TUF protects against more attacks than just HTTPS or regular trivial signing methods do (rollback attacks, freezes, mix and match attacks, and helps secure mirrors), and has little to do with HTTPS or raw "transport layer encryption". It absolutely compliments and suppliments HTTPS if you're using it for your downloads, it is not obsoleted by it. (Though, the subtext on the introduction page probably doesn't help this impression by saying "Like the S in HTTPS...")
Well, the reason TUF has to protect against all of the attacks is because it is choosing to support a varying set of requirements, including lack of SSL and insecure mirrors. Mender simply does not care about them, so it can be dramatically simpler:
- rollback attacks -- impossible since all comms are secure, and there are no untrusted mirrors
- freezes -- impossible, because SSL channel must be re-negotiated every time
- mix and match attacks -- nothing to mix+match, mender only does one file (rootfs)
- helps secure mirrors -- mender does not support 3rd party mirrors, so no need to secure them.
You can see it right on the TUF homepage: it claims to replace application, library package and system package managers. This is a lot of work, which requires a lot of complexity, and there is no need at all to pay that price if you do not need to.
