AnonCreds

AnonCreds (Anonymous Credentials) is a W3C Verifiable Credentials-compatible specification that enables the creation of zero-knowledge proofs for credential attributes. It allows a holder to cryptographically prove they possess a valid credential from a trusted issuer—such as a university degree or professional license—without revealing the credential itself or any unnecessary personal data. This selective disclosure is a core privacy feature, preventing correlation and tracking across different verification events. The protocol is built upon CL-signatures (Camenisch-Lysyanskaya), a form of blind signature that underpins its privacy guarantees.

The AnonCreds ecosystem operates on three primary roles: the Issuer (who creates and signs the credential), the Holder (who receives and stores the credential in a digital wallet), and the Verifier (who requests and checks proof from the holder). Credentials and their associated schemas (defining the data structure) and credential definitions (containing the issuer's public key) are typically written to a verifiable data registry, such as a Hyperledger Indy distributed ledger. This decentralized architecture ensures no single entity controls the credential infrastructure, enhancing security and interoperability.

A key technical innovation of AnonCreds is its support for predicate proofs and non-revocation proofs. A holder can prove that a number in their credential (e.g., their age) satisfies a condition like "over 21" without revealing the exact value. Simultaneously, a verifier can request a proof that the credential has not been revoked by the issuer, which is checked against a revocation registry on the ledger. This combination allows for complex, real-world verification scenarios while maintaining user privacy and issuer control.

Originally developed for the Hyperledger Indy project, AnonCreds has evolved into a W3C Community Group standard, promoting wider adoption beyond its initial SSI (Self-Sovereign Identity) context. Its JSON-LD-based data model ensures compatibility with the broader verifiable credentials ecosystem. Practical applications range from KYC/AML compliance in finance, where only necessary information is shared, to educational credentialing and employment background checks, providing a more secure and private alternative to traditional paper-based or centralized digital systems.

When compared to other credential formats like W3C Verifiable Credentials with Data Integrity Proofs (e.g., using JSON Web Tokens), AnonCreds is distinguished by its inherent support for zero-knowledge proofs and link-secret-based holder binding. The holder uses a master secret to generate proofs, ensuring credentials cannot be used by anyone else even if the data is copied. This makes AnonCreds particularly suited for high-assurance, privacy-sensitive use cases where minimizing data exposure and preventing credential correlation are paramount requirements.

AnonCreds is a compound word formed from Anon (short for anonymous) and Creds (short for credentials). This name was chosen to succinctly describe a specific type of verifiable credential system built on zero-knowledge proofs (ZKPs). The primary goal of this system is to allow an individual to prove a claim—such as being over 18 or holding a professional license—without revealing the underlying credential data or their identity, hence providing anonymous attestation.

The protocol originated from academic research into attribute-based credentials and anonymous credentials, most notably the work of cryptographers like Jan Camenisch and Anna Lysyanskaya. Their CL-signature scheme forms a foundational cryptographic primitive for AnonCreds. The technology was first implemented in a production blockchain context by the Hyperledger Indy project, which established it as a core component of its decentralized identity stack, designed for privacy-preserving interactions.

The evolution of AnonCreds is closely tied to the development of Decentralized Identifiers (DIDs) and Verifiable Credentials (VCs) standards at the World Wide Web Consortium (W3C). While the W3C VC Data Model provides a general framework, AnonCreds specifies a particular cryptographic suite and data format optimized for selective disclosure and issuer correlation resistance. This makes it a specific implementation choice within the broader verifiable credentials ecosystem.

A key differentiator in its origin story is its link to the Ursa cryptographic library. Hyperledger Ursa was created to provide a shared, secure repository of cryptographic implementations for distributed ledgers, and the AnonCreds cryptographic operations are implemented within this library. This separation ensures that the complex ZKP math is consistently and securely applied across different platforms adopting the standard.

Today, the term AnonCreds has transcended its initial Hyperledger Indy association. Managed by the AnonCreds Working Group under the Decentralized Identity Foundation (DIF), it is being standardized as a credential format interoperable across various blockchains and distributed ledgers. This move from a project-specific tool to a formal, community-driven standard underscores its recognized utility in achieving privacy-by-design in digital identity systems.

AnonCreds is a decentralized credential system that enables the issuance, holding, and verification of verifiable credentials while preserving user privacy. It leverages zero-knowledge proofs (ZKPs) and public-key cryptography to allow a credential holder to prove specific claims—like being over 18—without revealing the underlying credential data or their identity. This mechanism is built upon the Hyperledger Indy blockchain stack, which provides a public, permissioned ledger for storing decentralized identifiers (DIDs) and credential schemas, but not the private credential data itself.

The workflow involves three primary roles: the Issuer, the Holder, and the Verifier. An issuer, such as a university, creates a credential schema and definition, publishing their public key (from a cryptographic keypair) to a verifiable data registry like an Indy ledger. They then issue a signed credential to a holder. The holder stores this credential securely in their digital wallet. When a verifier, like an employer, requests proof of a claim, the holder generates a zero-knowledge proof derived from their credential. This proof cryptographically demonstrates the claim is valid and was signed by a trusted issuer, without exposing any extraneous personal data.

The core privacy technology is the CL signature scheme (Camenisch-Lysyanskaya), a type of blind signature that supports the creation of selective disclosure and predicate proofs. This allows for attribute-based credentials, where a single credential can contain multiple attributes. A holder can selectively disclose only the necessary attributes (e.g., "birth year > 2000") or prove predicates about hidden attributes (e.g., "age ≥ 21") using ZKPs. This prevents correlation between different presentations of the same credential, a critical feature for user anonymity across multiple interactions.

For verification, the verifier receives the holder's proof along with the relevant credential definition and issuer's public key from the ledger. The verifier's software cryptographically verifies the proof's validity against the public key and the stated policy. The entire process ensures data minimization and user sovereignty, as the holder controls their credentials and decides what to share. Unlike some systems, AnonCreds credentials are revocable; issuers can publish revocation registries on the ledger, allowing verifiers to check if a credential is still valid without learning the holder's identity.

AnonCreds is a foundational component of Self-Sovereign Identity (SSI) architectures. Its design is interoperable through the W3C Verifiable Credentials Data Model and is widely implemented in ecosystems like the Indy, Aries, and Ursa projects. A key differentiator is its reliance on a public permissioned ledger for trust anchors (DIDs and schemas), which avoids the need for direct issuer-verifier communication or centralized certificate authorities, establishing a decentralized trust model for digital credentials.