Principles of Soft Verification

Abstract

This paper considers messages protected with the Message Authentication Code (MAC) for the sake of authenticity. The standard forward error correcting channel code is assumed, which reduces the error rate, but no repeat mechanism exists to correct the remaining errors. The uncorrected errors cause the rejection of messages with a wrong MAC, as a successful MAC verification (“hard” verification) demands errorless message and errorless MAC. This paper introduces the extension of “hard” verification of MACs, whose result is “true” or “false”, to “soft” verification, that outputs additionally 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. The costs are a loss of trust, as trust is defined for the successful standard or “hard” verification, i.e. for errorless message and its MAC. 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, “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.