Password Decryption in C#

 public static string Decrypt(string strPasswordDecrypt)
 {
    byte[] sByBuf = Encoding.ASCII.GetBytes(strPasswordDecrypt);
    for (int nCtr = 0; nCtr < sByBuf.Length; nCtr++)
    sByBuf[nCtr] = Convert.ToByte(Convert.ToInt64(sByBuf[nCtr]) - 4);
    string strDecrptedword = Encoding.ASCII.GetString(sByBuf);
    return strPasswordDecrypt;
}