GPG error错误

当使用非官方debian源的时候，会出现gpg error的问题:

W: GPG error: http://ftp.sjtu.edu.cn ./ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 73E6B0FAA42A6CF5
W: You may want to run apt-get update to correct these problems

(1)倒数第二行的73E6B0FAA42A6CF5即为缺失的公钥，使用如下命令获得公钥：
gpg --keyserver pgp.mit.edu --recv-keys 73E6B0FAA42A6CF5
公钥服务器可更换为其他，如wwwkeys.eu.pgp.net，视自己的网络状况决定。
获得公钥后的输出如下：
Juno:/etc/apt# gpg --keyserver pgp.mit.edu --recv-keys 73E6B0FAA42A6CF5
gpg: requesting key A42A6CF5 from hkp server pgp.mit.edu
gpg: /root/.gnupg/trustdb.gpg: trustdb created
gpg: key A42A6CF5: public key "shame (beryl repository) <shame@sidux>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1

(2）将公钥导入apt
gpg --armor --export A42A6CF5 | apt-key add -

(3）再次运行apt-get update