How to unlock encrypted file system on Linux automatically

1. create a keyfile with random content

$ sudo su -
$ dd if=/dev/random of=/root/keyfile bs=1024 count=4

2. Add the file to LUKS

$ cryptsetyp luksAddKey /dev/sdb1 /root/keyfile

3. Edit /etc/crypttab

$ sdb1_crypt /dev/sdb1 /root/keyfile luks

4. Edit /etc/fstab

/dev/mapper/sdb1_crypt /media/Repository ext4 defaults 0 2

5. Reboot or Remount