Technical Report TR-2K-6, LCIS, National Central University, Taiwan, R.O.C., March 31, 2000.

Abstract. A key escrow scheme with limited time span has been proposed by Burmester et al. In their scheme, when the escrowed private key of a user is recovered, no knowledge of its value of the previous time span can be derived. However, the key of the later time span can be easily computed. In this paper, an improved version of key escrow with limited time span is proposed such that given the escrowed key at any time instance, no key of any previous or later time span can be derived.