<repo>/root.json does not have enough signatures signed with the appropriate keys

I get this error when doing `cabal update`:

```
Downloading the latest package list from hackage.haskell.org
<repo>/root.json does not have enough signatures signed with the appropriate
keys
```

This presumably means a problem with Hackage.

This error happens both with cabal git HEAD and with a released version of Cabal (1.25.0.0)
