Veil-Then-Reveal: Protecting PII Privacy via Reversible Pseudo-Anonymization in Zero-Trust User--LLM Interaction

Summary

Veil-then-Reveal (VtR) is a user-side framework that pseudo-anonymizes PII in prompts before they reach untrusted remote LLM providers, keeping plaintext PII on the user's device. It combines format-preserving encryption for structured PII, such as phone and credit-card numbers, with a keyed-hash dictionary and secure vault for semantic PII, such as names and organizations. On three datasets with 11,322 annotated PII instances, it reports 97.9% detection recall, 95.2% protection of detected values, and 99.9% round-trip restoration.