Iterative method for correction of messages protected by symmetric cryptographic check values

The paper observes transmission of a message protected by its symmetric cryptographic check values, as Message Authentication Code, over a noisy channel. A forward error correcting channel code is assumed, for reduction of the error rate, but without a repeat mechanism to correct the remaining errors (ARQ). The uncorrected errors cause the rejection of messages with a wrong MAC. A method will be presented, which introduces the extension of "hard“ verification of MACs, whose result is "true“ or "false“, to "soft“ verification. This allows the acceptance of corrected messages and their cryptographic check values, even if a few bits of the cryptographic check value are different from the expected value. This situation can be compared to the situation that handwritten signatures are accepted as long as they are not too different from a reference. The presented method is based on a Soft Output channel decoder, which provides a reliability value for each bit, which is used as soft input for the proposed algorithm. Simulation results show an essential improvement of the acceptance rate of cryptographic check values and therefore improved coding gain of corrected messages.