RSAUtil.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Text;

namespace RSADemo_WinForms
{
    public static class RsaUtil
    {
        public static string ByteArrayToString(byte[] ba)
        {
            if (ba == null)
                return "null";

            return BitConverter.ToString(ba).Replace("-", "");
        }

        // https://stackoverflow.com/questions/28406888/c-sharp-rsa-public-key-output-not-correct/28407693#28407693
        public static string ExportPublicKeyToPemString(RSACryptoServiceProvider csp)
        {
            var parameters = csp.ExportParameters(false);
            string base64;
            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);
                }

                base64 = Convert.ToBase64String(stream.GetBuffer(), 0, (int) stream.Length);
            }

            return $"-----BEGIN PUBLIC KEY-----\r\n{base64}\r\n-----END PUBLIC KEY-----";
        }

        public 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));
                }
            }
        }

        public 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]);
                }
            }
        }
    }
}