I am running Ubuntu 16.10 KDE on a laptop on a wifi network that somehow forces a proxy without the need for it to be configured. Whenever I try apt-get update I get the following error:

Get:1 http://archive.ubuntu.com/ubuntu xenial InRelease [2827 B]

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

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

Get:2 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [2835 B]

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

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

Get:3 http://archive.ubuntu.com/ubuntu xenial-security InRelease [2836 B]

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

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

Fetched 8498 B in 0s (11.7 kB/s)   

Reading package lists... Done

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

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

E: Some index files failed to download. They have been ignored, or old ones used instead.

Installing a package returns errors like this:

Err:12 http://archive.ubuntu.com/ubuntu xenial/universe amd64 cabextract amd64 1.6-1

  Hash Sum mismatch

I have already tried setting the http-proxy variable using my username and password to no avail. The proxy does the exact same thing.

Is there any way to prevent it from doing whatever it's doing?

Does anybody know what NOSPLIT is? (If you do, could you comment?)

This is not a duplicate of questions like these:

'apt-get update' fails when using proxy

Got NODATA issue: 'NODATA' (does the network require authentication?)

because their answers do not work.

Frankly, my organization doesn't care whether their proxy is interfering with my installing software, because I'm the only one trying to do it, so asking them is out.
Also, I don't have the same problem as the NODATA issue, because wget -O - http://extras.ubuntu.com/ubuntu/dists/trusty/InRelease returns

--2016-11-11 12:20:26--  http://extras.ubuntu.com/ubuntu/dists/trusty/InRelease

 Resolving extras.ubuntu.com (extras.ubuntu.com)... 91.189.92.152, 2001:67c:1360:8c01::23

Connecting to extras.ubuntu.com (extras.ubuntu.com)|91.189.92.152|:80... connected.

HTTP request sent, awaiting response... 404 Not Found

2016-11-11 12:20:27 ERROR 404: Not Found.