Traditionally, digital identity has operated through three models:

1. Centralized identity: Single organizations (like Facebook or Google) control user identities
2. Federated identity: Multiple organizations share identity information through trust agreements
3. Self-sovereign identity: Users directly control their own identity information

The evolution toward SSI was driven by several factors:

• Privacy concerns: Centralized identity providers accumulate extensive user data
• Vendor lock-in: Users cannot easily move identities between platforms
• Single points of failure: Centralized systems create vulnerability to breaches and outages
• Surveillance risks: Centralized identity enables tracking and profiling
• Lack of user control: Users cannot selectively disclose information

Early SSI implementations focused on blockchain-based identity systems, using distributed ledgers as decentralized registries for identifiers and credentials. However, these approaches faced challenges with scalability, privacy (public blockchains expose transaction patterns), and the requirement for global consensus on all identity operations.

KERI's Approach

KERI (Key Event Receipt Infrastructure) represents a specific architectural approach within the broader SSI landscape, distinguished by its security-first design philosophy. The KERI development team explicitly prioritizes:

1. Security first: Cryptographic integrity and verifiability as non-negotiable foundations
2. Confidentiality second: Protection of sensitive data content
3. Privacy third: Minimization of correlation and identification

This ordering reflects a deliberate architectural choice that security properties must be established at the foundational level before addressing higher-level privacy concerns. KERI positions itself as a "rather small subset" of all available SSI systems, focusing on doing specific things exceptionally well rather than attempting to be a universal SSI solution.

Key Differentiators

KERI's approach differs from other SSI systems in several fundamental ways:

Autonomic Identifiers (AIDs): KERI uses self-certifying identifiers that are cryptographically bound to key pairs through derivation codes. Unlike DIDs that may rely on external resolution infrastructure, AIDs are verifiable through the identifier itself and its associated Key Event Log (KEL).

Key Event Logs: Instead of storing identity state on blockchains or distributed ledgers, KERI maintains append-only logs of key events for each identifier. These logs are end-verifiable, meaning anyone can cryptographically verify the entire history without trusting intermediaries.

Witness Networks: Rather than requiring global consensus, KERI uses designated witnesses that provide receipts for key events. This enables scalability while maintaining security through duplicity detection rather than preventing duplicity through consensus.

Pre-rotation: KERI's key management includes cryptographic commitments to future rotation keys, providing post-quantum security and enabling recovery from key compromise without requiring trusted third parties.

Decoupled from Ledgers: While KERI can use ledgers as backers, it does not depend on them. Identifiers can be transferred off ledgers, providing true portability and avoiding blockchain-specific limitations.

Security Model

KERI's security-first approach manifests in several ways:

• Cryptographic root-of-trust: Trust derives from cryptographic verification, not institutional authority
• End-verifiability: All claims can be verified to their cryptographic roots without intermediaries
• Duplicity detection: Rather than preventing conflicting statements, KERI makes duplicity evident and detectable
• Ambient verifiability: Anyone, anywhere, anytime can verify identifier state

This model enables attributional trust - proving "who said what" through cryptographic signatures - as the foundation upon which other trust types can be built.

Practical Implications

The SSI system architecture, and KERI's specific implementation, has significant practical implications:

Use Cases

Organizational Identity: The vLEI (verifiable Legal Entity Identifier) ecosystem demonstrates SSI for organizational identity, enabling legal entities to issue cryptographically verifiable credentials about their structure, authorized representatives, and roles. This provides a trust layer for business-to-business interactions without requiring centralized identity providers.

Credential Ecosystems: SSI systems enable credential issuance, presentation, and verification without centralized registries. Issuers can create credentials, holders can store and selectively disclose them, and verifiers can validate them independently.

Supply Chain Provenance: Authentic data supply chains use SSI principles to track provenance of goods and data through verifiable chains of custody, with each participant maintaining sovereign control over their identifiers.

Cross-Border Identity: SSI systems enable identity portability across jurisdictions without requiring international agreements on centralized identity providers, as cryptographic verification works universally.

Benefits

User Control: Individuals and organizations maintain direct control over their identifiers and credentials, deciding what to disclose and to whom.

Privacy by Design: Selective disclosure mechanisms enable sharing only necessary information, minimizing data exposure and correlation risks.

Reduced Infrastructure Costs: Eliminating centralized identity providers reduces operational costs and removes single points of failure.

Interoperability: Cryptographic standards enable different implementations to work together without requiring business agreements between providers.

Resilience: Decentralized architecture provides resilience against outages, censorship, and attacks on centralized infrastructure.

Trade-offs

KERI's security-first approach involves deliberate trade-offs:

Complexity: Cryptographic verification and key management require more sophisticated implementations than simple username/password systems.

User Responsibility: Self-sovereign identity places key management responsibility on users, requiring secure key storage and backup mechanisms.

Privacy Limitations: The security-first approach means some privacy features may be layered on top rather than built into the foundation, accepting that perfect security, confidentiality, and privacy cannot be achieved simultaneously (the PAC Theorem).

Adoption Barriers: Moving from familiar centralized systems to decentralized architectures requires education and tooling to make SSI accessible to non-technical users.

Ecosystem Coordination: While SSI eliminates centralized control, it requires coordination on standards, schemas, and governance frameworks to enable interoperability.

Implementation Considerations

Building SSI systems requires addressing several practical challenges:

Key Management: Secure generation, storage, and rotation of cryptographic keys is fundamental. KERI's pre-rotation mechanism provides a path for key recovery, but implementations must still protect current keys.

Witness Infrastructure: KERI-based systems require witness networks for indirect mode operation. Deploying and maintaining witnesses involves operational considerations around availability, performance, and trust.

Credential Schemas: Interoperable credential ecosystems require shared schemas. The vLEI ecosystem demonstrates governance frameworks for schema management and versioning.

User Experience: Making cryptographic operations transparent to users while maintaining security requires careful UX design. Wallets, agents, and other tools must balance usability with security.

Governance: While SSI systems are decentralized, they still require governance for schemas, trust frameworks, and ecosystem rules. The vLEI Ecosystem Governance Framework demonstrates how governance can be layered on decentralized infrastructure.

The SSI system concept represents a fundamental rethinking of digital identity, moving from institutional trust to cryptographic verification. KERI's implementation demonstrates how security-first design can provide the foundation for scalable, verifiable identity systems that enable true user sovereignty while maintaining the verifiability and interoperability necessary for practical deployment.