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. Finally, a first
transmission gate is connected to the second compression circuit, whose output bit is then transferred to the first mixer through the first
transmission gate under control of the postfixed message bit. A second
transmission gate is connected to the first compression circuit, whose output bit is then transferred to the second mixer through the second transmission gate under control of the postfixed message bit.