do checksum integrity check in installers

[ashleygwilliams]

we make the checksums, let's use them in the installers to add a (thin) layer of security

[Gankra]

Note for implementors, this affects all the "fetching installers", which predate the checksum system and need to be updated to use them:

• shell
• powershell
• npm
• homebrew

There are two ways to approach this:

1. bake the checksum into the installer
2. fetch the checksum file and compare it

The homebrew installer already implements this feature (as expected by homebrew), and it uses the baking strategy. In general baking is faster and more robust, but it comes at the cost of preventing the user for re-uploading binaries or doing post-processing that we don't know about.

The in-tree partial support for signing windows artifacts effectively looks like such a "reupload", which results in us regenerating the checksums for signed files after installers have been built (because the installers are the things we want to sign, and we do all signing in one shot, absent a more complete design for signing that can be interleaved into the build). However this signing support currenly only modifies .msi and .ps1 files -- neither of which is fetched by another installer, so it's not a problem. However it would be reasonable in the future for us to support signing the binaries stored in fetchable archives.

In the future better integration of signing tools (where we interleave signing into the build instead of stamping it on at the end) will resolve all the tension here.

In the immediate term, baking is fine but will block "sign the contents of an archive". If we want to keep "sign the contents of the archive" on the table without fixing the signing design (or just want to let users edit things as long as they regen checksums), then fetching is needed.

I think we should just accept baking (homebrew has to use it anyway) and deal with signing stuff properly down the road (or we disable checksum stuff for signed binaries since they essentially have super-checksums stapled to themselves).

Note that for most installers there isn't really a material difference in security between the solutions, because you fetch the installer from the same server as the archives and everything uses https anyway. The checksums mostly exist to catch things like on-disk data corruption. (Homebrew does get a marginal security benefit here in that taps are a separate github repo, so there are in fact two sources of truth that need to be kept consistent by an attacker.)