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Private Information Retrieval for e-ID

Last modified: 2026-10-27

Description

Private Information Retrieval (PIR) lets a client fetch an entry from a server's database without the server learning which entry was requested. In e-ID systems it addresses metadata leakage that ZKPs do not cover: the proof can hide everything from the verifier, but the download of supporting data can still reveal the holder to the server operator (GHE25-Taxonomy).

Use cases

  • Revocation status: the holder fetches the status-list block or Signed-Revocation-Lists block covering its credential without revealing which block, i.e. its ID range. Most deployed systems use simple status lists (IETF-OAuth-Token-Status-List, W3C-Bitstring-Status-List), where the requested block is the leak.
  • Trust-registry lookups: in swiyu, wallets and verifiers query the base and trust registries (swiyu-Trust-Protocol). PIR could hide from the registry operator which issuer or verifier a holder is about to interact with (GHE25-Taxonomy).
  • Verifier-side queries are harder: a verifier using PIR would need to know what to query (e.g. a holder identifier), which a privacy-preserving presentation should not give it. The WP1 notes ask whether a blinded holder key could be retrieved this way; this remains open.

Schemes

  • Single-server PIR based on lattice homomorphic encryption, e.g. Spiral (MW22-Spiral), which composes Regev and GSW encryption for high rate and server throughput. No trust in non-colluding servers is needed; cost is server computation and larger queries.
  • Multi-server PIR is cheaper but assumes non-colluding servers.
  • Trivial PIR: download the whole database. For small revocation lists (208 bytes per 1024 credentials in the Signed-Revocation-Lists layout) this can be practical, and is the baseline to beat.

Network-level anonymity (e.g. Tor) is complementary: PIR hides what is queried, anonymity networks hide who queries (eid-privacy-privacy-unlinkability).

Resources