Use manifest file for master key, branches and packs (REPO FORMAT CHANGE)
Generate a Repository ID and verify it in the manifest (REPO FORMAT CHANGE)

This is a simplification and defends at someone maliciously switching
around different sign+encrypted manifest files of the same user. This
way we verify the repository we read is the repository we want.

Repo ID is not secret. Only requirement is that the same user does not
generate the same repo id more than onece.
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