众所周知，Linux虽然比Windows的安全系数要高，但是经常更新还是个好习惯（至少近期就爆出了不少关于SSL的安全漏洞值得大家重视！），但是在Ubuntu和Debian下运行apt-get update刷新更新源的时候却经常遇到“There is no public key available for the following key IDs”的问题，具体表现为以下错误提示：

W:There is no public key available for the following key IDs: 3B4FE6ACC0B21F32,
W:There is no public key available for the following key IDs: 3B4FE6ACC0B21F32,

解决方法如下，

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3B4FE6ACC0B21F32

额外的，如果有如下问题，是因为加入了natty-backports源，删掉即可

W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/natty-backports/main/source/Sources  404  Not Found [IP: 91.189.91.13 80],
W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/natty-backports/restricted/source/Sources  404  Not Found [IP: 91.189.91.13 80],
W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/natty-backports/universe/source/Sources  404  Not Found [IP: 91.189.91.13 80],
W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/natty-backports/multiverse/source/Sources  404  Not Found [IP: 91.189.91.13 80],

运行如下命令：

sudo sed -i '/natty\-backports/d' /etc/apt/sources.list{,.d/*}