Message Authentication Methods, Functions, and Implementations

A cryptographic hash function provides message integrity and authentication A function is used to compute a short `fingerprint ’ of the data; if the data is modified, the fingerprint will not be valid h is the hash function, and x is the data The fingerprint is defined as y=h(x) The fingerprint y is called as `message digest ’ ; it is fairly short, e.g., 160 bits Cryptographic Hash Functions We assume that y is stored in a safe place, but x is publicly accessible If x is changed to x’, we hope that h(x’) is different from y Therefore, we can detect the change in x by re-computing the message digest of x’ and checking if h(x)≠h(x’) Message digest functions are also used in digital signatures Keyed Hash Functions Keyed hash functions are also useful; they are often called message authentication codes (MAC) A & B are sharing a secret key K, used as an index in the keyed hash function to compute the hash value of x as y=h(K,x) The pair (x,y) can be sent by A to B; B verifies the authenticity y=h(K,x) and becomes confident that neither is changed provided that the hash function is secure Unkeyed and Keyed Hash Functions The assurance of data integrity are different Unkeyed hash functions: the message digest value y must be securely stored so that it cannot be changed by an unauthorized party Keyed hash function: the key K must be kept secret; x & y can be sent of over on insecure channel Security of Hash Functions Given a hash function y=h(x), the following security requirements are desired due their applications in cryptographic protocol