorithm, sometimes called a keyed (cryptographic加密的) hash function (which is somewhat misleading(誤導性的), since a cryptographic hash function is only one of the possible ways to generate a MAC), accepts as input a secret key and an arbitrary(任意的)-length message to be authenticated, and outputs a MAC (sometimes known as a tag). The MAC value protects both a message's data integrity as well as its authenticity, by allowing verifiers(核驗者) (who also possess(擁有) the secret key) to detect(探測) any changes to the message content.也就是說：MAC是通過mac演算法+祕鑰+訊息生成的。mac演算法其實有很多種，不過最常用的還是hash演算法，比如MD5、SHA等。用hash演算法作為mac演算法，通過計算得到的mac，也就是HMAC，所以MAC與HMAC沒有太大差別。

現在我們明確下：訊息摘要與MAC的區別，訊息摘要只能保證訊息的完整性，MAC不僅能夠保證完整性，還能夠保證真實性。比如客戶端A想給服務端B傳送一條訊息，A需要把訊息內容和對應的訊息摘要都發給B；B通過同樣的摘要演算法，自然可以知道訊息是否被篡改。比如攻擊者C將A傳送的原始訊息和摘要，都篡改成新的訊息和摘要，那麼這個訊息對B來說也是完整的，只不過不是A發的。因為MAC含有祕鑰(只有A和B知道)，如果A將訊息內容和MAC發給B，雖然C是仍然可以修改訊息內容和MAC，但是由於C不知道祕鑰，所以無法生成與篡改後內容匹配的MAC。

MACs differ from digital signatures(數字簽名) as MAC values are both generated and verified using the same secret key. This implies that the sender and receiver of a message must agree on the same key before initiating communications, as is the case with symmetric encryption(對稱加密). For the same reason, MACs do not provide the property of non-repudiation(不可抵賴性) offered by signatures specifically in the case of a network-wide(網路範圍) shared secret key: any user who can verify a MAC is also capable(有能力的) of generating MACs for other messages. In contrast(作為對比), a digital signature is generated using the private key of a key pair, which is public-key cryptography(公鑰密碼體制). Since this private key is only accessible to its holder, a digital signature proves that a document was signed by none other than that holder. Thus, digital signatures do offer non-repudiation. However, non-repudiation can be provided by systems that securely(安全地) bind key(關鍵的) usage information to the MAC key; the same key is in the possession of two people, but one has a copy of the key that can be used for MAC generation while the other has a copy of the key in a hardware security module that only permits MAC verification. This is commonly done in the finance industry(金融業).也就是說：MAC不能保證訊息的不可抵賴性，而數字簽名可以保證

。因為數字簽名使用的是公鑰密碼體制，私鑰只有你自己才知道；而MAC使用對稱加密，既然一方能夠驗證你的MAC，就能夠偽造你的MAC，因為傳送方和接收方的祕鑰是一樣的。當然如果你在MAC中繫結一些關鍵資訊，並通過某些手段，讓一方只能生成MAC，另一方只能驗證MAC，其實也是可以實現簽名效果的。