public class Program
{
  public static void Main(string[] args)
  {
    CreatePem("Alice");
    CreatePem("Bob");
    using (var alice = new RijndaelSite("Alice"))
    using (var bob = new RijndaelSite("Bob"))
    {
      alice.Send(bob, "Мама мыла раму");
      bob.Send(alice, "Рыбий жир победит постмодернизм.");
    }
  }

  static void CreatePem(string name)
  {
    var keyPair = DotNetUtilities.GetRsaKeyPair(new RSACryptoServiceProvider().ExportParameters(true));
    using (var fs = File.Create(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, $"{name}.pem")))
    using (var tw = new StreamWriter(fs))
    {
      var pw = new Org.BouncyCastle.OpenSsl.PemWriter(tw);
      pw.WriteObject(keyPair.Private);
    }
  }
}

class RijndaelSite: CryptoSite<RijndaelManaged>
{
  public RijndaelSite(string name) : base(name)
  {
  }
}

class CryptoSite<T> : IDisposable where T: SymmetricAlgorithm, new()
{
  readonly RSA _rsaKey;
  public byte[] PublicKey { get; }
  public string Name { get; }
  public CryptoSite(string name)
  {
    if (string.IsNullOrEmpty(name))
      throw new ArgumentException(nameof(name));
    Name = name;
    AsymmetricCipherKeyPair keyPair; 
    using (var reader = File.OpenText(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, $"{name}.pem")))
      keyPair = (AsymmetricCipherKeyPair) new Org.BouncyCastle.OpenSsl.PemReader(reader).ReadObject();
    _rsaKey = DotNetUtilities.ToRSA((RsaPrivateCrtKeyParameters) keyPair.Private);
    PublicKey = ((RSACryptoServiceProvider)_rsaKey).ExportCspBlob(false);
  }

  public void Send(CryptoSite<T> cryptoSite, string plainText)
  {
    using(var provider = new RSACryptoServiceProvider())
    using (var sa = new T())
    {
      provider.ImportCspBlob(cryptoSite.PublicKey);
      var keyFormatter = new RSAPKCS1KeyExchangeFormatter(provider);
      using (var ciphertext = new MemoryStream())
      using (var cs = new CryptoStream(ciphertext, sa.CreateEncryptor(), CryptoStreamMode.Write))
      {
        var plaintextMessage = Encoding.UTF8.GetBytes(plainText);
        cs.Write(plaintextMessage, 0, plaintextMessage.Length);
        cs.Close();
        Console.WriteLine("{0} send to {1} message {2}", Name, cryptoSite.Name, plainText);
        cryptoSite.Receive(sa.IV, keyFormatter.CreateKeyExchange(sa.Key, typeof(T)), ciphertext.ToArray());
      }
    }
  }

  public void Receive(byte[] iv, byte[] encryptedSessionKey, byte[] encryptedMessage)
  {
    using (var sa = new T())
    {
      sa.IV = iv;
      var keyDeformatter = new RSAPKCS1KeyExchangeDeformatter(_rsaKey);
      sa.Key = keyDeformatter.DecryptKeyExchange(encryptedSessionKey);
      using (var plaintext = new MemoryStream())
      using (var cs = new CryptoStream(plaintext, sa.CreateDecryptor(), CryptoStreamMode.Write))
      {
        cs.Write(encryptedMessage, 0, encryptedMessage.Length);
        cs.Close();
        Console.WriteLine("{0} received: {1}", Name, Encoding.UTF8.GetString(plaintext.ToArray()));
      }
    }
  }

  public void Dispose() => _rsaKey.Dispose();
}