Get:1 http://archive.ubuntu.com/ubuntu xenial InRelease [247 kB]         
Get:2 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Get:3 http://archive.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Err:2 http://security.ubuntu.com/ubuntu xenial-security InRelease
  At least one invalid signature was encountered.
Ign:1 http://archive.ubuntu.com/ubuntu xenial InRelease      
Ign:3 http://archive.ubuntu.com/ubuntu xenial-security InRelease
Fetched 451 kB in 2s (193 kB/s)
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://security.ubuntu.com/ubuntu xenial-security InRelease: At least one invalid signature was encountered.
W: GPG error: http://archive.ubuntu.com/ubuntu xenial InRelease: At least one invalid signature was encountered.
W: The repository 'http://archive.ubuntu.com/ubuntu xenial InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://archive.ubuntu.com/ubuntu xenial-security InRelease: At least one invalid signature was encountered.
W: The repository 'http://archive.ubuntu.com/ubuntu xenial-security InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/xenial-security/InRelease  At least one invalid signature was encountered.
W: Some index files failed to download. They have been ignored, or old ones used instead.