Authenticate from onprem windows service to Sharepoint Online

Question

I'm building a Windows service that will run in a customer network.

This service should upload files to some SharePoint Online document library.

I'm struggling about authentication.

I could store the user/password of a fake user in the tenant in my service configuration, but I don't like the idea of storing user credentials.

I'm trying to understand alternatives. As far as I understand, I may be able to create some app to get an AppId and a secret. I created one in My apps page, but I didn't found how to setup my ClientContext (or rest call) to use these values.

I also didn't found how to grant this application permission in specific libraries in my tenant.

All other documentation I found relate to Addin development.

Everything is mixing in my head.

What is the correct way to grant a windows service permission to some SP365 site ?

Answer 1

If you used CSOM, you can do authentication as code below.

    public static void setOnlineCredential(ClientContext clientContext,string userName,string password)
    {
        //set the user name and password
        SecureString secureString = new SecureString();
        foreach (char c in password.ToCharArray())
        {
            secureString.AppendChar(c);
        }
        clientContext.Credentials = new SharePointOnlineCredentials(userName, secureString);           
    }

If you used System.Net to call REST API, you can do authentication as code below

    /// <summary>
    /// Get JSON data of list via REST API
    /// </summary>
    /// <param name="webUri"></param>
    /// <param name="credentials"></param>
    /// <param name="listTitle"></param>
    /// <returns></returns>
    public static JToken GetList(Uri webUri, string userName, string password, string listTitle)
    {
        SecureString secureString = new SecureString();

        foreach (char c in password.ToCharArray())
        {
            secureString.AppendChar(c);
        }

        using (WebClient client = new WebClient())
        {
            client.Headers.Add("X-FORMS_BASED_AUTH_ACCEPTED", "f");
            client.Credentials = new SharePointOnlineCredentials(userName, secureString);
            client.Headers.Add(HttpRequestHeader.ContentType, "application/json;odata=verbose");
            client.Headers.Add(HttpRequestHeader.Accept, "application/json;odata=verbose");
            Uri endpointUri = new Uri(webUri + "/_api/web/lists/getbytitle('" + listTitle + "')");
            string result = client.DownloadString(endpointUri);

            JObject jo = (JObject)JsonConvert.DeserializeObject(result);
            JToken jToken = jo["d"];
            return jToken;
        }
    }