StrToMD5 кириллица

using System.Security.Cryptography;
using System.Text;
using System.Windows.Forms;

namespace md5_1
{
    public partial class FMD5 : Form
    {
        public FMD5()
        {
            InitializeComponent();
        }

        public string ComputeStringMD5Hash(string instr)
        {
            string strHash = string.Empty;

            foreach (byte b in new MD5CryptoServiceProvider().ComputeHash(Encoding.Default.GetBytes(instr)))
            {
                strHash += b.ToString("X2");
            }
            return strHash.ToLower();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            label1.Text = ComputeStringMD5Hash(textBox1.Text);
        }
    }
}