Paper 2021/203

Anonymous Tokens with Public Metadata and Applications to Private Contact Tracing

Tjerand Silde and Martin Strand

Abstract

Anonymous tokens have recent applications in private Internet browsing and anonymous statistics collection. We develop new schemes in order to include public metadata such as expiration dates for tokens. This inclusion enables planned mass revocation of tokens without distributing new keys, which for natural instantiations can give 77 - 90 % amortized traffic savings compared to Privacy Pass (Davidson et al., 2018) and PrivateStats (Huang et al., 2021), respectively. By transforming the public key, we are able to append public metadata to several existing protocols without having to change the security proofs in any substantial way. Additional contributions include expanded definitions and a description of how anonymous tokens can improve the privacy in dp3t-like digital contact tracing applications. We also show how to create efficient and conceptually simple tokens with public metadata and public verifiability from pairings.