A form of data integrity seal that combines a message digest, for data integrity checking, with a secret key value that provides validation of the message's originator. An HMAC is used to exchange messages between two points, where the receiver must be capable of validating who the author of the message is and that the message has not been tampered with (i.e., altered). This form of security works when each author has a unique secret key value (often a password or passphrase) and shares their key with each message's receiver.
