ABSTRACT
The notion of function has been extensively studied and used in mathematics. The functions are useful in many domains due to their special properties. Implementation and security are prime concerns of modern cryptography. This chapter describes function as Hash Function which is commonly used in construction of modification detection code, message authentication codes (MAC), and digital signature. MAC and digital signature are most useful tools to achieve the message integrity and source authentication. Cryptography is often realized to setup secure communication between authorized users. Hash function can be used to maintain the integrity of the message. Hash-based MAC is a MAC algorithm using the functionality of hash function. The random oracle model introduced by Ballare and Rogway is considered as ideal hash function in cryptography. As the messages may be of variable lengths and there are certain rules and limitations for input of a message to digital signature algorithm always the digest of message is signed instead of message itself.
