  Formal methods have been largely thought of in the context of safety-critical
systems, where they have achieved major acceptance. Tens of millions of people
trust their lives every day to such systems, based on formal proofs rather than
``we haven't found a bug'' (yet!). Why is ``we haven't found a bug'' an
acceptable basis for systems trusted with hundreds of millions of people's
personal data?
  This paper looks at some of the issues in CyberSecurity, and the extent to
which formal methods, ranging from ``fully verified'' to better tool support,
could help. Alas The Royal Society (2016) only recommended formal methods in
the limited context of ``safety critical applications'': we suggest this is too
limited.
