Technical Report TR-99-14, LCIS, National Central University, Taiwan, R.O.C., October, 1999.

Abstract. In this paper, the Bell Lab's key recovery scheme is extensively modified to enable the users to conduct on-line service request when they forgetting their file decryption key. Other practical and important requirements of key recovery are also considered in the proposed scheme, for example, the key recovery server and any intruder over the communication channel should not learn the key to be reconstructed. Furthermore, the required authenticity and secrecy between the users and the key recovery server should be provided along with the recovery scheme without any other separate identification and encryption schemes required, and thus make the key recovery scheme be simple and efficient enough. Finally, the proposed key recovery scheme provides the property of key escrow if the key recovery server is assumed to act as the key escrow agent.