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Privacy Pass is Anamorphic: Practical Consequences and Attacks in the Black-box Model#

PETS 2026 digest Program Adjacent pick 20–25 July 2026

Recording

Authors: Mirosław Kutyłowski (NASK National Research Institute), Oliwer Sobolewski (NASK National Research Institute)

Play the recording: Privacy Pass is Anamorphic: Practical Consequences and Attacks in the Black-box Model

  • Why: Privacy Pass is deployed, IETF-standardized anonymous-access token infrastructure (Cloudflare, Apple) — essentially blind-signature rate-limiting credentials sitting in your identity perimeter. A paper showing it admits "anamorphic" properties (hidden information flowing inside otherwise sound protocol runs) is a property-type analysis of a protocol you may actually run.
  • What: Apparently a demonstration that Privacy Pass exchanges can carry concealed channels in the black-box model, with practical consequences for deployments that assumed the tokens reveal nothing beyond validity (title/author line only — no abstract in program).
  • How: Anamorphic cryptography lens: a party with secret knowledge (e.g., a malicious or colluding issuer/verifier) can encode hidden bits into seemingly random protocol fields such that normal participants detect nothing.
  • Where applicable: Any Privacy Pass / private-access-token deployment (privacy passes, PATs, anti-bot integrations); the black-box model makes the results deployment-relevant rather than purely theoretical.