X509V3CertificateGenerator certificateGenerator = new X509V3CertificateGenerator();           
SecureRandom secureRandom = new SecureRandom();           
ECKeyPairGenerator keyGenerator = new ECKeyPairGenerator();           
keyGenerator.Init(new KeyGenerationParameters(secureRandom, 256));           
AsymmetricCipherKeyPair asymmetricKeyPair = keyGenerator.GenerateKeyPair();            certificateGenerator.SetSignatureAlgorithm("SHA512withECDSA");                       
certificateGenerator.SetIssuerDN(new X509Name("CN=" + CN));           
certificateGenerator.SetSubjectDN(new X509Name("CN=" + CN));            certificateGenerator.SetNotBefore(DateTime.Now);            certificateGenerator.SetPublicKey(asymmetricKeyPair.Public);           
DateTime expirationDate = DateTime.Now;           
expirationDate = expirationDate.AddMonths(6);           
certificateGenerator.SetNotAfter(expirationDate);           
StringBuilder serialNumberBuf = new StringBuilder();           
Random random = new Random();           
serialNumberBuf.Append(random.Next(5, 20));           
serialNumberBuf.Append(random.Next(1, 10));           
serialNumberBuf.Append(random.Next(8, 15));           
certificateGenerator.SetSerialNumber(new BigInteger(new System.Text.ASCIIEncoding().GetBytes(serialNumberBuf.ToString())));
certificateGenerator.AddExtension(X509Extensions.KeyUsage, false, new Org.BouncyCastle.Asn1.X509.KeyUsage(Org.BouncyCastle.Asn1.X509.KeyUsage.DigitalSignature | Org.BouncyCastle.Asn1.X509.KeyUsage.KeyAgreement));           
ArrayList purpouses = new ArrayList();           
purpouses.Add(KeyPurposeID.IdKPSmartCardLogon);           
purpouses.Add(KeyPurposeID.IdKPIpsecUser);           
Org.BouncyCastle.Asn1.X509.ExtendedKeyUsage extended_usage = new Org.BouncyCastle.Asn1.X509.ExtendedKeyUsage(purpouses);            certificateGenerator.AddExtension(X509Extensions.ExtendedKeyUsage, false, extended_usage);           
return certificateGenerator.Generate(asymmetricKeyPair.Private, secureRandom).GetEncoded();

StreamReader sr = new StreamReader(new FileStream(inputFileName, FileMode.Open, FileAccess.Read));
ContentInfo contentInfo = new ContentInfo(new System.Text.ASCIIEncoding().GetBytes(sr.ReadToEnd()));
sr.Close();           
SignedCms signedCms = new SignedCms(contentInfo);           
CmsSigner cmsSigner = new CmsSigner(x509cert);           
signedCms.ComputeSignature(cmsSigner);           
StreamWriter streamWriter = new StreamWriter(new FileStream(inputFileName+"_sgn", FileMode.OpenOrCreate, FileAccess.Write));           
streamWriter.Write(System.Text.Encoding.ASCII.GetString(signedCms.Encode()));           
streamWriter.Close();