Credential Format

In the context of Decentralized Identity (DID) and Verifiable Credentials (VCs), a credential format is a formalized data schema that dictates how claims—such as a person's name, age, or professional certification—are packaged into a cryptographically secure, machine-readable document. It specifies the syntax for the credential's metadata, the structure of the claims, the supported proof mechanisms (like digital signatures or zero-knowledge proofs), and the rules for revocation. Common formats include the W3C Verifiable Credentials Data Model, JSON Web Tokens (JWT), and JSON-LD with Linked Data Proofs. The choice of format determines interoperability, proof expressiveness, and privacy features.

The evolution of credential formats reflects a shift from centralized, proprietary systems to open, interoperable standards. Early formats were often simple signed JSON or XML blobs tied to a specific issuer's system. Modern formats, like those built on the W3C VC Data Model, are designed for portability across different ecosystems, known as trust frameworks. They enable features like selective disclosure, where a user can reveal only specific attributes from a credential (e.g., proving they are over 21 without revealing their exact birthdate), and cryptographic agility, allowing for the use of various signature suites like EdDSA, BBS+ signatures, or zk-SNARKs.

Selecting the appropriate credential format is a critical architectural decision. For instance, the JWT VC format is compact and widely supported in web infrastructure, making it suitable for simple attestations. In contrast, JSON-LD with BBS+ signatures supports advanced privacy-preserving capabilities, enabling predicate proofs and unlinkable presentations. The format also defines how a Verifiable Presentation—a holder's package of one or more credentials for a verifier—is constructed. As the ecosystem matures, formats like SD-JWT (Selective Disclosure JWT) and MDOC (ISO 18013-5 mobile driver's license) are emerging to address specific use cases in digital identity and access management.

A credential format is the technical blueprint for a digital credential, specifying its serialization, cryptographic proof mechanism, and core data model. Common formats include W3C Verifiable Credentials (VCs), which use JSON-LD or JWT serializations, and AnonCreds, which is optimized for selective disclosure and zero-knowledge proofs using CL signatures. The choice of format dictates interoperability, the types of cryptographic proofs available (e.g., digital signatures vs. zero-knowledge proofs), and the complexity of the verification process. It is a foundational layer separate from the credential schema, which defines the actual data fields.

The primary function of a credential format is to ensure cryptographic verifiability and data integrity. When an issuer creates a credential, they package the claims according to the format's rules and bind them to the holder's Decentralized Identifier (DID) using a cryptographic signature or proof. This creates a tamper-evident package. Formats like SD-JWT (Selective Disclosure JWT) extend this by allowing holders to reveal only specific claims from a signed credential, enhancing privacy. The format also defines how revocation status—whether a credential is still valid—is communicated, often through a revocation registry or status list.

Interoperability between different trust ecosystems hinges on supporting common credential formats. For instance, a credential issued in the AnonCreds format within a Hyperledger Indy ecosystem may not be directly verifiable by a verifier's system that only supports W3C VC-JWT. Format translators or wallets that support multiple formats are often required to bridge these gaps. Standardization bodies like the W3C and Decentralized Identity Foundation (DIF) work to define and promote interoperable formats, with the W3C VC Data Model serving as a widely adopted conceptual framework that various formats implement.

When selecting a format, issuers must balance requirements for privacy, proof expressiveness, and computational overhead. Zero-knowledge proof-based formats like AnonCreds or BBS+ signatures enable sophisticated privacy-preserving features but can be more complex to implement. Simpler JWT-based formats offer ease of use and broad library support but may reveal all credential data upon presentation. The format also influences holder autonomy; some formats require interaction with the issuer during presentation, while others support holder-bound credentials that can be presented offline.

A credential format is the technical blueprint for a verifiable credential, specifying the exact schema for its data fields, digital signatures, and metadata. Common formats include W3C Verifiable Credentials (VCs), which use JSON-LD and Linked Data Proofs, and JSON Web Tokens (JWTs), which use a compact, URL-safe token structure. The choice of format dictates the credential's interoperability, cryptographic proof type (e.g., EdDSA, ECDSA), and how it is processed by verifiers. Selecting the right format is a foundational decision that impacts the entire credential lifecycle.

The W3C Verifiable Credentials Data Model is the most widely adopted standard, promoting semantic interoperability. It structures a credential into core components: the credentialSubject (the claims about the holder), issuer, issuanceDate, and optional expirationDate. Proofs are attached using Linked Data Proofs, which can be implemented with various cryptographic suites like Ed25519Signature2020. This format's use of JSON-LD allows for extensible, context-aware data that machines can unambiguously interpret, making it ideal for complex, interconnected credential ecosystems.

In contrast, the JWT-based format encodes the credential claims and header into a compact string, signed using the JSON Web Signature (JWS) standard. This results in a smaller payload, often preferred for high-throughput or constrained environments like mobile apps. While efficient, JWT credentials typically lack the built-in semantic context of JSON-LD, which can limit their ability to express complex relationships between data points without additional约定.

Beyond these, specialized formats exist for specific use cases. AnonCreds, originally developed for Hyperledger Indy, uses CL-signatures to enable advanced privacy features like selective disclosure and predicate proofs without revealing the underlying data. SD-JWT (Selective Disclosure JWT) is an emerging IETF standard that combines the compactness of JWTs with the ability to selectively reveal individual claims, offering a balance between privacy and efficiency.

The format also defines the presentation layer—how a credential is packaged for verification. A Verifiable Presentation bundles one or more credentials, often from different issuers, under a single proof from the holder. The presentation format must be compatible with the original credential formats. This decoupling allows a holder to present a W3C VC in a JWT-based presentation, or vice-versa, using format-translation layers, though this can add complexity to the verification process.

Ultimately, the credential format is not just a serialization choice; it enforces the trust model and privacy guarantees. Developers must evaluate formats based on requirements for cryptographic agility, proof succinctness, selective disclosure capabilities, and ecosystem support. The ongoing evolution of standards aims to consolidate the strengths of different approaches, with projects like the W3C VC-JWT specification working to bring JWT's efficiency into the broader W3C VC ecosystem.