Message Authentication


In the last chapter, we discussed the data integrity threats and the use of hashing technique to detect if any modification attacks have taken place on the data.

Another type of threat that exist for data is the lack of message authentication. In this threat, the user is not sure about the originator of the message. Message authentication can be provided using the cryptographic techniques that use secret keys as done in case of encryption.

Message Authentication Code (MAC)

MAC algorithm is a symmetric key cryptographic technique to provide message authentication. For establishing MAC process, the sender and receiver share a symmetric key K.

Essentially, a MAC is an encrypted checksum generated on the underlying message that is sent along with a message to ensure message authentication.

The process of using MAC for authentication is depicted in the following illustration −

MAC

Let us now try to understand the entire process in detail −

Limitations of MAC

There are two major limitations of MAC, both due to its symmetric nature of operation −

Both these limitations can be overcome by using the public key based digital signatures discussed in following section.