I have headache for hours why it's say invalid signature eventhough I tried set default keyring in the repository. But after googling for some keyword I just found out that it's a BUG!!! Great!!! It's problem with the ISP's cache so how to AVOID it. Follow the instruction below.
use the same apt-get update but put one of these options after update
$ sudo apt-get update -o Acquire::http::No-Cache=True
$ sudo apt-get update -o Acquire::BrokenProxy=true
Then it's working again!