Sigsaly: Churchill's top secret hotline to the President

A great example of a one-time pad!

There is an illustration for the encryption process in Appendix B here: https://www.nsa.gov/portals/75/documents/about/cryptologic-h...

Also, worth reading about is Turing's Delilah speech encryption system: https://spectrum.ieee.org/alan-turings-delilah

* US president