Scenario 8.4: Verify Signature

This scenario describes the complete verification of digital signatures. Proper verification includes:

• Signature verification - Cryptographic validation
• Certificate verification - Chain, revocation, validity
• Timestamp verification - If present
• Policy verification - Intended usage

using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;
using System.Security.Cryptography;
using System.Security.Cryptography.Pkcs;
 
using var ctx = PqCryptoContext.Initialize();
 
public class SignatureVerifier
{
    public VerificationResult Verify(
        byte[] data,
        byte[] signature,
        X509Certificate2 signerCert,
        VerificationOptions options)
    {
        var result = new VerificationResult();
 
        try
        {
            // 1. Cryptographic signature verification
            result.SignatureValid = ctx.VerifyData(
                data: data,
                signature: signature,
                certificate: signerCert,
                hashAlgorithm: options.HashAlgorithm,
                mode: options.CryptoMode
            );
 
            if (!result.SignatureValid)
            {
                result.Error = "Cryptographic signature invalid";
                return result;
            }
 
            // 2. Validate certificate chain
            var chain = new X509Chain();
            ConfigureChainPolicy(chain.ChainPolicy, options);
 
            result.CertificateValid = chain.Build(signerCert);
            result.ChainStatus = chain.ChainStatus
                .Select(s => s.Status.ToString())
                .ToList();
 
            if (!result.CertificateValid)
            {
                result.Error = $"Certificate invalid: {string.Join(", ", result.ChainStatus)}";
                return result;
            }
 
            // 3. Verify Extended Key Usage
            if (options.RequiredEku != null)
            {
                result.EkuValid = HasExtendedKeyUsage(signerCert, options.RequiredEku);
                if (!result.EkuValid)
                {
                    result.Error = $"Required EKU ({options.RequiredEku}) missing";
                    return result;
                }
            }
 
            // 4. Verify signature time
            var signatureTime = options.SignatureTime ?? DateTimeOffset.UtcNow;
            result.TimeValid = signatureTime >= signerCert.NotBefore &&
                               signatureTime <= signerCert.NotAfter;
 
            if (!result.TimeValid)
            {
                result.Error = "Certificate was not valid at signature time";
                return result;
            }
 
            result.IsValid = true;
            result.SignerSubject = signerCert.Subject;
            result.SignatureTime = signatureTime;
        }
        catch (Exception ex)
        {
            result.IsValid = false;
            result.Error = ex.Message;
        }
 
        return result;
    }
}

public VerificationResult VerifyCms(byte[] signedData, byte[]? originalContent = null)
{
    var result = new VerificationResult();
 
    try
    {
        // Parse CMS
        var signedCms = new SignedCms();
 
        if (originalContent != null)
        {
            // Detached signature
            var contentInfo = new ContentInfo(originalContent);
            signedCms = new SignedCms(contentInfo, true);
        }
 
        signedCms.Decode(signedData);
 
        // Verify each signature
        foreach (var signerInfo in signedCms.SignerInfos)
        {
            // Cryptographic verification
            signerInfo.CheckSignature(verifySignatureOnly: false);
 
            // Signer certificate
            var cert = signerInfo.Certificate;
            result.SignerSubject = cert?.Subject;
 
            // Verify timestamp (if present)
            var timestampAttr = signerInfo.UnsignedAttributes
                .Cast<CryptographicAttributeObject>()
                .FirstOrDefault(a => a.Oid.Value == "1.2.840.113549.1.9.16.2.14");
 
            if (timestampAttr != null)
            {
                var tsToken = Rfc3161TimestampToken.Decode(
                    timestampAttr.Values[0].RawData, out _);
 
                result.TimestampPresent = true;
                result.TimestampTime = tsToken.TokenInfo.Timestamp;
 
                // Validate timestamp
                var signatureBytes = signerInfo.GetSignature();
                var signatureHash = SHA256.HashData(signatureBytes);
 
                result.TimestampValid = tsToken.VerifySignatureForHash(
                    signatureHash,
                    HashAlgorithmName.SHA256,
                    out _,
                    extraCandidates: null
                );
            }
 
            // Signing Time attribute
            var signingTimeAttr = signerInfo.SignedAttributes
                .Cast<CryptographicAttributeObject>()
                .FirstOrDefault(a => a.Oid.Value == "1.2.840.113549.1.9.5");
 
            if (signingTimeAttr != null)
            {
                var pkcs9Time = new Pkcs9SigningTime(signingTimeAttr.Values[0].RawData);
                result.SignatureTime = pkcs9Time.SigningTime;
            }
        }
 
        result.IsValid = true;
        result.SignatureValid = true;
        result.CertificateValid = true;
    }
    catch (CryptographicException ex)
    {
        result.IsValid = false;
        result.Error = ex.Message;
    }
 
    return result;
}

public class VerificationResult
{
    public bool IsValid { get; set; }
    public bool SignatureValid { get; set; }
    public bool CertificateValid { get; set; }
    public bool TimeValid { get; set; }
    public bool EkuValid { get; set; } = true;
    public bool TimestampPresent { get; set; }
    public bool TimestampValid { get; set; }
 
    public string? SignerSubject { get; set; }
    public DateTimeOffset? SignatureTime { get; set; }
    public DateTimeOffset? TimestampTime { get; set; }
    public List<string> ChainStatus { get; set; } = new();
    public string? Error { get; set; }
 
    public override string ToString()
    {
        var sb = new StringBuilder();
        sb.AppendLine($"Signature valid: {IsValid}");
        sb.AppendLine($"  Crypto verification: {SignatureValid}");
        sb.AppendLine($"  Certificate: {CertificateValid}");
        sb.AppendLine($"  Time verification: {TimeValid}");
 
        if (SignerSubject != null)
            sb.AppendLine($"  Signer: {SignerSubject}");
 
        if (SignatureTime.HasValue)
            sb.AppendLine($"  Signature time: {SignatureTime:yyyy-MM-dd HH:mm:ss}");
 
        if (TimestampPresent)
        {
            sb.AppendLine($"  Timestamp: {(TimestampValid ? "valid" : "invalid")}");
            if (TimestampTime.HasValue)
                sb.AppendLine($"  Timestamp time: {TimestampTime:yyyy-MM-dd HH:mm:ss}");
        }
 
        if (Error != null)
            sb.AppendLine($"  ERROR: {Error}");
 
        return sb.ToString();
    }
}

public class BatchVerifier
{
    public async Task<Dictionary<string, VerificationResult>> VerifyDirectory(
        string directory,
        X509Certificate2Collection trustedCerts)
    {
        var results = new Dictionary<string, VerificationResult>();
        var verifier = new SignatureVerifier();
 
        // Find all .sig files
        var sigFiles = Directory.GetFiles(directory, "*.sig", SearchOption.AllDirectories);
 
        foreach (var sigFile in sigFiles)
        {
            // Find original file
            var originalFile = sigFile.Replace(".sig", "");
            if (!File.Exists(originalFile))
            {
                results[sigFile] = new VerificationResult
                {
                    IsValid = false,
                    Error = "Original file not found"
                };
                continue;
            }
 
            // Find certificate
            var certFile = sigFile.Replace(".sig", ".crt");
            X509Certificate2 signerCert;
 
            if (File.Exists(certFile))
            {
                signerCert = new X509Certificate2(certFile);
            }
            else
            {
                // Search in trusted certs (simplified)
                signerCert = trustedCerts[0];
            }
 
            // Verify
            var data = await File.ReadAllBytesAsync(originalFile);
            var signature = await File.ReadAllBytesAsync(sigFile);
 
            results[sigFile] = verifier.Verify(
                data,
                signature,
                signerCert,
                new VerificationOptions
                {
                    HashAlgorithm = HashAlgorithmName.SHA256,
                    CryptoMode = CryptoMode.Hybrid
                }
            );
        }
 
        return results;
    }
}

« ← 8.3 Timestamp | ↑ Signatures Overview | → All Scenarios »

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