String to SecureString and SecureString to String converter

SecureStringHelperExtensions.cs

static class SecureStringConverter
    {
        public static SecureString ConvertToSecureString(this string password)
        {
            if (password == null)
                throw new ArgumentNullException("password");

            unsafe
            {
                fixed (char* passwordChars = password)
                {
                    var securePassword = new SecureString(passwordChars, password.Length);
                    securePassword.MakeReadOnly();
                    return securePassword;
                }
            }
        }

        public static string ConvertToString(this System.Security.SecureString str)
        {
            IntPtr bstr = Marshal.SecureStringToBSTR(str);
            byte b = 1;
            int i = 0;
            string s = "";
            while (((char)b) != '\0')
            {
                b = Marshal.ReadByte(bstr, i);
                i += 2;
                s += (char)b;
            }
            Marshal.FreeBSTR(bstr);
            return s;
        }
    }