当我这样做时会发生这种情况apt-get update：
Get:1 http://security.debian.org stretch/updates InRelease [68.2 kB]
Ign:1 http://security.debian.org stretch/updates InRelease
Get:2 http://ftp.bme.hu/debian stretch InRelease [175 kB]
Get:3 http://ftp.cz.debian.org/debian stretch-updates InRelease [88.5 kB]
Ign:2 http://ftp.bme.hu/debian stretch InRelease
Ign:3 http://ftp.cz.debian.org/debian stretch-updates InRelease
Fetched 332 kB in 1s (175 kB/s)
Reading package lists... Done
W: GPG error: http://security.debian.org stretch/updates InRelease: At least one invalid signature was encountered.
W: The repository 'http://security.debian.org stretch/updates InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially …