How to Programmatically Generate an X509 Certificate Using Only C#

Is it possible to programmatically generate an X509 certificate using only C#?

Just to clarify, an X.509 certificate does not contain the private key. The word certificate is sometimes misused to represent the combination of the certificate and the private key, but they are two distinct entities. The whole point of using certificates is to send them more or less openly, without sending the private key, which must be kept secret. An X509Certificate2 object may have a private key associated with it (via its PrivateKey property), but that's only a convenience as part of the design of this class.

In your first BouncyCastle code example, newCert is really just the certificate and DotNetUtilities.ToX509Certificate(newCert) is built from the certificate only.

Considering that the PKCS#12 format requires the presence of a private key, I'm quite surprised that the following part even works (considering you're calling it on a certificate which can't possibly know the private key):

.Export(System.Security.Cryptography.X509Certificates.X509ContentType.Pkcs12,
    "password");

(gen.Generate(kp.Private) signs the certificate using the private key, but doesn't put the private key in the certificate, which wouldn't make sense.)

If you want your method to return both the certificate and the private key you could either:

• Return an X509Certificate2 object in which you've initialized the PrivateKey property
• Build a PKCS#12 store and returns its byte[] content (as if it was a file). Step 3 in the link you've sent (mirror) explains how to build a PKCS#12 store.

Returning the byte[] (DER) structure for the X.509 certificate itself will not contain the private key.

If your main concern (according to your test case) is to check that the certificate was built from an RSA key-pair, you can check the type of its public key instead.

How can I create an X509 Store key programmatically for use in encryption?

Since you apparently encrypt an AES key, it can be assumed that the (maximum) length of your data is 32 bytes (AES-256). The error message can be reproduced, e.g. if the message is too large for the key used.

For RSA, the length of a message must not exceed the key length. In fact, it is even smaller, because a part of the allowed length is reserved for padding. For OAEP this part depends on the digest used. For SHA512 the reserved part (130 bytes) is so large that a 1024 bits (128 bytes) key is not sufficient, with a 2048 bits (256 bytes) key the message may still be 126 bytes large, see here.

You create an RSA key with

var rsa = RSA.Create(); 

which uses a default value for the key length, s. RSA.Create(). This default value also depends on the .NET version. Under .NET Framework 4.8 a 1024 bits key is created on my machine (nowadays too short), under .NET Core 3.1 a 2048 bits key is created. You can check the key length with rsa.KeySize.

Probably a key that is too short for your purposes is generated in your environment. It is always better not to rely on defaults, but to specify the values explicitly. The Create method has an overload that makes this possible, see RSA.Create(Int32). You should use this overload and create a key of (at least 2048 bits).

Alternatively, the bug could theoretically be eliminated with another digest (e.g. SHA256). Independent of this, a key with 1024 bits should not be used nowadays (12/2020) for security reasons, the length should be at least 2048 bits (see here).

Generate and Sign Certificate Request using pure .net Framework

Short answer: You can starting in .NET Framework 4.7.2.

This functionality was originally added to .NET Core 2.0 in the form of the CertificateRequest class, which can build a PKCS#10 certification signing request or an X.509 (self-signed or chained) public key certificate.

The classes for that feature were made available in .NET Framework 4.7.2.

using (RSA parent = RSA.Create(4096))
using (RSA rsa = RSA.Create(2048))
{
    CertificateRequest parentReq = new CertificateRequest(
        "CN=Experimental Issuing Authority",
        parent,
        HashAlgorithmName.SHA256,
        RSASignaturePadding.Pkcs1);

    parentReq.CertificateExtensions.Add(
        new X509BasicConstraintsExtension(true, false, 0, true));

    parentReq.CertificateExtensions.Add(
        new X509SubjectKeyIdentifierExtension(parentReq.PublicKey, false));

    using (X509Certificate2 parentCert = parentReq.CreateSelfSigned(
        DateTimeOffset.UtcNow.AddDays(-45),
        DateTimeOffset.UtcNow.AddDays(365)))
    {
        CertificateRequest req = new CertificateRequest(
            "CN=Valid-Looking Timestamp Authority",
            rsa,
            HashAlgorithmName.SHA256,
            RSASignaturePadding.Pkcs1);

        req.CertificateExtensions.Add(
            new X509BasicConstraintsExtension(false, false, 0, false));

        req.CertificateExtensions.Add(
            new X509KeyUsageExtension(
                X509KeyUsageFlags.DigitalSignature | X509KeyUsageFlags.NonRepudiation,
                false));

        req.CertificateExtensions.Add(
            new X509EnhancedKeyUsageExtension(
                new OidCollection
                {
                    new Oid("1.3.6.1.5.5.7.3.8")
                },
                true));

        req.CertificateExtensions.Add(
            new X509SubjectKeyIdentifierExtension(req.PublicKey, false));

        using (X509Certificate2 cert = req.Create(
            parentCert,
            DateTimeOffset.UtcNow.AddDays(-1),
            DateTimeOffset.UtcNow.AddDays(90),
            new byte[] { 1, 2, 3, 4 }))
        {
            // Do something with these certs, like export them to PFX,
            // or add them to an X509Store, or whatever.
        }
    }
}

Longer answer if you're stuck on older versions: To accomplish your goal without adding any new P/Invokes, you would need to read and understand the following documents:

• ITU-T X.680-201508, the ASN.1 language
• IETF RFC 5280 or ITU-T X.509, the documents that explain the fields in X.509 certificates.
• IETF RFC 2986, explains the PKCS#10 certification signing request
• ITU-T X.690, explains the BER encoding family for ASN.1 (including DER) which tells you how to read and write bytes to achieve the semantic meaning from X.509 / PKCS#10.

And then you could write a DER writer/reader, and just emit the bytes for what you want.

How can I create a self-signed certificate using C#?

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 (e.g. Microsoft's implementation).

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