Credential Exchange Protocol

The software components that implement the protocols. An Aries Agent is a service that runs the protocols on behalf of an identity owner. Key elements include:

• Edge Agent: A user-facing wallet (mobile/app) that interacts with the holder.
• Cloud Agent: A always-online agent that provides backend services for an edge agent.
• Mediator: A specialized cloud agent that routes messages for edge agents behind firewalls or without public IP addresses.
• Verifiable Data Registry: The underlying ledger (e.g., Indy, Ethereum) used to anchor and resolve Decentralized Identifiers (DIDs).

The entity that creates and digitally signs verifiable credentials. An issuer is the authoritative source for a specific claim, such as a university issuing a diploma or a government issuing a digital ID. Their role is to:

• Define the credential schema (structure and data fields).
• Apply their cryptographic signature, binding the credential to the holder.
• Publish their Decentralized Identifier (DID) and public keys to a verifiable data registry for trust.

The entity, often an individual or organization, that receives and controls verifiable credentials from issuers. The holder stores credentials in a digital wallet and presents them to verifiers. Their key capabilities include:

• Selective Disclosure: Choosing which specific claims from a credential to share.
• Credential Management: Organizing, storing, and revoking consent for their credentials.
• Presentation Creation: Generating a verifiable presentation for a specific verification request.

The entity that requests and validates credentials presented by a holder. A verifier's goal is to assess if the presented proofs satisfy their business or access policies. Their verification process involves:

• Requesting specific credentials or claims.
• Checking the cryptographic signatures from the issuer(s).
• Ensuring credentials are not revoked (e.g., by checking a revocation registry).
• Validating the credential's schema and that it meets policy requirements.

The trusted system that acts as a source for Decentralized Identifiers (DIDs), public keys, schemas, and revocation statuses. It provides the necessary data for issuers, holders, and verifiers to interact. Common implementations include:

• Distributed Ledgers (e.g., blockchain) for immutable DID registration.
• Decentralized Networks (e.g., peer-to-peer or overlay networks).
• Traditional Databases in more centralized architectures. It is a foundational component for establishing trust without a central authority.

The software component, controlled by the holder, that securely stores verifiable credentials and manages cryptographic keys. It is the primary interface for holders to interact with the protocol. Core functions include:

• Secure Storage: Encrypting and storing private keys and credential data.
• Interaction Agent: Communicating with issuers and verifiers via standardized protocols (e.g., DIDComm).
• Presentation Logic: Helping the holder construct and sign verifiable presentations in response to a verifier's request.

The set of rules, standards, and legal agreements that define how the other actors interact within a specific ecosystem. While not a technical actor, it is a critical role that establishes:

• Accreditation Rules: Criteria for becoming a recognized issuer.
• Technical Specifications: Which cryptographic suites and data formats are accepted.
• Liability & Compliance: Legal frameworks for disputes and data protection (e.g., GDPR). Examples include the W3C Verifiable Credentials Data Model and ecosystem-specific governance authorities.

A core privacy feature that allows a user to prove a specific claim from a credential without revealing the entire document. This is often implemented using zero-knowledge proofs (ZKPs).

• Example: Proving you are over 21 by revealing only a birthdate > 21 years ago predicate, not your exact date of birth.
• Mechanism: Uses cryptographic commitments and ZK-SNARKs or BBS+ signatures to generate a minimal, verifiable proof.

The W3C standard data model that structures credentials to be cryptographically secure, privacy-respecting, and machine-verifiable. It is the foundational object exchanged by these protocols.

• Key Components: Issuer (signs the VC), Holder (stores and presents the VC), Verifier (requests and checks the VC).
• Security: Relies on Decentralized Identifiers (DIDs) and digital signatures (e.g., Ed25519) to ensure authenticity and integrity, preventing forgery.

Protocols must defend against malicious actors who intercept or reuse credential presentations.

• Replay Attacks: Prevented using cryptographic nonces and audience restrictions tied to the verifier's DID.
• Man-in-the-Middle: Mitigated by establishing authenticated, confidential channels (e.g., DIDComm) for the exchange.
• Mimicry: The verifier must cryptographically validate the issuer's signature and check the credential status against a revocation registry.

The protocol architecture ensures the credential holder (user) maintains control over their data, a principle known as self-sovereign identity (SSI).

• User Consent: The holder must explicitly consent to any presentation request.
• Data Minimization: The protocol should only transmit the minimal data required for verification (see Selective Disclosure).
• Storage: Credentials are stored in a digital wallet under the holder's sole control, not in a central database.

A foundational technology for privacy-preserving credential exchange. DIDs are globally unique identifiers controlled by the subject (user, organization, thing) without a central registry.

• Privacy Benefit: DIDs enable pseudonymous interactions; a user can have different DIDs for different contexts to prevent correlation.
• Verification: DIDs resolve to DID Documents containing public keys, enabling any party to verify signatures without prior relationship.

A critical security mechanism for invalidating credentials before their expiration date (e.g., if a driver's license is suspended).

• Common Methods:
  • Status Lists: Bitstring-based registries (W3C Status List 2021) where a credential's status is indicated by the state of a bit.
  • Revocation Registries: Used in Hyperledger Indy/Aries, where the issuer publishes cryptographic accumulators.
• Privacy Consideration: Status checks should not reveal which specific credential a verifier is checking.