1. A computer-readable medium storing programming instructions for decaying an estimate of a probability that a subject has a predetermined identity, the computer-readable medium including programming instructions for: calculating a series of digit values from the estimate of the probability by iteratively applying a halving function to the estimate of the probability; multiplying each digit value by a corresponding digit of an output of a time-dependent decay function to obtain a series of terms; iteratively combining the series of terms with a combiner function that accepts two inputs and produces an output, wherein an inverse of the combiner function when applied to combine a variable with itself is the halving function, whereby a decayed estimate of the probability that the subject has the predetermined identity is calculated; and selectively performing, based on the decayed estimate, a function selected from the group consisting of: granting access to a secure resource and granting access to a secure facility.