Stephan van Rooij

using System;
using System.Security.Cryptography.X509Certificates;
using System.Security.Cryptography;

using Org.BouncyCastle.X509;
using Org.BouncyCastle.Utilities;
using Org.BouncyCastle.Math;
using Org.BouncyCastle.Security;
using Org.BouncyCastle.Asn1.X509;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Generators;
using Org.BouncyCastle.Crypto.Operators;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Pkcs;

namespace Selfsigned
{
    public static class Certificates {
        public static X509Certificate2 GenerateCertificate(string subject) {

            var random = new SecureRandom();
            var certificateGenerator = new X509V3CertificateGenerator();

            var serialNumber = BigIntegers.CreateRandomInRange(BigInteger.One, BigInteger.ValueOf(Int64.MaxValue), random);
            certificateGenerator.SetSerialNumber(serialNumber);

            certificateGenerator.SetIssuerDN(new X509Name($"C=NL, O=SomeCompany, CN={subject}"));
            certificateGenerator.SetSubjectDN(new X509Name($"C=NL, O=SomeCompany, CN={subject}"));
            certificateGenerator.SetNotBefore(DateTime.UtcNow.Date);
            certificateGenerator.SetNotAfter(DateTime.UtcNow.Date.AddYears(1));

            const int strength = 2048;
            var keyGenerationParameters = new KeyGenerationParameters(random, strength);
            var keyPairGenerator = new RsaKeyPairGenerator();
            keyPairGenerator.Init(keyGenerationParameters);

            var subjectKeyPair = keyPairGenerator.GenerateKeyPair();
            certificateGenerator.SetPublicKey(subjectKeyPair.Public);

            var issuerKeyPair = subjectKeyPair;
            const string signatureAlgorithm = "SHA256WithRSA";
            var signatureFactory = new Asn1SignatureFactory(signatureAlgorithm,issuerKeyPair.Private);
            var bouncyCert = certificateGenerator.Generate(signatureFactory);

            // Lets convert it to X509Certificate2
            X509Certificate2 certificate;

            Pkcs12Store store = new Pkcs12StoreBuilder().Build();
            store.SetKeyEntry($"{subject}_key",new AsymmetricKeyEntry(subjectKeyPair.Private), new [] {new X509CertificateEntry(bouncyCert)});
            string exportpw = Guid.NewGuid().ToString("x");

            using(var ms = new System.IO.MemoryStream()){
                store.Save(ms,exportpw.ToCharArray(),random);
                certificate = new X509Certificate2(ms.ToArray(),exportpw,X509KeyStorageFlags.Exportable);
            }

            //Console.WriteLine($"Generated cert with thumbprint {certificate.Thumbprint}");
            return certificate;
        }

        // You can also load the certificate from to CurrentUser store
        private X509Certificate2 LoadCertificate(string subject) {
            var userStore = new X509Store(StoreName.My,StoreLocation.CurrentUser);
            userStore.Open(OpenFlags.OpenExistingOnly);
            if(userStore.IsOpen){
                var collection = userStore.Certificates.Find(X509FindType.FindBySubjectName,Environment.MachineName,false);
                if(collection.Count > 0) {
                    return collection[0];
                }
                userStore.Close();
            }
            return null;
        }

        // Or store the certificate to the local store.
        private void SaveCertificate(X509Certificate2 certificate) {
            var userStore = new X509Store(StoreName.My,StoreLocation.CurrentUser);
            userStore.Open(OpenFlags.ReadWrite);
            userStore.Add(certificate);
            userStore.Close();
        }
    }
}