First of all i have surfed through all previous questions related to this error, but nothing worked. Below is my output of sudo apt-get update:

following an old post i ended up changing my whole sources.list file.

Get:1 http://archive.canonical.com/ubuntu xenial InRelease [3,537 B]

Err:1 http://archive.canonical.com/ubuntu xenial InRelease                     

  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)

Get:2 http://archive.ubuntu.com/ubuntu xenial InRelease [3,534 B]              

Err:2 http://archive.ubuntu.com/ubuntu xenial InRelease

  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)

Get:3 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [3,542 B]

Err:3 http://archive.ubuntu.com/ubuntu xenial-updates InRelease

  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)

Get:4 http://archive.ubuntu.com/ubuntu xenial-backports InRelease [3,544 B]

Err:4 http://archive.ubuntu.com/ubuntu xenial-backports InRelease

  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)

Get:5 http://archive.ubuntu.com/ubuntu xenial-security InRelease [3,543 B]

Err:5 http://archive.ubuntu.com/ubuntu xenial-security InRelease

  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)

Reading package lists... Done  

N: See apt-secure(8) manpage for repository creation and user configuration details.

N: Updating from such a repository can't be done securely, and is therefore disabled by default.

E: The repository 'http://archive.canonical.com/ubuntu xenial InRelease' is not signed.

E: Failed to fetch http://archive.canonical.com/ubuntu/dists/xenial/InRelease  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)

N: See apt-secure(8) manpage for repository creation and user configuration details.

N: Updating from such a repository can't be done securely, and is therefore disabled by default.

E: The repository 'http://archive.ubuntu.com/ubuntu xenial InRelease' is not signed.

E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/xenial/InRelease  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)

N: See apt-secure(8) manpage for repository creation and user configuration details.

N: Updating from such a repository can't be done securely, and is therefore disabled by default.

E: The repository 'http://archive.ubuntu.com/ubuntu xenial-updates InRelease' is not signed.

E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/xenial-updates/InRelease  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)

E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/xenial-backports/InRelease  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)

E: The repository 'http://archive.ubuntu.com/ubuntu xenial-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.

E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/xenial-security/InRelease  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)

E: The repository 'http://archive.ubuntu.com/ubuntu xenial-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.