Authentication to SharePoint using secure authentication and CSOM in a .NET application

I’m using CSOM for making REST API calls to a SharePoint list from a C# application. This needs credentials (of course), however I’ve found that it needs a username and a password in plain text. Something like code below:

using (ClientContext context = new ClientContext("https://site.sharepoint.com"))
{
    context.AuthenticationMode = ClientAuthenticationMode.FormsAuthentication; 
    context.FormsAuthenticationLoginInfo = new FormsAuthenticationLoginInfo("[email protected]", "P@ssw0rd");
    // Rest of the code
    context.ExecuteQuery(); // <-- error on this line
}

I’ve two issues with this code:

1. However I use the correct credentials, it’s given me this error on the marked line:

   The user’s login name or password is not valid.

2. Security. It’s not secure to add the credentials in plain text in the code.

I’ve also tried code below, but I get a 403 response (also with the correct credentials) and it’s not secure too.

using (ClientContext context = new ClientContext("https://site.sharepoint.com"))
{
    context.Credentials = new NetworkCredential("[email protected]", "P@ssw0rd");
    // Rest of the code
    context.ExecuteQuery(); // <-- error on this line
}

In the header stand this:

Access denied. Before opening files in this location: you must first browse to the web site and select the option to login automatically.

How can I solve this, especially the second issue?

 SecureString se = new SecureString();
 foreach (var cc in password)
 {
     se.AppendChar(cc);
 }
 var cre = new SharePointOnlineCredentials("[email protected]", se);
 using (ClientContext context = new ClientContext("https://site.sharepoint.com"))
 {
     context.Credentials = cre;
     // Rest of the code
     context.ExecuteQuery();
 }