Here is a relevant section of the gnupg manual: https://www.gnupg.org/gph/en/manual.html#AEN385 https://www.gnupg.org/gph/en/manual.html#AEN385 I think the co

Here is a relevant section of the gnupg manual: https://www.gnupg.org/gph/en/manual.html#AEN385 https://www.gnupg.org/gph/en/manual.html#AEN385 I think the concept was developed by Hal Finney; IIRC there is somewhere an email in which he describes it.