Create a CSR in C# using an explicit RSA key-pair

Question

Using the OpenSSL libraries one can create a CSR (certificate signing request) by doing this:

openssl genrsa -out rsa.key 1024
openssl req -new -key rsa.key -out output.csr -config config.txt

where config.txt contains the distinguished name to use in the certificate.

I would like to do something similar under Windows using C#. However, the method createPKCS10 does not require you to supply an RSA key.

Is there a way to get C# to generate an explicit RSA private key and then use that private key to create the CSR?

Answer 1

You can use the OpenSSL.NET library to accomplish this task. The following routines should be what you need:

public static void Main() 
{
    Console.Write(GenerateCsr(GenerateRsaKeyPair()));
}

/// <summary>
/// Generates a 2048 bit RSA key pair.
/// </summary>
/// <returns>The key container</returns>
public static CryptoKey GenerateRsaKeyPair()
{
    using(var rsa = new RSA())
    {
        rsa.GenerateKeys(2048, 0x10021, null, null);
        return new CryptoKey(rsa);
    }
}

/// <summary>
/// Generates a CSR file content using to the hard-coded details and the given key.
/// </summary>
/// /// <param name="key">RSA key to be used</param>
/// <returns>The CSR file content</returns>
public static string GenerateCsr(CryptoKey key)
{
    using (var subject = new X509Name
    {
        SerialNumber = "1234567890",
        Organization = "My Company"
        // Add more details here...
    })
    {
        using (var req = new X509Request(0, subject, key))
        {
            return req.PEM;
        }
    }
}

Answer 2

Here is the code used to generate the .CSR file in C#. I am using Bouncy castle library.

        var subjectName = "CN=www.copanyName.com,O=Company Name,OU=Department,T=Area,ST=State,C=Country";

        // Create new Object for Issuer and Subject
        var issuer = new X509Name(subjectName);
        var subject = new X509Name(subjectName);

        // Generate the key Value Pair, which in our case is a public Key
        var randomGenerator = new CryptoApiRandomGenerator();
        var random = new SecureRandom(randomGenerator);
        AsymmetricCipherKeyPair subjectKeyPair = default(AsymmetricCipherKeyPair);
        const int strength = 2048;
        var keyGenerationParameters = new KeyGenerationParameters(random, strength);

        var keyPairGenerator = new RsaKeyPairGenerator();
        keyPairGenerator.Init(keyGenerationParameters);
        subjectKeyPair = keyPairGenerator.GenerateKeyPair();
        AsymmetricCipherKeyPair issuerKeyPair = subjectKeyPair;

        //PKCS #10 Certificate Signing Request
        Pkcs10CertificationRequest csr = new Pkcs10CertificationRequest("SHA1WITHRSA", subject, issuerKeyPair.Public, null, issuerKeyPair.Private);

        //Convert BouncyCastle CSR to .PEM file.
        StringBuilder CSRPem = new StringBuilder();
        PemWriter CSRPemWriter = new PemWriter(new StringWriter(CSRPem));
        CSRPemWriter.WriteObject(csr);
        CSRPemWriter.Writer.Flush();

        //get CSR text
        var CSRtext = CSRPem.ToString();

        // Write content into a Txt file
        using (StreamWriter f = new StreamWriter(@"C:\Cert_TEST\DemoCSR.txt"))
        {
            f.Write(CSRtext);
        }