When the user first logs in, he fetches his TGT from the KDC along with the session key. I get why Sa (session key) is encrypted with Ka (user's key). But why is TGT also encrypted with Ka. TGT is already pretty unbreakable on its own because it's encrypted with Kkdc (KDC's private key). So even if an intruder gets hold of it, it can practically do nothing with it.