/usr/lib/x86_64-linux-gnu/libcidn.so is in libc6-dev 2.15-0ubuntu10.18.
This file is owned by root:root, with mode 0o777.
It is a symlink to /lib/x86_64-linux-gnu/libcidn.so.1
/lib/x86_64-linux-gnu/libcidn.so.1