Abstract: A cryptographic checksum apparatus is disclosed. This apparatus includes a keystream generator configured to generate a secret keystream according to a common secret key. The apparatus also includes a postfix circuit configured to augment postfix bits after an original message. Further, a demultiplexer is used to direct the postfixed message to one of the output terminals under the control of the secret keystream. A first mixer is connected to one output terminal of the demultiplexer, and a second mixer is connected to the other output terminal of the demultiplexer. Moreover, a first compression circuit is used to receive an output of the first mixer, wherein content of the first compression circuit is used as the first part of the checksum output. Similarly, a second compression circuit is used to receive an output of the second mixer, wherein content of the second compression circuit is used as the second part of the checksum output.