The public key can be derived from the secret key, so there's no need for the user to supply it separately.
http://hydra.nixos.org/build/32085949
This is currently only used by the Hydra queue runner rework, but like eff5021eaa it presumably will be useful for the C++ rewrite of nix-push and download-from-binary-cache. (@shlevy)
eff5021eaa