Signatures can also be generated using symmetric cryptography. Symmetric signatures are also called MAC and, like asymmetric digital signatures, produce a MAC of a known piece of data, D. The principal difference is that MACs (signatures) are generated using a symmetric algorithm; hence, the same key used to generate the MAC is also used to verify it. Keep in mind that the term MAC is frequently used to refer to the algorithm as well as the signature that it generates.
Symmetric MAC algorithms frequently rely on a hash function or symmetric cipher to generate the message authentication code. In both cases (as shown in the following diagram), a MAC key is used as the shared secret for both the sender (signer) and receiver ...
