You're downloading form a third-party volunteer. What protocol can provide privacy given that condition?
The HTTPS everywhere movements are attempting to make privacy the default rather than the exception, but are of course done knowing that the server will always know whats up. The point is to make it so that only the two parties concerned, the server and the client, comprehend the communication rather than the entire world
1. I gain access to a router near you.
2. I rent a sizable server at the hetzner/ovh/… location that's closest to you and volunteer to run a mirror for the OS you're using.
Both are somewhat uncertain (your traffic might flow via a different route, you might be load-balanced to a different mirror) but the uncertainty seems comparable. Option 2 seems so much easier that I have a real problem seeing the point of even attempting option 1 if all I want is the information option 2 would give. Perhaps someone can explain?
However, any argument against using encryption for privacy for APT can equally be applied to any other traffic. Do you trust your public internet route enough to let your traffic run authenticated, but unencrypted? Chats, news, bank statements, software updates?
Even if content cannot be modified, it can still be blocked or made public. There are quite a few nosy governments that would like to know or block certain types of content, software packages included.
As for privacy, eh? It is visible you're connecting to a debian mirror and what size of update list you're getting. Barring that, indexing packages by size is trivial.
You want true privacy, you'd have to use Tor or such.
For example, they might want to know what versions you're running (by looking at what updates you _didn't_ download) so they can target you or lots of people at once.
Erm, what?
The number of people, who can listen to (much less — modify) your traffic is very small. It is basically your ISP (who is supposed to offers you services in good faith, not spy on you) and a number of engineers, who maintain Internet backbone. That's far from "everyone". Some SSL evangelists make it sound like everyone's traffic is permanently broadcasted to everyone else in the world, but it is not.
As for "vulnerable packages", the most certain sign, that someone does not install security updates, is lack of traffic between them and update servers. But that's orthogonal to use of encryption.
for example, overlay onion routing, size blurring by appending random length random bits,... with oblivious transfer even the APT-server does not know what you downloaded (but that would require a large amount of information..., nevertheless oblivious transfer might still be a useful tool when used as a primitive, perhaps just to send a list of bootstrap addresses for p2p hosting of the signed files etc...)