PemReader.cs

using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.OpenSsl;
using Org.BouncyCastle.Security;
using System;
using System.IO;
using System.Security.Cryptography;

namespace WindowsFormsApplication5
{
    public static class PemReaderB
    {
        static RSACryptoServiceProvider GetRsaProviderFromPem(string pemstr)
        {
            var cspParameters = new CspParameters { KeyContainerName = "MyKeyContainer" };
            var rsaKey = new RSACryptoServiceProvider(cspParameters);

            Func<RSACryptoServiceProvider, RsaPrivateCrtKeyParameters, RSACryptoServiceProvider> MakePrivateRCSP = (rcsp, rkp) =>
            {
                var rsaParameters = DotNetUtilities.ToRSAParameters(rkp);
                rcsp.ImportParameters(rsaParameters);
                return rsaKey;
            };

            var reader = new PemReader(new StringReader(pemstr));
            var kp = reader.ReadObject();

            return MakePrivateRCSP(rsaKey, (((RsaPrivateCrtKeyParameters)kp)));
        }

        public static RSACryptoServiceProvider GetRsaProviderFromPemFile(string pemfile)
        {
            return GetRsaProviderFromPem(File.ReadAllText(pemfile).Trim());
        }
    }
}