Step-by-step migration from classical to hybrid/post-quantum cryptography.

Phase 1          Phase 2          Phase 3          Phase 4
Classic    ->    Hybrid     ->    Hybrid+    ->    PostQuantum
(RSA only)       (RSA+ML-DSA)    (Validation)    (ML-DSA only)

Goal: Install library, remain in Classic mode.

// No change to existing behavior
CryptoConfig.DefaultMode = CryptoMode.Classic;

• Install NuGet package
• Deploy OpenSSL 3.6 → Installation
• Run existing tests (must continue to pass)

Goal: New certificates are PQ-protected, old ones continue to work.

// Activate hybrid mode
CryptoConfig.DefaultMode = CryptoMode.Hybrid;

What happens:

• New certificates: RSA signature + ML-DSA signature (X.509 extension)
• Old certificates: Continue to be accepted
• Legacy clients: Ignore PQ extension, validate only RSA

Goal: PQ signatures are actively verified (not just generated).

// Build chain with PQ validation
var chain = new X509Chain();
bool valid = chain.Build(cert, CryptoMode.Hybrid);
 
// Check if PQ signature is present
if (cert.HasPqSignature())
{
    bool pqValid = cert.VerifyPqSignature();
}

Goal: Only PQ algorithms, maximum security.

CryptoConfig.DefaultMode = CryptoMode.PostQuantum;

• Strategy - Timeline and risk assessment
• Security - Threat model

Wolfgang van der Stille @ EMSR DATA d.o.o. - Post-Quantum Cryptography Professional