Reading package lists... Done W: There is no public key available for the following key IDs: 7638D0442B90D010 W: There is no public key available for the following key IDs: 7638D0442B90D010 W: There is no public key available for the following key IDs: 9D6D8F6BC857C906
最近安装vanish想从apt自动安装，按照官方命令，添加vanish的源之后，竟然不行了。
好吧，卸载，结果apt-key del之前添加的GPG key之后，apt-get update出现这种错误：
Reading package lists... Done
W: There is no public key available for the following key IDs:
7638D0442B90D010
W: There is no public key available for the following key IDs:
7638D0442B90D010
W: There is no public key available for the following key IDs:
9D6D8F6BC857C906
搜索之后，得到解答：
apt-get install debian-keyring debian-archive-keyring
apt-get update
问题解决了。