Convert RSACryptoServiceProvider ToXmlString RsaXML W3C XKMS format to Public Private key PEM base64 string

RSACryptoServiceProviderHelper.cs

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;

public class RSACryptoServiceProviderHelper
{
    // code from here:
    // - https://stackoverflow.com/questions/23734792/c-sharp-export-private-public-rsa-key-from-rsacryptoserviceprovider-to-pem-strin
    // - https://stackoverflow.com/questions/28406888/c-sharp-rsa-public-key-output-not-correct/28407693#28407693

    public static string ExportPrivateKey(RSACryptoServiceProvider csp)
    {
        var sb = new StringBuilder();
        var outputStream = new StringWriter(sb);

        if (csp.PublicOnly) throw new ArgumentException("CSP does not contain a private key", "csp");
        var parameters = csp.ExportParameters(true);
        using (var stream = new MemoryStream())
        {
            var writer = new BinaryWriter(stream);
            writer.Write((byte)0x30); // SEQUENCE
            using (var innerStream = new MemoryStream())
            {
                var innerWriter = new BinaryWriter(innerStream);
                EncodeIntegerBigEndian(innerWriter, new byte[] { 0x00 }); // Version
                EncodeIntegerBigEndian(innerWriter, parameters.Modulus);
                EncodeIntegerBigEndian(innerWriter, parameters.Exponent);
                EncodeIntegerBigEndian(innerWriter, parameters.D);
                EncodeIntegerBigEndian(innerWriter, parameters.P);
                EncodeIntegerBigEndian(innerWriter, parameters.Q);
                EncodeIntegerBigEndian(innerWriter, parameters.DP);
                EncodeIntegerBigEndian(innerWriter, parameters.DQ);
                EncodeIntegerBigEndian(innerWriter, parameters.InverseQ);
                var length = (int)innerStream.Length;
                EncodeLength(writer, length);
                writer.Write(innerStream.GetBuffer(), 0, length);
            }

            var base64 = Convert.ToBase64String(stream.GetBuffer(), 0, (int)stream.Length).ToCharArray();
            outputStream.WriteLine("-----BEGIN RSA PRIVATE KEY-----");
            // Output as Base64 with lines chopped at 64 characters
            for (var i = 0; i < base64.Length; i += 64)
            {
                outputStream.WriteLine(base64, i, Math.Min(64, base64.Length - i));
            }
            outputStream.WriteLine("-----END RSA PRIVATE KEY-----");
        }

        return sb.ToString();
    }

    public static string ExportPublicKey(RSACryptoServiceProvider csp)
    {
        var sb = new StringBuilder();
        var outputStream = new StringWriter(sb);

        var parameters = csp.ExportParameters(false);
        using (var stream = new MemoryStream())
        {
            var writer = new BinaryWriter(stream);
            writer.Write((byte)0x30); // SEQUENCE
            using (var innerStream = new MemoryStream())
            {
                var innerWriter = new BinaryWriter(innerStream);
                innerWriter.Write((byte)0x30); // SEQUENCE
                EncodeLength(innerWriter, 13);
                innerWriter.Write((byte)0x06); // OBJECT IDENTIFIER
                var rsaEncryptionOid = new byte[] { 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x01, 0x01, 0x01 };
                EncodeLength(innerWriter, rsaEncryptionOid.Length);
                innerWriter.Write(rsaEncryptionOid);
                innerWriter.Write((byte)0x05); // NULL
                EncodeLength(innerWriter, 0);
                innerWriter.Write((byte)0x03); // BIT STRING
                using (var bitStringStream = new MemoryStream())
                {
                    var bitStringWriter = new BinaryWriter(bitStringStream);
                    bitStringWriter.Write((byte)0x00); // # of unused bits
                    bitStringWriter.Write((byte)0x30); // SEQUENCE
                    using (var paramsStream = new MemoryStream())
                    {
                        var paramsWriter = new BinaryWriter(paramsStream);
                        EncodeIntegerBigEndian(paramsWriter, parameters.Modulus); // Modulus
                        EncodeIntegerBigEndian(paramsWriter, parameters.Exponent); // Exponent
                        var paramsLength = (int)paramsStream.Length;
                        EncodeLength(bitStringWriter, paramsLength);
                        bitStringWriter.Write(paramsStream.GetBuffer(), 0, paramsLength);
                    }
                    var bitStringLength = (int)bitStringStream.Length;
                    EncodeLength(innerWriter, bitStringLength);
                    innerWriter.Write(bitStringStream.GetBuffer(), 0, bitStringLength);
                }
                var length = (int)innerStream.Length;
                EncodeLength(writer, length);
                writer.Write(innerStream.GetBuffer(), 0, length);
            }

            var base64 = Convert.ToBase64String(stream.GetBuffer(), 0, (int)stream.Length).ToCharArray();
            outputStream.WriteLine("-----BEGIN PUBLIC KEY-----");
            for (var i = 0; i < base64.Length; i += 64)
            {
                outputStream.WriteLine(base64, i, Math.Min(64, base64.Length - i));
            }
            outputStream.WriteLine("-----END PUBLIC KEY-----");
        }

        return sb.ToString();
    }

    private static void EncodeLength(BinaryWriter stream, int length)
    {
        if (length < 0) throw new ArgumentOutOfRangeException("length", "Length must be non-negative");
        if (length < 0x80)
        {
            // Short form
            stream.Write((byte)length);
        }
        else
        {
            // Long form
            var temp = length;
            var bytesRequired = 0;
            while (temp > 0)
            {
                temp >>= 8;
                bytesRequired++;
            }
            stream.Write((byte)(bytesRequired | 0x80));
            for (var i = bytesRequired - 1; i >= 0; i--)
            {
                stream.Write((byte)(length >> (8 * i) & 0xff));
            }
        }
    }

    private static void EncodeIntegerBigEndian(BinaryWriter stream, byte[] value, bool forceUnsigned = true)
    {
        stream.Write((byte)0x02); // INTEGER
        var prefixZeros = 0;
        for (var i = 0; i < value.Length; i++)
        {
            if (value[i] != 0) break;
            prefixZeros++;
        }
        if (value.Length - prefixZeros == 0)
        {
            EncodeLength(stream, 1);
            stream.Write((byte)0);
        }
        else
        {
            if (forceUnsigned && value[prefixZeros] > 0x7f)
            {
                // Add a prefix zero to force unsigned if the MSB is 1
                EncodeLength(stream, value.Length - prefixZeros + 1);
                stream.Write((byte)0);
            }
            else
            {
                EncodeLength(stream, value.Length - prefixZeros);
            }
            for (var i = prefixZeros; i < value.Length; i++)
            {
                stream.Write(value[i]);
            }
        }
    }
}

code from here:

• https://stackoverflow.com/questions/23734792/c-sharp-export-private-public-rsa-key-from-rsacryptoserviceprovider-to-pem-strin
• https://stackoverflow.com/questions/28406888/c-sharp-rsa-public-key-output-not-correct/28407693#28407693

info on formats: https://www.cryptosys.net/pki/rsakeyformats.html