While trying to run `apt-get update` on `linux mint 17.3` I am getting following error.

```
W: GPG error: http://extra.linuxmint.com rosa Release: The following 
signatures were invalid: BADSIG 3EE67F3D0FF405B2 Clement Lefebvre (Linux 
Mint Package Repository v1) <root@linuxmint.com>

```

I tried adding key `3EE67F3D0FF405B2` as mentioned on [this post](https://unix.stackexchange.com/questions/97493/invalid-signatures-when-running-apt-get-update) but the key remains unchanged and there no expired key in the list either.

The content of my official source package list file is as follows

```
deb http://free.nchc.org.tw/linuxmint/packages rosa main upstream import 

deb http://extra.linuxmint.com rosa main

deb http://np.archive.ubuntu.com/ubuntu trusty main restricted universe multiverse
deb http://np.archive.ubuntu.com/ubuntu trusty-updates main restricted universe multiverse

deb http://security.ubuntu.com/ubuntu/ trusty-security main restricted universe multiverse
deb http://archive.canonical.com/ubuntu/ trusty partner

```

The mirror site `http://free.nchc.org.tw/linuxmint/packages` and extra repo site `http://extra.linuxmint.com` have identical structure. What is causing the error and how to resolve it?

Also is there alternative mirror for `http://extra.linuxmint.com rosa main`??