Issue with an “invalid issuer or signature” response from Sharepoint Online via Salesforce Apex REST

Question

I'm trying to make REST callouts from Salesforce to Sharepoint and I'm getting an "Invalid issuer or signature" response from my code. I tried generating a self-signed certificate on Salesforce and uploading the .crt file in my Sharepoint app, but the same error message still persists. Is there something I'm missing here?

Something else I don't understand is that when I upload the Salesforce certificate using the "Upload Public Key", I get a Private Key type generated for that public cert with a private key number. Maybe this a mislabeling on the App Registration Portal itself?

Salesforce Apex REST code:

public class AuthCallout {
   public static void basicAuthCallout(){
     HttpRequest req = new HttpRequest();
     req.setMethod('GET');
     req.setEndpoint('callout:Sharepoint_Online/_api/web/list');
}
}

The Sharepoint_Online Named Credential uses the self-signed certificate and Oauth2.0 with the "Files.Read.All Sites.Read.All offline_access" scope. The authentication status says Authenticated, so I know that part is fine. From SF, I downloaded the public .crt file and reuploaded it to my registered app in "apps.dev.microsoft.com".

Answer 1

Changing my Named Credential URL to point to Microsoft Graph and then my Sharepoint Tenant now gives me a 'Bad Request' "Inner Error", so it appears to have worked in getting me inside Sharepoint Online.