Sharepoint Authentication Helpers

I'm working on a rather large SharePoint project that's going to contain helper classes for a variety of item levels (sites, webs, lists, items, etc').

When writing code I'm expecting that the same site and authentication will span across multiple contexts (as in you might use a list helper, and then an item helper with the same credentials). For this reason I've built a credentials class to hold onto those.

Also since site names are going to be used in every helper I've built a "base helper class to hold the information.

Here are my current classes:

My authentication class

/// <summary>
/// SharePoint Class for doing quick authentication tests with a SharePoint Online Server
/// </summary>
public class SPAuthentication
{
    /// <summary>
    /// Initialize authentication class with connection information
    /// </summary>
    /// <param name="username">The <paramref name="username"/> (@domain or @onmicrosoft.com) for connecting to SharePoint Online</param>
    /// <param name="password"></param>
    /// <exception cref="CryptographicException">An error occurred when encrypting the parameter <paramref name="password"/> into a secure string</exception>
    /// <exception cref="ObjectDisposedException">This secure string has already been disposed.</exception>
    /// <exception cref="ArgumentOutOfRangeException">Performing this operation would make the length of this secure string greater than 65536 characters.</exception>
    public SPAuthentication(string username, string password)
    {
        var securePassword = new SecureString();

        // use password input to create a secure string for use with SharePoint Online
        foreach (var c in password.ToCharArray()) securePassword.AppendChar(c);

        Credentials = new SharePointOnlineCredentials(username, securePassword);
    }

    /// <summary>
    /// Container to hold SharePoint credentials after instantiating class. This prevents the user having to repeatably fetch the user/pass and recreate a credentials class
    /// </summary>
    public readonly SharePointOnlineCredentials Credentials;
}

My base helper class

/// <summary>
/// A base helper class for use with other SharePoint related helper.
/// </summary>
public class SPBaseHelper
{
    /// <summary>
    /// Backing field for SiteUrl property
    /// </summary>
    private string _spSiteUrl;

    /// <summary>
    /// Property for storing the SharePoint site to be used for operations
    /// </summary>
    public string SiteUrl
    {
        get { return _spSiteUrl; }
        set
        {
            _spSiteUrl = value;
            _spSiteUrl = _spSiteUrl?.Replace(@"\", @"/");
        }
    }

    /// <summary>
    /// Current SHarePoint authentication information.
    /// </summary>
    internal SharePointOnlineCredentials SPCredentials;

}

The foundation of my first helper class (lists)

/// <summary>
/// A helper class for use with SharePoint lists using the client object model
/// </summary>
public class SPListHelper : SPBaseHelper
{
    /// <summary>
    /// Instantiate new helper class with
    /// </summary>
    /// <param name="auth">SharePoint Class for doing quick authentication tests with a SharePoint Online Server</param>
    public SPListHelper(SPAuthentication auth)
    {
        SPCredentials = auth.Credentials;
    }

    /// <summary>
    /// Perform a test connection to a SharePoint Online Site
    /// </summary>
    /// <returns>Returns the site name that the class was opened with</returns>
    public string TestConnect()
    {
        if (SiteUrl == null) return "No site specified.";

        using (var ctx = new ClientContext(SiteUrl))
        {
            ctx.Credentials = SPCredentials;

            var web = ctx.Web;

            ctx.Load(web);
            ctx.ExecuteQuery();

            return web.Title;
        }
    }
}

Is this going to be too difficult for those that follow? Is there a better approach I could change to this early on into a project, what general improvements could I make?