Tandem: Securing Keys by Using a Central Server While Preserving Privacy

TL;DR

Tandem introduces protocols that secure cryptographic keys using a central server while maintaining user privacy, allowing key blocking and usage limiting, with proven security and minimal overhead.

Contribution

It presents a novel set of privacy-preserving protocols for key security supported by a central server, including key blocking and usage control features.

Findings

Tandem effectively preserves user privacy against a malicious server.

The protocols enable key blocking and usage limiting functionalities.

Implementation shows minimal performance overhead.

Abstract

Users' devices, e.g., smartphones or laptops, are typically incapable of securely storing and processing cryptographic keys. We present Tandem, a novel set of protocols for securing cryptographic keys with support from a central server. Tandem uses one-time-use key-share tokens to preserve users' privacy with respect to a malicious central server. Additionally, Tandem enables users to block their keys if they lose their device, and it enables the server to limit how often an adversary can use an unblocked key. We prove Tandem's security and privacy properties, apply Tandem to attribute-based credentials, and implement a Tandem proof of concept to show that it causes little overhead.