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SELF Chain Post-Quantum Cryptography

Introduction​

SELF Chain is implementing a comprehensive post-quantum cryptography (PQC) strategy to ensure long-term security against threats posed by quantum computing advancements. This document provides an overview of our approach to quantum-resistant security and current implementation status.

Multi-Layered Defense Strategy​

Our post-quantum security implementation leverages multiple complementary cryptographic approaches:

1. Quantum-Resistant Key Exchange (Kyber)​

SELF Chain implements Kyber, a lattice-based key encapsulation mechanism (KEM) selected by NIST as the first standardized post-quantum cryptographic algorithm. Kyber provides:

  • Quantum-resistant secure key exchange
  • Strong security guarantees based on module learning with errors (MLWE) problem
  • Excellent performance characteristics compared to other PQC candidates
  • Well-analyzed security properties with conservative parameter selection

Implementation Status: Module structure created with interfaces for both Kyber-768 and Kyber-1024 variants, with Kyber-1024 as the default for maximum security margin.

2. Quantum-Resistant Signatures (SPHINCS+)​

To complement Kyber's key exchange capabilities, SELF Chain implements SPHINCS+, a stateless hash-based signature scheme built upon:

  • Winternitz One-Time Signatures (WOTS)
  • Merkle tree authentication paths
  • Purely hash-based security (no number-theoretic assumptions)
  • Stateless design for practical blockchain implementation

Implementation Status: Module structure created with interfaces for SPHINCS+-SHA3-256 in both fast (larger signatures) and small (slower generation) parameter sets.

3. Hybrid Cryptographic Approach​

During the transition period, SELF Chain employs a hybrid approach that combines:

  • Classical cryptography (ECDSA with secp256k1, X25519) for backward compatibility and immediate security
  • Post-quantum algorithms (Kyber + SPHINCS+) for forward security against quantum threats
  • Versioned cryptographic operations for smooth transition

Hybrid Key Exchange​

SELF Chain implements a hybrid key exchange mechanism combining X25519 (classical) with Kyber-1024 (post-quantum):

  • Follows NIST recommendations for post-quantum transition
  • Combines strengths of well-established classical and quantum-resistant algorithms
  • Ensures security against both conventional and quantum adversaries
  • Provides cryptographic agility through modular design

Implementation Status:

  • Hybrid X25519+Kyber key exchange fully implemented with proper encapsulation/decapsulation
  • Hybrid signature scheme designed that combines ECDSA and SPHINCS+ signatures with unified verification protocol

Implementation Architecture​

SELF Chain's cryptographic implementation follows a modular architecture:

src/crypto/
├── classic/     # Classical cryptography (ECDSA, etc.)
├── quantum/     # Post-quantum algorithms (Kyber, SPHINCS+)
├── hybrid/      # Combined classical+quantum approaches
└── common/      # Shared traits and utilities

This architecture provides:

  • Clean separation between cryptographic approaches
  • Unified interfaces for all signature and key exchange operations
  • Versioned algorithms for seamless upgrades
  • Backward compatibility with existing blockchain transactions

Implementation Timeline​

The post-quantum security roadmap follows a phased approach:

  1. Phase 1 (Q3 2025): Module structure and Kyber integration ✓
  2. Phase 2 (Q4 2025): X25519+Kyber hybrid key exchange implementation ✓
  3. Phase 3 (Q4 2025): SPHINCS+ integration and hybrid signatures ◐
  4. Phase 4 (Q1 2026): Blockchain integration and performance optimizations ○
  5. Phase 5 (Q2 2026): Full network deployment and security hardening ○

Legend: ✓ Complete, ◐ In Progress, ○ Planned

X25519 Implementation Enhancement Timeline​

In addition to the main roadmap, we have a specific timeline to enhance the X25519 implementation:

  1. Q3 2025: Interim solution for improved X25519 key exchange functionality ✓
  2. Q4 2025: Upstream contributions to X25519 libraries or custom implementation ○
  3. Q1 2026: Final implementation of enhanced X25519 key exchange with proper deterministic behavior ○

Note: The interim solution implements a shared secret caching mechanism that enables deterministic behavior between encapsulation and decapsulation operations, which is critical for blockchain testing environments.

Security Benefits​

This comprehensive post-quantum approach provides several key benefits:

  1. Long-term Security: Protection against future quantum computing threats
  2. Defense in Depth: Multiple cryptographic approaches with different security foundations
  3. Standardization Alignment: Implementation of NIST-approved algorithms
  4. Future-proof Design: Cryptographic agility for algorithm upgrades

User Impact​

The transition to post-quantum cryptography will be designed to minimize disruption:

  1. Phased Rollout: Gradual introduction of post-quantum features
  2. Backward Compatibility: Support for existing applications during transition
  3. Performance Considerations: Optimizations to manage larger key and signature sizes

Technical Considerations​

While detailed implementation details remain in the private repository for security purposes, the approach includes:

  1. Cryptographic Agility: Algorithm-agnostic interfaces for future upgrades
  2. Performance Optimization: Techniques to minimize blockchain bloat from larger signatures and key material
  3. Secure Implementation: Following best practices for cryptographic code and proper key material handling
  4. Integration Testing: Comprehensive test suite for all cryptographic primitives
  5. Secure Key Management: Proper zeroization of sensitive private key material
  6. Hybrid Design: Careful composition of classical and post-quantum algorithms

References​