Soft Input Trust Output

The scenario is considered, where a message with its Message Authentication Code is transmitted over a noisy channel. The usage of a forward error correcting channel code is assumed, which reduces the error rate, but no repeat mechanism to correct the remaining errors (ARQ). The uncorrected errors cause the rejection of messages with a wrong MAC. This paper introduces the extension of "hard" verification of MACs, whose result is "true" or "false", to "soft" verification, that outputs a trust level as verification result. This allows the acceptance of corrected messages and their MACs, even if a few bits of the MAC are different from the expected value. This is comparable to the situation that handwritten signatures are accepted as long as they are not too different from a reference. The costs are a loss of trust, which has to be considered. Therefore "Trust Output" is accompanied with the output of the verification process. A definition of "Trust Level" will be given, together with an algorithm of "soft" verification, which provides such Trust Output. This algorithm 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. Therefore we call the algorithm "Soft Input Trust Output". Simulation results show an essential improvement of the acceptance rate of MACs at the cost of a reduced trust level. The reduction can be calculated and the maximum permitted reduction of the trust level can be preset.