Anonymous Credit Tokens

This document specifies Anonymous Credit Tokens (ACT), a privacy- preserving authentication protocol that enables numerical credit systems without tracking individual clients. Based on keyed- verification anonymous credentials and privately verifiable BBS-style signatures, the protocol allows issuers to grant tokens containing credits that clients can later spend anonymously with that issuer. The protocol's key features include: (1) unlinkable transactions - the issuer cannot correlate credit issuance with spending, or link multiple spends by the same client, (2) partial spending - clients can spend a portion of their credits and receive anonymous change, and (3) double-spend prevention through cryptographic nullifiers that preserve privacy while ensuring each token is used only once. Anonymous Credit Tokens are designed for modern web services requiring rate limiting, usage-based billing, or resource allocation while respecting user privacy. Example applications include rate limiting and API credits. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF.