apt-get update出现xxx is not signed

1、问题

root@DESKTOP-Q86FNIK:~# apt update
Get:1 http://mirrors.163.com/ubuntu bionic InRelease [242 kB]
Err:1 http://mirrors.163.com/ubuntu bionic InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
Get:2 http://mirrors.163.com/ubuntu bionic-security InRelease [88.7 kB]
Err:2 http://mirrors.163.com/ubuntu bionic-security InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
Get:3 http://mirrors.163.com/ubuntu bionic-updates InRelease [88.7 kB]
Err:3 http://mirrors.163.com/ubuntu bionic-updates InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
Get:4 http://mirrors.163.com/ubuntu bionic-proposed InRelease [242 kB]
Err:4 http://mirrors.163.com/ubuntu bionic-proposed InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
Get:5 http://mirrors.163.com/ubuntu bionic-backports InRelease [83.3 kB]
Err:5 http://mirrors.163.com/ubuntu bionic-backports InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
Reading package lists... Done
W: GPG error: http://mirrors.163.com/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://mirrors.163.com/ubuntu bionic 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: http://mirrors.163.com/ubuntu bionic-security InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://mirrors.163.com/ubuntu bionic-security 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: http://mirrors.163.com/ubuntu bionic-updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://mirrors.163.com/ubuntu bionic-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: http://mirrors.163.com/ubuntu bionic-proposed InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://mirrors.163.com/ubuntu bionic-proposed 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: http://mirrors.163.com/ubuntu bionic-backports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://mirrors.163.com/ubuntu bionic-backports 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.

2、解决办法

主要是由于没有公钥，无法验证下列签名而导致的错误，参考1解决，下载导入公钥

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

# 此处的8B48AD6246925553、7638D0442B90D010是错误提示中的：NO_PUBKEY 3B4FE6ACC0B21F32