Hybrid Cryptography Strategy

QNSP uses hybrid cryptography combining classical and post-quantum algorithms.

Why hybrid

• Defense in depth: If one algorithm is broken, the other provides protection
• Transition period: Gradual migration from classical to PQC
• Compliance: Some standards still require classical algorithms

Hybrid modes

Signatures

Dual signatures with both algorithms:

• Classical: Ed25519
• PQC: Dilithium (ML-DSA)

Both must verify for acceptance.

Key encapsulation

Combined KEM:

• Classical: X25519
• PQC: Kyber (ML-KEM)

Shared secret derived from both.

Encryption

Layered encryption:

• Outer layer: Classical (AES-256-GCM)
• Inner layer: PQC-derived key

Configuration

Hybrid mode is default. Pure PQC mode available:

CRYPTO_MODE=hybrid  # default
CRYPTO_MODE=pqc     # PQC only

Migration path

1. Current: Hybrid mode (classical + PQC)
2. Transition: Monitor PQC standardization
3. Future: Pure PQC when classical deprecated

Performance

Hybrid adds overhead:

• Signature size: ~2.5x larger
• Verification time: ~1.5x slower
• Key exchange: ~2x slower

Acceptable for security-critical operations.