I want to shrink the partition to be able to install a windows OS, but i installed ubuntu 15.04 with encryption of the logical volume, gparted doesn't support crypt-luks so what do i do?
Support for resizing LUKS partitions is being added right now to KDE Partition Manager. Grow support is already done in unreleased git versions and shrinking will be added soon. It will probably take a while until it reaches distributions and in particular Ubuntu but it will work at some point.
Growing and shrinking LUKS in action: https://www.youtube.com/watch?v=ZwiyCC9gvHM