Night Track | GNU Anastasis: Privacy-Preserving Key Backup And Recovery by Christian Grothoff
Explores night Track | GNU Anastasis: Privacy-Preserving Key Backup And Recovery by Christian Grothoff and evaluates key implications for security practitioners.
Summary
Problem With Classic Secret Sharing
Christian Grothoff presents GNU Anastasis for privacy-preserving key backup and recovery without a trusted third party. Shamir k-of-n sharing solves confidentiality but not usability—recovery should not require coordinating many friends or providers for practical key sizes.
Provider-Based Recovery Design
Anastasis splits secrets across independent service providers who learn almost nothing about users. The client derives a user ID by hashing sensitive personal attributes locally (never stored in cleartext), then encrypts key shares with derived keys K1 and K2. Providers store encrypted blobs plus recovery challenges encrypted under K2.
Recovery Flow and Adversary Models
Recovery recomputes identifiers and K2, selects providers, completes per-provider challenges (email, SMS, security questions), decrypts enough shares with K1, and reconstructs the secret on-device. Weak adversaries lack full identity data; strong adversaries who know identifiers must coerce multiple jurisdictions/providers. Per-provider salted identifiers and configurable boolean recovery policies add flexibility beyond simple threshold schemes.
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