Encrypt QueryString parameter in DataList control in ASP.Net

Here I have created sample as per your requirement.

HTML

<div>
    <asp:DataList ID="datalistproduct" runat="server" RepeatColumns="4" RepeatLayout="Flow"
        OnItemDataBound="datalistproduct_ItemDataBound">
        <ItemTemplate>
            <asp:HyperLink ID="hlProductId" NavigateUrl='<%# Eval("Product_Id","/ProductDetails.aspx?pitem={0}") %>'
                runat="server" Text="View Product" />
        </ItemTemplate>
    </asp:DataList>
</div>

Code

protected void Page_Load(object sender, EventArgs e)
{
    DataTable dt = new DataTable();
    dt.Columns.Add("Product_Id");
    dt.Rows.Add("a456d");
    dt.Rows.Add("fr85d");
    datalistproduct.DataSource = dt;
    datalistproduct.DataBind();
}

protected void datalistproduct_ItemDataBound(object sender, DataListItemEventArgs e)
{
    if (e.Item.ItemType == ListItemType.Item)
    {
        string productId = ((HyperLink)e.Item.FindControl("hlProductId")).NavigateUrl;
        productId = productId.Split('?')[1];
        productId = productId.Substring(6, productId.Length - 6);
        string encryptedProductId = Encrypt(productId);
        ((HyperLink)e.Item.FindControl("hlProductId")).NavigateUrl = string.Format("/ProductDetails.aspx?pitem={0}", encryptedProductId);
    }
}

private string Encrypt(string clearText)
{
    string EncryptionKey = "MAKV2SPBNI99212";
    byte[] clearBytes = Encoding.Unicode.GetBytes(clearText);
    using (Aes encryptor = Aes.Create())
    {
        Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });
        encryptor.Key = pdb.GetBytes(32);
        encryptor.IV = pdb.GetBytes(16);
        using (MemoryStream ms = new MemoryStream())
        {
            using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateEncryptor(), CryptoStreamMode.Write))
            {
                cs.Write(clearBytes, 0, clearBytes.Length);
                cs.Close();
            }
            clearText = Convert.ToBase64String(ms.ToArray());
        }
    }
    return clearText;
}

I hope this will help you out.