Salsa20 шифрование каждого N-го символа

string text = "Привет! Hello!";
byte[] key = {117, 211, 146, 162, 230, 86, 207, 172, 3, 183, 170, 209, 16, 176, 21, 236, 94, 99, 85, 105, 120, 80, 208, 113, 59, 79, 207, 45, 198, 87, 227, 161};
byte[] iv = {184, 68, 238, 192, 150, 147, 73, 174};
 
// Шифровка
string encrypted;
using (var salsa = new Salsa20())
using (var mstream_out = new MemoryStream())
{
    salsa.Key = key;
    salsa.IV = iv;
    
    using (var cstream = new CryptoStream(mstream_out, salsa.CreateEncryptor(), CryptoStreamMode.Write))
    {
        var bytes = Encoding.UTF8.GetBytes(text);
        cstream.Write(bytes, 0, bytes.Length);
    }
    encrypted = Convert.ToBase64String(mstream_out.ToArray());
}
 
// Дешифровка
string decryptedText;
using (var salsa = new Salsa20())
using (var mstream_out = new MemoryStream())
{
    salsa.Key = key;
    salsa.IV = iv;
 
    using (var mstream_in = new MemoryStream(Convert.FromBase64String(encrypted)))
    using (var cstream = new CryptoStream(mstream_in, salsa.CreateDecryptor(), CryptoStreamMode.Read))
    {
        cstream.CopyTo(mstream_out);
    }
    decryptedText = Encoding.UTF8.GetString(mstream_out.ToArray());
}