如何使用 C# 创建自签名证书？

Question

I need to create a self-signed certificate (for local encryption - it's not used to secure communications), using C#.

I've seen some implementations that use P/Invoke with Crypt32.dll , but they are complicated and it's hard to update the parameters - and I would also like to avoid P/Invoke if at all possible.

I don't need something that is cross platform - running only on Windows is good enough for me.

Ideally, the result would be an X509Certificate2 object that I can use to insert into the Windows certificate store or export to a PFX file.

Answer 1

Since .NET 4.7.2 you can create self-signed certs using System.Security.Cryptography.X509Certificates.CertificateRequest .

For example:

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

public class CertificateUtil
{
    static void MakeCert()
    {
        var ecdsa = ECDsa.Create(); // generate asymmetric key pair
        var req = new CertificateRequest("cn=foobar", ecdsa, HashAlgorithmName.SHA256);
        var cert = req.CreateSelfSigned(DateTimeOffset.Now, DateTimeOffset.Now.AddYears(5));

        // Create PFX (PKCS #12) with private key
        File.WriteAllBytes("c:\\temp\\mycert.pfx", cert.Export(X509ContentType.Pfx, "P@55w0rd"));

        // Create Base 64 encoded CER (public key only)
        File.WriteAllText("c:\\temp\\mycert.cer",
            "-----BEGIN CERTIFICATE-----\r\n"
            + Convert.ToBase64String(cert.Export(X509ContentType.Cert), Base64FormattingOptions.InsertLineBreaks)
            + "\r\n-----END CERTIFICATE-----");
    }
}

Answer 2

This implementation uses the CX509CertificateRequestCertificate COM object (and friends - MSDN doc ) from certenroll.dll to create a self signed certificate request and sign it.

The example below is pretty straight forward (if you ignore the bits of COM stuff that goes on here) and there are a few parts of the code that are really optional (such as EKU) which are none-the-less useful and easy to adapt to your use.

public static X509Certificate2 CreateSelfSignedCertificate(string subjectName)
{
    // create DN for subject and issuer
    var dn = new CX500DistinguishedName();
    dn.Encode("CN=" + subjectName, X500NameFlags.XCN_CERT_NAME_STR_NONE);

    // create a new private key for the certificate
    CX509PrivateKey privateKey = new CX509PrivateKey();
    privateKey.ProviderName = "Microsoft Base Cryptographic Provider v1.0";
    privateKey.MachineContext = true;
    privateKey.Length = 2048;
    privateKey.KeySpec = X509KeySpec.XCN_AT_SIGNATURE; // use is not limited
    privateKey.ExportPolicy = X509PrivateKeyExportFlags.XCN_NCRYPT_ALLOW_PLAINTEXT_EXPORT_FLAG;
    privateKey.Create();

    // Use the stronger SHA512 hashing algorithm
    var hashobj = new CObjectId();
    hashobj.InitializeFromAlgorithmName(ObjectIdGroupId.XCN_CRYPT_HASH_ALG_OID_GROUP_ID,
        ObjectIdPublicKeyFlags.XCN_CRYPT_OID_INFO_PUBKEY_ANY, 
        AlgorithmFlags.AlgorithmFlagsNone, "SHA512");

    // add extended key usage if you want - look at MSDN for a list of possible OIDs
    var oid = new CObjectId();
    oid.InitializeFromValue("1.3.6.1.5.5.7.3.1"); // SSL server
    var oidlist = new CObjectIds();
    oidlist.Add(oid);
    var eku = new CX509ExtensionEnhancedKeyUsage();
    eku.InitializeEncode(oidlist); 

    // Create the self signing request
    var cert = new CX509CertificateRequestCertificate();
    cert.InitializeFromPrivateKey(X509CertificateEnrollmentContext.ContextMachine, privateKey, "");
    cert.Subject = dn;
    cert.Issuer = dn; // the issuer and the subject are the same
    cert.NotBefore = DateTime.Now;
    // this cert expires immediately. Change to whatever makes sense for you
    cert.NotAfter = DateTime.Now; 
    cert.X509Extensions.Add((CX509Extension)eku); // add the EKU
    cert.HashAlgorithm = hashobj; // Specify the hashing algorithm
    cert.Encode(); // encode the certificate

    // Do the final enrollment process
    var enroll = new CX509Enrollment();
    enroll.InitializeFromRequest(cert); // load the certificate
    enroll.CertificateFriendlyName = subjectName; // Optional: add a friendly name
    string csr = enroll.CreateRequest(); // Output the request in base64
    // and install it back as the response
    enroll.InstallResponse(InstallResponseRestrictionFlags.AllowUntrustedCertificate,
        csr, EncodingType.XCN_CRYPT_STRING_BASE64, ""); // no password
    // output a base64 encoded PKCS#12 so we can import it back to the .Net security classes
    var base64encoded = enroll.CreatePFX("", // no password, this is for internal consumption
        PFXExportOptions.PFXExportChainWithRoot);

    // instantiate the target class with the PKCS#12 data (and the empty password)
    return new System.Security.Cryptography.X509Certificates.X509Certificate2(
        System.Convert.FromBase64String(base64encoded), "", 
        // mark the private key as exportable (this is usually what you want to do)
        System.Security.Cryptography.X509Certificates.X509KeyStorageFlags.Exportable
    );
}

The result can be added to a certificate store using X509Store or exported using the X509Certificate2 methods.

For a fully managed and not tied to Microsoft's platform, and if you're OK with Mono's licensing, then you can look at X509CertificateBuilder from Mono.Security . Mono.Security is standalone from Mono, in that it doesn't need the rest of Mono to run and can be used in any compliant .Net environment (eg Microsoft's implementation).

Answer 3

Another option is to use the CLR Security extensions library from CodePlex, which implements a helper function to generate self-signed X.509 certificates:

X509Certificate2 cert = CngKey.CreateSelfSignedCertificate(subjectName);

You can also look at the implementation of that function (in CngKeyExtensionMethods.cs ) to see how to create the self-signed certificate explicitly in managed code.

Answer 4

You can use the free PluralSight.Crypto library to simplify programmatic creation of self-signed X.509 certificates:

    using (CryptContext ctx = new CryptContext())
    {
        ctx.Open();

        X509Certificate2 cert = ctx.CreateSelfSignedCertificate(
            new SelfSignedCertProperties
            {
                IsPrivateKeyExportable = true,
                KeyBitLength = 4096,
                Name = new X500DistinguishedName("cn=localhost"),
                ValidFrom = DateTime.Today.AddDays(-1),
                ValidTo = DateTime.Today.AddYears(1),
            });

        X509Certificate2UI.DisplayCertificate(cert);
    }

PluralSight.Crypto requires .NET 3.5 or later.

Answer 5

If it helps anyone else, I needed to generate a test certificate in PEM format (so needed crt and key files), using the answer from Duncan Smart , I produced the following...

public static void MakeCert(string certFilename, string keyFilename)
{
    const string CRT_HEADER = "-----BEGIN CERTIFICATE-----\n";
    const string CRT_FOOTER = "\n-----END CERTIFICATE-----";

    const string KEY_HEADER = "-----BEGIN RSA PRIVATE KEY-----\n";
    const string KEY_FOOTER = "\n-----END RSA PRIVATE KEY-----";

    using var rsa = RSA.Create();
    var certRequest = new CertificateRequest("cn=test", rsa, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);

    // We're just going to create a temporary certificate, that won't be valid for long
    var certificate = certRequest.CreateSelfSigned(DateTimeOffset.Now, DateTimeOffset.Now.AddDays(1));

    // export the private key
    var privateKey = Convert.ToBase64String(rsa.ExportRSAPrivateKey(), Base64FormattingOptions.InsertLineBreaks);

    File.WriteAllText(keyFilename, KEY_HEADER + privateKey + KEY_FOOTER);

    // Export the certificate
    var exportData = certificate.Export(X509ContentType.Cert);

    var crt = Convert.ToBase64String(exportData, Base64FormattingOptions.InsertLineBreaks);
    File.WriteAllText(certFilename, CRT_HEADER + crt + CRT_FOOTER);
}

Answer 6

Extending 0909EMs answer with SubjectAlternativeNames based on code found here: Understanding self-signed certificates in c#

        public static void MakeCert(string certFilename, string keyFilename)
        {
            const string CRT_HEADER = "-----BEGIN CERTIFICATE-----\n";
            const string CRT_FOOTER = "\n-----END CERTIFICATE-----";

            const string KEY_HEADER = "-----BEGIN RSA PRIVATE KEY-----\n";
            const string KEY_FOOTER = "\n-----END RSA PRIVATE KEY-----";

            using var rsa = RSA.Create();
            var certRequest = new CertificateRequest("cn=test", rsa, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);

            // Adding SubjectAlternativeNames (SAN)
            var subjectAlternativeNames = new SubjectAlternativeNameBuilder();
            subjectAlternativeNames .AddDnsName("test");
            certRequest.CertificateExtensions.Add(subjectAlternativeNames.Build());

            // We're just going to create a temporary certificate, that won't be valid for long
            var certificate = certRequest.CreateSelfSigned(DateTimeOffset.Now, DateTimeOffset.Now.AddDays(1));

            // export the private key
            var privateKey = Convert.ToBase64String(rsa.ExportRSAPrivateKey(), Base64FormattingOptions.InsertLineBreaks);

            File.WriteAllText(keyFilename, KEY_HEADER + privateKey + KEY_FOOTER);

            // Export the certificate
            var exportData = certificate.Export(X509ContentType.Cert);

            var crt = Convert.ToBase64String(exportData, Base64FormattingOptions.InsertLineBreaks);
            File.WriteAllText(certFilename, CRT_HEADER + crt + CRT_FOOTER);
        }

And for definition of the usage of a key using X509KeyUsageExtension look here https://stackoverflow.com/a/48210587/226278

Answer 7

This is the Powershell version on how to create a certificate. You can use it by executing the command. Check https://technet.microsoft.com/itpro/powershell/windows/pkiclient/new-selfsignedcertificate

Edit: forgot to say that after you create the certificate, you can use the Windows "manage computer certificates" program, to export the certificate to .CER or other type.