在 Ubuntu 系统上执行 sudo apt-get update; sudo apt-get upgrade
命令更新软件列表时出错,报错信息如下:
Reading package lists... Done
W: GPG error: https://mirrors.tuna.tsinghua.edu.cn/ubuntu trusty-updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu trusty-updates InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: https://mirrors.tuna.tsinghua.edu.cn/ubuntu trusty-backports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
请问,该怎么做才能正确地完成软件更新呢 ?