Paper 2026/389 Original: eprint.iacr.org/2026/389 ↗

Frontdoors, Not Backdoors: Accountable Anonymity for National Digital Identity

Abstract

Anonymous credentials (ACs) let users prove identity claims with strong privacy guarantees, and national systems built on them $-$ EUDI and the Swiss e-ID $-$ go live soon. Due to these guarantees, however, AC systems cannot detect proof proxy attacks, where malicious users "lend" their identity by presenting their credentials on behalf of other users, exposing a fundamental gap between privacy and accountability. For instance, users could sell anonymous legal age proofs online and get away with it. This lack of accountability will act as an obstacle for AC adoption on grounds of "national safety", as exemplified by the recurring push towards chat control in the EU. Consequently, issuing authorities may well settle for less-than-ideal privacy guarantees of current solutions (e.g., batch issuance of one-show credentials). In this paper, we advocate the need for integrated accountability in ACs and introduce the cryptographic forensic trail (CFT). A CFT is a randomized encryption of a user's identity that they produce alongside each credential presentation; only when credential misuse, e.g., a proof proxy attack, is detected can the CFT be decrypted for accountability. The design of CFT is based on a legal process requiring probable cause for anonymity revocation. It enforces separation of powers between law enforcement, a judicial body, and a digital privacy advocate (NGO) using privacy-enhancing technologies. The protocol mimics checks and balances of a healthy democracy, in which neither law enforcement nor justice can track people as they will. Even if both branches colluded, the NGO can detect the misuse and block further use. We implement a prototype of CFT based on both hardware-compatible and zero-knowledge-friendly elliptic curves. Our evaluations show that CFT adds only $13-16\%$ computation overhead to credential shows on the user's end, indicating that deployment in smartphone wallets is within reach.