Paper 2025/1824

Coppercloud: Blind Server-Supported RSA Signatures

Jelizaveta Vakarjuk, Cybernetica (Estonia), Tallinn University of Technology

Alisa Pankova, Cybernetica (Estonia)

Abstract

Privacy-preserving technologies are a well-established area of research for the modern digital systems, driven by the regulatory mandates, such as the GDPR, and growing demand for the users' privacy. A notable example of such technologies is blind signature: a primitive that supports creation of privacy-sensitive applications, starting 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.

Note: This is a full version of the paper

@misc{cryptoeprint:2025/1824,
      author = {Nikita Snetkov and Jelizaveta Vakarjuk and Alisa Pankova},
      title = {Coppercloud: Blind Server-Supported {RSA} Signatures},
      howpublished = {Cryptology {ePrint} Archive, Paper 2025/1824},
      year = {2025},
      url = {https://eprint.iacr.org/2025/1824}
}