This scenario describes digitally signing documents with Post-Quantum secure algorithms (ML-DSA). Digital signatures ensure:

• Authenticity - Sender is verified
• Integrity - Document has not been altered
• Non-Repudiation - Signature cannot be denied

Supported Formats:

• Detached Signature (separate .sig file)
• CMS/PKCS#7 (embedded or detached)
• PDF Digital Signature (PAdES)

using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;
using System.Security.Cryptography;
 
using var ctx = PqCryptoContext.Initialize();
 
// Load signing key
var signingKey = ctx.LoadPrivateKey("signing.key.pem", "KeyPassword!");
var signingCert = ctx.LoadCertificate("signing.crt.pem");
 
// Load document
var document = File.ReadAllBytes("contract.pdf");
 
// Calculate hash
var hash = SHA256.HashData(document);
 
// Sign with ML-DSA
var signature = ctx.SignData(
    data: hash,
    privateKey: signingKey,
    hashAlgorithm: HashAlgorithmName.SHA256,
    mode: CryptoMode.Hybrid  // RSA + ML-DSA parallel
);
 
// Save signature
File.WriteAllBytes("contract.pdf.sig", signature);
 
// Signature metadata
var sigInfo = new SignatureInfo
{
    Algorithm = "ML-DSA-65 + RSA-PSS (Hybrid)",
    SignedAt = DateTimeOffset.UtcNow,
    Signer = signingCert.Subject,
    DocumentHash = Convert.ToHexString(hash),
    SignatureFile = "contract.pdf.sig"
};
 
File.WriteAllText("contract.pdf.sig.json",
    JsonSerializer.Serialize(sigInfo, new JsonSerializerOptions { WriteIndented = true }));
 
Console.WriteLine($"Document signed:");
Console.WriteLine($"  Signer: {signingCert.Subject}");
Console.WriteLine($"  Hash: {Convert.ToHexString(hash).Substring(0, 16)}...");
Console.WriteLine($"  Signature: {signature.Length} Bytes");

using System.Security.Cryptography.Pkcs;
 
using var ctx = PqCryptoContext.Initialize();
 
var signingCert = ctx.LoadCertificate("signing.crt.pem");
var signingKey = ctx.LoadPrivateKey("signing.key.pem", "KeyPassword!");
 
// Document
var document = File.ReadAllBytes("contract.pdf");
 
// Create CMS ContentInfo
var contentInfo = new ContentInfo(document);
 
// Create SignedCms
var signedCms = new SignedCms(contentInfo, detached: false);  // Embedded
 
// Configure signer info
var signer = new CmsSigner(signingCert)
{
    DigestAlgorithm = new Oid("2.16.840.1.101.3.4.2.1"),  // SHA-256
    IncludeOption = X509IncludeOption.WholeChain
};
 
// Add signed attributes
signer.SignedAttributes.Add(new Pkcs9SigningTime(DateTimeOffset.UtcNow));
 
// Sign with PQ extension (Hybrid)
signedCms.ComputeSignature(signer, mode: CryptoMode.Hybrid);
 
// Export CMS container
var signedData = signedCms.Encode();
File.WriteAllBytes("contract.pdf.p7s", signedData);
 
Console.WriteLine($"CMS signature created: {signedData.Length} Bytes");

using var ctx = PqCryptoContext.Initialize();
 
// Load document and signature
var document = File.ReadAllBytes("contract.pdf");
var signature = File.ReadAllBytes("contract.pdf.sig");
 
// Load signer certificate
var signerCert = ctx.LoadCertificate("signing.crt.pem");
 
// Calculate hash
var hash = SHA256.HashData(document);
 
// Verify signature
bool isValid = ctx.VerifyData(
    data: hash,
    signature: signature,
    certificate: signerCert,
    hashAlgorithm: HashAlgorithmName.SHA256,
    mode: CryptoMode.Hybrid
);
 
Console.WriteLine($"Signature valid: {isValid}");
 
if (isValid)
{
    // Validate certificate chain
    var chain = new X509Chain();
    chain.ChainPolicy.RevocationMode = X509RevocationMode.Online;
    bool chainValid = chain.Build(signerCert);
 
    Console.WriteLine($"Certificate valid: {chainValid}");
    Console.WriteLine($"  Signer: {signerCert.Subject}");
    Console.WriteLine($"  Valid until: {signerCert.NotAfter:yyyy-MM-dd}");
}

public class BatchSigner
{
    private readonly AsymmetricAlgorithm _signingKey;
    private readonly X509Certificate2 _signingCert;
    private readonly PqCryptoContext _ctx;
 
    public BatchSigner(string certPath, string keyPath, string password)
    {
        _ctx = PqCryptoContext.Initialize();
        _signingCert = _ctx.LoadCertificate(certPath);
        _signingKey = _ctx.LoadPrivateKey(keyPath, password);
    }
 
    public async Task<IEnumerable<SignatureResult>> SignBatch(IEnumerable<string> filePaths)
    {
        var results = new List<SignatureResult>();
 
        foreach (var filePath in filePaths)
        {
            try
            {
                var document = await File.ReadAllBytesAsync(filePath);
                var hash = SHA256.HashData(document);
 
                var signature = _ctx.SignData(
                    hash,
                    _signingKey,
                    HashAlgorithmName.SHA256,
                    CryptoMode.Hybrid
                );
 
                var sigPath = filePath + ".sig";
                await File.WriteAllBytesAsync(sigPath, signature);
 
                results.Add(new SignatureResult
                {
                    FilePath = filePath,
                    SignaturePath = sigPath,
                    Success = true,
                    SignedAt = DateTimeOffset.UtcNow
                });
            }
            catch (Exception ex)
            {
                results.Add(new SignatureResult
                {
                    FilePath = filePath,
                    Success = false,
                    Error = ex.Message
                });
            }
        }
 
        return results;
    }
}

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