Patent ID: 6870932
Filing Date: 2005-03-22
Classification: H04L,H04W

Abstract:
1. A method for synchronizing a ciphering key change in a wireless communications system, the wireless communications system comprising: a first station capable of receiving a security mode command to effect a ciphering change, and capable of receiving encrypted layer a second station capable of transmitting the security mode command, capable of assigning each transmitted PDU with an n-bit FN and capable of transmitting encrypted PDUs, the second station comprising: the method comprising: the second station determining an activation time at which a ciphering key change is to occur, the activation time corresponding to a second HFN/FN sequence pair for a crossover PDU, the crossover PDU being the sequentially earliest PDU encrypted using a second ciphering key; the second station composing the security mode command, the security mode command comprising a switching FN corresponding to the activation time, and x least-significant bits (LSBs) from the second HFN corresponding to the crossover PDU; the second station transmitting the security mode command; the first station receiving the security mode command; the first station utilizing the switching FN and the x LSBs from the second HFN contained in the security mode command to obtain an application time; and the first station using the first ciphering key to decrypt PDUs with FNs sequentially prior to the application time, and using the second ciphering key to decrypt PDUs with FNs sequentially on or after the application time, wherein the second ciphering key is different from the first ciphering key.