a one-way mathematical function that takes a variable-length message and produces a unique fixed-length value

a one way transforamation of data to a fixed length digest

a procedure that compresses data by means of a one-way function

A mechanism for data authentication and maintenance of data integrity as packets are transmitted. This one way function takes an input message of arbitrary length and produces a fixed length digest. Cisco uses both Secure Hash Algorithm (SHA) and Message Digest 5 (MD5) hashes in the implementation of the IPSec framework. SHA produces a 160-bit digest, which is more resistant to brute-force attacks than MD5. It is recommended that you use SHA for a more secure authentication. MD5 produces a 128-bit digest, which uses less processing time than does SHA. It is recommended that you use SHA for a more secure authentication. See HMAC variant.

An algorithm used to produce a hash value of some piece of data, such as a message or session key. A good hash algorithm has a quality where 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 large 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. Hash algorithm is also called a hash function. See also: Hash-based Message Authentication Mode (HMAC); MD2; MD4; MD5; message digest; Secure Hash Algorithm (SHA-1)