* 现象：
$ sudo apt-get update时警告如下：
W: GPG error: http://ppa.launchpad.net precise Release: The following
signatures couldn't be verified because the public key is not
available: NO_PUBKEY 7FB8BEE0A1F196A8
* 原因：
apt中的repo缺少public key
* 解决：
下载public key
jb@H39:~/.config$ gpg --keyserver subkeys.pgp.net --recv-keys 7FB8BEE0A1F196A8
gpg: requesting key A1F196A8 from hkp server subkeys.pgp.net
gpg: key A1F196A8: public key "Launchpad PPA for Pidgin Developers" imported
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: Total number processed: 1
gpg: imported: 1 (RSA: 1)
导入下载的public key
jb@H39:~/.config$ gpg --export --armor 7FB8BEE0A1F196A8 | sudo apt-key add -
OK
再次$ sudo apt-get update
上面的警告消除