US Patent 7697690 Windowed backward key rotation

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.