/usr/lib/x86_64-linux-gnu/libsignal-protocol-c.so.2 is in libsignal-protocol-c2 2.3.1+git20171007-3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libsignal-protocol-c.so.2.3.1