Post-WW2, the allies sold enigma machines to developing countries, without mentioning they had broken the code.
Source: https://en.wikipedia.org/wiki/Enigma_machine
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/).
Post-WW2, the allies sold enigma machines to developing countries, without mentioning they had broken the code.
Source: https://en.wikipedia.org/wiki/Enigma_machine
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/).