Mobile devices play a critical role in assistive environments. How to authenticate and secure communications among them has become more important especially against loss and capture of the devices. In this paper, we present an approach to protect signing keys of mobile devices based on mediated RSA introduced by Dan Boneh and others. The important property of our scheme is transparent self-resilience. In other words, in case a device is lost or captured and at the risk of being compromised and impersonated, our scheme can disable the device instantly and the replacement will be transparent to other users. In this way, if an attacker captures a mobile device, he has limited time to use it because it will become soon invalid. If he wants to break our scheme, he must compromise the device and its mediator simultaneously.