It goes without saying that there are some secrets which need to be kept, for whatever reason.
Friends of ours who work for a company involved in the manufacture of items for the space industry, have to
sign a letter of confidentiality, and don’t even talk to us about what they do.

This video of a Ch.4 programme, sums up a lot of the absurdity surrounding secrecy.

https://www.youtube.com/watch?v=itqMwuB8gOk