Here is quick tip, if you want to obtain more that one key of same portal. Its easy only thing you should do is drop the Portal key that you have before hacking it. So there is possibility that you will obtain a key from hacking, and after it just pickup portal key that you dropped. This way you can obtain as much keys as you want. Remember portal hack limit is 4 times in 4 hours period, and cool-down time is 5 minutes.