RSA加密前端JS加密，后端asp.net解密，报异常

function GetChangeStr() 
{
            debugger;
            var pwdStr = document.getElementById("txtPassWordStr").value;
            var uidStr= $("#<%=txtUserStr.ClientID%>").val();
            if (!pwdStr || pwdStr.length == 0 || !uidStr || uidStr.length==0) {
                return false;
            }
            var keyE = "<%=PubRsaKeyE%>";
            var keyM = "<%=PubRsaKeyM%>";
            setMaxDigits(131);
            var key = new RSAKeyPair(keyE, "", keyM);
            var pwdRtn = encryptedString(key, pwdStr);
            var uidRtn = encryptedString(key, uidStr);
            $("#<%=txtPassword.ClientID%>").val(pwdRtn);
            $("#<%=txtUser.ClientID%>").val(uidRtn);
}

        try
        {
            RSACryptoServiceProvider rsa = (RSACryptoServiceProvider)Cache.Get("privateKey");
            byte[] result = rsa.Decrypt(HexStringToBytes(password), false);
            password = Encoding.UTF8.GetString(result);

            result = rsa.Decrypt(HexStringToBytes(userName), false);
            userName = Encoding.UTF8.GetString(result);
        }
        catch (Exception ex)
        {
            ShowMessage("身份验证失败！", "您登陆的帐号或密码错误!","");
            return;
        }