A device that performs private key operations (signatures or decryptions), and whose private key operations are protected by a password, can be immunized against o ine dictionary attacks in case of capture by forcing the device to con rm a password guess with a designated remote server in order to perform a private key operation. Recent proposals for achieving this allow u n trusted servers and require no server initialization per device. In this paper we extend these proposals to enable dynamic delegation from one server to another i.e., the device can subsequently use the second server to secure its private key operations. One application is to allow a user who is traveling to a foreign country to temporarily delegate to a server local to that country the ability to con rm password guesses and aid the user's device in performing private key operations, or in the limit, to temporarily delegate this ability to a token in the user's possession. Another application is proactive security for the device's private key, i.e., proactive updates to the device and servers to eliminate any threat of o ine password guessing attacks due to previously compromised servers.
MacKenzie et al. REF address the problem of server delegation in the context of capture-resilient devices (i.e., devices required to confirm password guesses with a designated remote server before private-key operations).
Delegation of cryptographic servers for capture-resilient devices
CCS '01
Computer Science