Clearsigned file isn't valid, from 01.org package?

The installer contains the wrong repository url. To fix this you need:

1. Start the installer and try to install, you got error, close installer.

2. Open console and type:

   sudo -H gedit /etc/apt/sources.list.d/intellinuxgraphics.list
   

3. Replace text with text below, don't close gedit just leave it:

   deb https://download.01.org/gfx/ubuntu/14.04/main/ trusty main #Intel Graphics drivers
   

4. Start the installer again, press Begin button, press Install button and fast switch to gedit and hold CTRL+S.

   sudo apt-get update may yield:
   

   GPG error: https://download.01.org trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A902DDA375E52366
   

   Fix it with:

   sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys A902DDA375E52366
   

Here's the easier solution for those with problems:

1. Open Nautilus(file explorer), go to Edit -> Preferences -> Behavior, and make sure that under Executable text files, Ask each time is selected! (if not, select it)
2. Close the window
3. Open Nautilus (if not already open)
4. Right click New document > Empty document
5. Name it script.sh

6. Open it and enter this:

   #! /bin/bash
   while [ 1 ]; do
       sudo sed --in-place 's/http:/https:/g' /etc/apt/sources.list.d/intellinuxgraphics.list
       sleep 1
   done
   

7. Save it and close it

8. Right click on script.sh and select Permissions
9. Check Allow executing this file as program
10. Close the window
11. Double click it
12. Select Run in terminal
13. Enter password
14. DO NOT close the terminal
15. Begin installation, and everything should work

I also got Failed to fetch download.01.org/gfx/ubuntu/14.04/main/pool/main/i/… Size mismatch error and this fixed the problem. You can delete script file and close the terminal when the installation is finished.

Edit: There is simpler way:

1. Enter this in terminal:

   #! /bin/bash
   while [ 1 ]; do
       sudo sed --in-place 's/http:/https:/g' /etc/apt/sources.list.d/intellinuxgraphics.list
       sleep 1
   done
   

2. Enter password

3. Run the installer