How to generate CSR with existing private and public key via .net7?

I’ll say right away that before writing I tried different options from this site. However, none of them came up. Initial data: there is a private and public key, for example and not for real case (suppose they are in a txt file or even in a line, it does not matter). If you need a different format for the public key (der, pem, crt), tell me, I can convert them.

-----BEGIN RSA PUBLIC KEY-----
MEgCQQCo9+BpMRYQ/dL3DS2CyJxRF+j6ctbT3/Qp84+KeFhnii7NT7fELilKUSnx
S30WAvQCCo2yU1orfgqr41mM70MBAgMBAAE=
-----END RSA PUBLIC KEY-----

There is a requirement: this code (and CSR generation) must work equally on both Linux and Windows. Purpose: csr saved on disk as a file.

The main problem is that I am not particularly strong in cryptography and it is still difficult to choose a tool. I watched both BouncyCastle and System.Security.Cryptography. It seems like a simple task for an experienced. Please, help.

I tried something like this (BouncyCastle):

byte[] derKeyBytes = File.ReadAllBytes("key.der");
AsymmetricKeyParameter publicKey = PublicKeyFactory.CreateKey(derKeyBytes);

and this

var rsaPublicKey = RSA.Create();
rsaPublicKey.ImportFromPem(publicKeyString);

in the first case there was an error, in the second it was not possible to reach the final result

Solution

I did it. Use.

publicParameter and privateParameter are strings just like in the question above. With headers.

private static string GenerateCsrPem(string hostname, string publicParameter, string privateParameter)
    {
        AsymmetricKeyParameter publicKeyParameter;
        AsymmetricKeyParameter privateKeyParameter;
        using (TextReader publicPem = new StringReader(publicParameter))
        {
            var publicPemReader = new PemReader(publicPem);
            var publicPemObject = publicPemReader.ReadObject();
            publicKeyParameter = (AsymmetricKeyParameter)publicPemObject;
        }
        using (TextReader privatePem = new StringReader(privateParameter))
        {
            var privatePemReader = new PemReader(privatePem);
            var privatePemObject = privatePemReader.ReadObject();
            privateKeyParameter = ((AsymmetricCipherKeyPair)privatePemObject).Private;
        }
        
        var asymmetricCipherKeyPair = new AsymmetricCipherKeyPair(publicKeyParameter, privateKeyParameter);
        var values = new Dictionary<DerObjectIdentifier, string> {
            {X509Name.CN, hostname}, //domain name
            {X509Name.O, "SS"},
            {X509Name.C, "JP"},
        };
        var subject = new X509Name(values.Keys.Reverse().ToList(), values);
        var csr = new Pkcs10CertificationRequest(
            "SHA256withRSA",
            subject,
            asymmetricCipherKeyPair.Public,
            null,
            asymmetricCipherKeyPair.Private); 
         
        var csrPem = new StringBuilder();

        var csrPemToWrite = csrPem.ToString();
        return csrPemToWrite;   
    }