http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/300-walking-onions.html
Since generating a signature takes
around 20 usec on my mediocre laptop, authorities should be able to
generate this many signatures fairly easily.
Another possibility is to use a **threshold signature** on each SNIP,
so that the authorities collaboratively generate a short signature
that the clients can verify.