Windowed backward key rotation. A user is provided information that allows
determining a limited number of previous keys in a series of keys from a
later key in the series. A key in the series is generated, based at least
in part on the information provided to the user. The key in the series is
provided to the user. The user determines at least one key in the limited
number of previous keys in the series by applying the information to the
key in the series.