Here is a relevant section of the gnupg manual:<p><a href="https:&#x2F;&#x2F;www.gnupg.org&#x2F;gph&#x2F;en&#x2F;manual.html#AEN385" rel="nofollow">https:&#x2F;&#x2F;www.gnupg.org&#x2F;gph&#x2F;en&#x2F;manual.html#AEN385</a><p>I think the concept was developed by Hal Finney; IIRC there is somewhere an email in which he describes it.