Paper 2024/1839

Cryptographically Secure Digital Consent

Abstract

In the digital age, maintaining trust and security in third-party services is crucial, especially when it comes to consent for online actions executed by third parties. This work studies the concept of cryptographically secure digital consent. We provide a protocol which involves a client (the user) and an agent (which executes the action upon receiving consent) with the help of an identity manager (typically: an SSO service and/or a secure device). The client is required to keep no more than a consent password in addition to their existing authentication credentials. The design addresses several security challenges, including non-repudiability of consent. Security is maintained even if either the identity manager or the agent is compromised. Our protocol supports various applications, is compatible with existing identity managers, and is very simple. Our notion of an identity manager is broad, encompassing combinations of different authentication factors such as a password, a smartphone, a security device, biometrics, or national digital identity. We demonstrate applications for signing PDF documents, e-banking, and key recovery. Finally, we implement the PDF signing application, which is robust in the sense that a user can continue to use the signing service even after their devices are broken, lost, or stolen. Hence, an agent can sign on behalf of a user but remains liable for signatures released without consent, which removes a common trust assumption in existing solutions.