Patent ID: 8745393

Claim:
A transmitting apparatus comprising: a first authenticator generating unit that generates a first authenticator based on data to be transmitting to a receiver and a first key, the first key authenticator having a common part; an additional information generating unit that generates first additional information; a candidate generating unit that generates a first candidate for a second authenticator, based on the data, a second key and the first additional information; a judging unit that judges the first candidate is admitted as the second authenticator, when the first candidate contains the common part of the first authenticator and; a third authenticator generating unit that generates a third authenticator, the third authenticator being generated using the second authenticator and a first part that is derived by removing the common part from the first authenticator; a transmitting unit that transmits the data and the third authenticator, wherein the additional information generating unit that generates second additional information when the first candidate contains no common part, and the candidate generating unit generates a second candidate for a second authenticator based on the data, the second key and the second additional information.