  In order to apply canonical labelling of graphs and isomorphism checking in
interactive theorem provers, these checking algorithms must either be
mechanically verified or their results must be verifiable by independent
checkers. We analyze a state-of-the-art algorithm for canonical labelling of
graphs (described by McKay and Piperno) and formulate it in terms of a formal
proof system. We provide an implementation that can export a proof that the
obtained graph is the canonical form of a given graph. Such proofs are then
verified by our independent checker and can be used to confirm that two given
graphs are not isomorphic.
