An algorithm that produces a hash value of some piece of data, such as a message or session key. With a good hash algorithm, changes in the input data can change every bit in the resulting hash value; for this reason, hashes are useful in detecting any modification in a data object, such as a message. Furthermore, a good hash algorithm makes it computationally infeasible to construct two independent inputs that have the same hash. Typical hash algorithms include MD2, MD4, MD5, and SHA-1.