Post-WW2, the allies sold enigma machines to developing countries, without mentioning they had broken the code.
Note: some people speculate the purchasing countries knew this and accepted the risk (https://www.reddit.com/r/AskHistorians/comments/5cpqrc/after_ww2_the_uk_government_sold_enigma_machines/).
The British kept radar secret during WW2, and attributed pilots’ ability to shoot down planes in the dark to vitamin A.
Cultures of dangerous pathogens (seems a bit weird to call this a secret, but it has a lot of attributes of secrets)
Tools and knowledge to build better gene-editing technology
Concrete applications of existing technology that could be highly dangerous
Drone manufacturing technology?
Methods of terrorism
Potential critical weakpoints in infrastructure
Other terrorism ideas
(Originally posted by Ruby)
“Drone manufacturing” might be unnecessary or something, or more accurately, you can figure out how to manufacture a drone so long as you can obtain one. I believe many systems can be reverse-engineered.
Though maybe less algorithms and training data. Google’s current search engine algorithm might actually be secret? I’ve heard it is supposed to be, but I don’t know.
Well, that’s the reason why governments try really hard to prevent foreign powers getting access to their drones and airplanes and other technologies.
Have “tools and knowledge to build better gene-editing technology” been kept secret? I had the impression developments were paraded out at conferences and in papers, etc.
My sense is that there is significant government research here that is being kept confidential, with parallel private sector research that is mostly public