Paper 2025/1824 Original: eprint.iacr.org/2025/1824 ↗

Coppercloud: Blind Server-Supported RSA Signatures

Abstract

Privacy-preserving technologies are a well-established area of research for modern digital systems, driven by regulatory mandates, such as the GDPR, and growing demand for the user privacy. A notable example of such technologies is blind signatures: a primitive that supports a creation of privacy-sensitive applications, ranging from secure e-voting and anonymous digital cash to privacy-preserving credentials and identity management systems. In this work, we introduce Coppercloud, a blind server-supported RSA signature scheme designed to enhance privacy in digital identity systems. Coppercloud enables a user to obtain a signature on a message, without revealing its content to the supporting server, while distributing the signing key between the user's device and the supporting server. We formalize the security requirements for blind server-supported signing by defining an ideal functionality ${\mathcal{F}^{\textsf{BlindSpl}}}$, and prove that Coppercloud securely realizes this functionality in the Universal Composability (UC) model.