Indy AnonCreds

Indy AnonCreds (Anonymous Credentials) is a W3C Verifiable Credentials-compatible data format and protocol that enables cryptographic, privacy-preserving proofs of identity attributes. It allows a holder to prove they possess a credential from a trusted issuer—such as a university degree or government ID—without revealing the credential itself or any unnecessary personal data. This is achieved through zero-knowledge proofs (ZKPs), specifically CL signatures, which enable selective disclosure and predicate proofs (e.g., proving one is over 21 without revealing their birth date).

The protocol's architecture is built around three core roles: the issuer (who creates and signs credentials), the holder (who receives and stores credentials in their digital wallet), and the verifier (who requests and cryptographically checks proofs). Credentials are issued against a DID (Decentralized Identifier) stored on a verifiable data registry, typically a permissioned blockchain like the Indy ledger. This creates a trust model where verifiers can cryptographically confirm the issuer's DID and credential schema without needing to query a central database, enhancing both privacy and decentralization.

A key innovation of AnonCreds is linkability control. Holders can generate unique, unlinkable correlation identifiers for each interaction, preventing verifiers from tracking their activity across different services. This stands in contrast to some public credential systems. The original Indy AnonCreds 1.0 uses CL signatures and is implemented in the Hyperledger Aries framework. AnonCreds 2.0 and the newer AnonCreds 3.0 specification extend this model to work on any decentralized web ledger (like cheqd or Ethereum) by utilizing W3C DID Core and BBS+ signatures, which support more complex zero-knowledge proof capabilities.

The primary use cases for Indy AnonCreds are in self-sovereign identity (SSI) ecosystems requiring high-assurance privacy. This includes enterprise access control, where employees prove employment status; educational credential verification, where graduates share diplomas; and regulated industries like finance (KYC) and healthcare, where proving compliance or eligibility without exposing full records is critical. Its design directly addresses the principles of data minimization and user consent central to modern data protection regulations like GDPR.

While powerful, AnonCreds presents implementation complexities, such as managing revocation registries (to invalidate credentials without compromising privacy) and the computational overhead of ZKPs. It is often compared to other credential formats like W3C JSON Web Tokens (JWTs) or SD-JWT-VCs, which are simpler but offer less sophisticated privacy guarantees. The ongoing development of AnonCreds 3.0 aims to improve interoperability and performance, solidifying its role as a core cryptographic trust layer for decentralized identity networks.

AnonCreds is a compound term derived from Anonymous Credentials, a cryptographic protocol standard developed for the Hyperledger Indy distributed ledger. The name was coined by the project's architects to succinctly describe a system that enables the issuance, holding, and verification of verifiable credentials with strong privacy guarantees. Unlike simpler attestations, AnonCreds allows a holder to prove predicates about their credentials (e.g., "I am over 21") without revealing the credential itself or creating a correlatable identifier, hence the emphasis on anonymity.

The protocol's origins are deeply tied to academic research into zero-knowledge proofs and attribute-based credentials, notably the Idemix (Identity Mixer) system developed by IBM Research and the U-Prove technology from Microsoft. Hyperledger Indy's implementation synthesized these concepts into a practical, ledger-anchored system. The "Creds" portion of the name signifies its focus on a specific, reusable data format—the W3C Verifiable Credentials data model—making it a credentialing system first and foremost.

As a core component of the Self-Sovereign Identity (SSI) stack, AnonCreds was designed to solve the fundamental privacy shortcomings of traditional PKI and blockchain-based identity systems. Its development was driven by the need for selective disclosure, unlinkability, and minimal disclosure, principles that are baked into its name. The term has become a proper noun within the SSI lexicon, specifically referring to the ZKP-based credential implementation native to the Indy ecosystem, distinguishing it from other signature-based approaches like JSON Web Tokens (JWT) or BBS+ signatures used in other verifiable credential systems.

Indy AnonCreds, short for Anonymous Credentials, is a zero-knowledge proof (ZKP)-based credential system originally developed for the Hyperledger Indy distributed ledger. Its core innovation is allowing a user to prove specific claims from a credential—like being over 21—without revealing the entire credential or any other identifying information. This is achieved through the use of Camenisch-Lysyanskaya (CL) signatures, a cryptographic scheme that enables selective disclosure and predicate proofs. The system operates on a public, permissioned blockchain (the Indy ledger) which stores only the essential cryptographic anchors—schemas, credential definitions, and revocation registries—needed for trust and verification, not the personal data itself.

The workflow involves three key roles: the issuer, the holder, and the verifier. An issuer (e.g., a university) creates a credential schema defining the attribute names (e.g., name, degree, graduation_date) and publishes a corresponding credential definition to the ledger, which contains its public key. When issuing a credential to a holder, the issuer creates a signed credential object containing the attribute values and a cryptographic link to the credential definition. The holder stores this credential securely in their digital wallet. Crucially, the holder can generate a presentation or proof for a verifier (e.g., an employer) that discloses only the required attributes or proves statements about them (e.g., degree == "Bachelor of Science"), while the verifier uses the ledger to check the issuer's public key and credential status.

A critical feature is revocation. Issuers can revoke credentials by updating a revocation registry on the ledger. During verification, the holder must prove non-revocation as part of their zero-knowledge proof, typically by providing a cryptographic witness that their credential is not on the current revocation list. This allows for credential lifecycle management without compromising the holder's privacy. The entire protocol ensures data minimization and unlinkability, meaning multiple presentations of the same credential cannot be correlated by different verifiers, providing strong protection against tracking and profiling.

The architecture is ledger-agnostic in its later specifications (AnonCreds 1.0), allowing the core cryptographic protocols to be implemented on other verifiable data registries beyond Hyperledger Indy. This has led to its adoption in ecosystems like the Cheqd network and its standardization work within the Decentralized Identity Foundation (DIF). The system's design directly enables self-sovereign identity (SSI) principles, giving individuals control over their verifiable data and facilitating trusted digital interactions without reliance on centralized identity providers.

Indy AnonCreds refers to the original, canonical implementation of anonymous credentials developed for the Hyperledger Indy distributed ledger, establishing the core data model, cryptographic protocols, and revocation mechanisms that defined the AnonCreds standard. It is a W3C-CCG-registered credential format specifically designed for privacy-preserving, selective disclosure of attributes and unlinkable proofs across multiple interactions. The protocol leverages CL signatures (Camenisch-Lysyanskaya) and zero-knowledge proofs to enable a holder to cryptographically prove possession of credentials from one or more issuers without revealing the credential itself or creating correlatable transactions.

The architecture is intrinsically linked to the Indy ledger, which acts as a public utility for publishing DIDs, schema definitions, credential definitions, and revocation registries. This design means a verifier can independently check the credential's validity—issuer identity, schema structure, and revocation status—by querying the immutable ledger, without contacting the issuer directly. This decentralized trust model was a foundational innovation, moving away from centralized credential validators. However, this tight coupling also meant traditional AnonCreds were not natively portable to other blockchain networks or non-ledger environments.

The legacy of Indy AnonCreds is profound, serving as the practical blueprint for decentralized identity. Its concrete implementation solved real-world problems around interoperability (through standardized object types like cred_def_id), cryptographic agility, and privacy-by-design. The specific constructs it introduced—such as link secrets for credential binding, predicate proofs for proving attributes meet numerical thresholds (e.g., age >= 21), and non-correlation guarantees—became required reading for any subsequent credential system. Its limitations, particularly ledger dependency, directly inspired the development of AnonCreds v2 and the W3C Verifiable Credentials Data Model.

Today, while AnonCreds v2 (or AnonCreds on JSON-LD) evolves the standard to be ledger-agnostic and more flexible, the original Indy AnonCreds remains in widespread production use. Major ecosystems like the European Blockchain Services Infrastructure (EBSI) and numerous SSI deployments are built upon its proven, battle-tested codebase. Understanding Indy AnonCreds is essential for grasping the historical constraints, design trade-offs, and cryptographic primitives that continue to underpin the field of verifiable credentials and self-sovereign identity (SSI).