AnonCreds Specification

AnonCreds is a privacy-preserving verifiable credential specification that enables the issuance and presentation of credentials with strong privacy guarantees. It implements a complete "Trust Triangle" model involving issuers, holders, and verifiers.

The specification's key privacy-preserving capabilities include:

• Zero-Knowledge Proofs (ZKPs) to prevent correlation across interactions
• Selective disclosure of credential attributes
• Predicate proofs for numeric claims without revealing values
• Blind signatures during issuance to enhance holder privacy
• Revocation support without revealing correlatable identifiers

Core technical features:

• Uses CL-Signatures (Camenisch-Lysyanskaya) as the underlying cryptographic scheme
• Implements a link secret mechanism for credential binding
• Supports revocation through cryptographic accumulators
• Enables predicate proofs for numeric attributes
• Provides selective disclosure of credential attributes

The protocol involves three main data flows:

1. Setup Flow:

   • Schema publication by Schema Publisher
   • Credential Definition creation by Issuer
   • Link secret generation by Holder

2. Issuance Flow:

   • Credential offer from Issuer to Holder
   • Credential request with blinded link secret
   • Credential issuance with signature

3. Presentation Flow:

   • Presentation request from Verifier
   • Proof generation by Holder
   • Verification by Verifier

Key Technical Specifications:

• Uses RSA cryptography with 2048-bit keys
• Implements CL-Signatures for credential signatures
• Employs BN254 elliptic curve for revocation
• Uses SHA-256 for hashing operations
• Requires secure storage of private keys and link secrets

Implementation Requirements:

• Support for large number arithmetic
• Cryptographic accumulator operations
• Zero-knowledge proof generation and verification
• Secure key and secret management
• Revocation registry handling

Security Considerations:

• Protection of issuer private keys
• Secure storage of holder link secrets
• Prevention of correlation through identifiers
• Mitigation of replay attacks
• Protection against key compromise

Interoperability Features:

• W3C Verifiable Credentials data model compatibility
• Support for multiple VDR implementations
• Standardized encoding rules for attributes
• Common revocation mechanism
• Unified proof format

The specification is widely adopted in self-sovereign identity implementations and continues to evolve through community-driven development.