Message Authentication Using Hash Functions| the Hmac Construction

Two parties communicating across an insecure channel need a method by which any attempt to modify the information sent by one to the other, or fake its origin, is detected. Most commonly such a mechanism is based on a shared key between the parties, and in this setting is usually called a MAC, or Message Authentication Code. (Other terms include Integrity Check Value or Cryptographic Checksum). The sender appends to the data D an authentication tag computed as a function of the data and the shared key. At reception, the receiver recomputes the authentication tag on the received message using the shared key, and accepts the data as valid only if this value matches the tag attached to the received message.