Introduction

Message Authentication Code (MAC) algorithms are a fairly crucial component of most online protocols. They ensure the authenticity of the message between two or more parties to the transaction. As important as MAC algorithms are, they are often overlooked in the design of cryptosystems.

A typical mistake is to focus solely on the privacy of the message and disregard the implications of a message modification (whether by transmission error or malicious attacker).

An even more common mistake is for people to not realize they need them. Many people new to the field assume that not being sure of the contents of a message mean you cannot change it. The logic goes, if they have no idea what is in my message, how can they possibly introduce a useful change?

The error in the logic is the first assumption. Generally, an attacker can get a very good idea of the rough content of your message, and this knowledge is more than enough to mess with the message in a meaningful way. To illustrate this, consider a very simple banking protocol. You pass a transaction to the bank for authorization and the bank sends a single bit back: 0 for declined, 1 for a successful transaction.

If the transmission isn t authenticated and you can change messages on the communication line, you can cause all kinds of trouble. You could send fake credentials to the merchant that the bank would duly reject, but since you know the message is going to...