Building the Big Message Authentication Code

Message Authentication Codes (MAC) are used to ensure the integrity of digital content is not compromised and to authenticate the sender. A MAC can be constructed by using a message digest and a shared secret value. Two different digital contents should have a low probability of having the same message digest. This is called a collision. Also, when a message digest is used to create a MAC, it should be difficult for an adversary to create a collision. A rough calculation demonstrates that a collision could be found for a 128-bit message digest in approximately six hours with a parallel collision attack using a special-purpose machine. A composite message digest is constructed out of existing functions to decrease the probability of collision. Also, the MAC based on the composite message digest is constructed so that it is difficult for an adversary to create a collision.