A possible procedure is described by Moxie here:<p><a href="https:&#x2F;&#x2F;moderncrypto.org&#x2F;mail-archive&#x2F;messaging&#x2F;2014&#x2F;001022.html" rel="nofollow">https:&#x2F;&#x2F;moderncrypto.org&#x2F;mail-archive&#x2F;messaging&#x2F;2014&#x2F;001022....</a>