This proposal doesn't require a new protocol. An ESMTP extension would
do just fine. I think we should kill two birds with one stone:
implement an extension in which the receiving server picks 2 primes of
sufficient length, multiplies them, and asks the sending server to
factor them.
I proposed something like this ("...it could be as simply as passing
the sender a product of primes, etc. and demanding that he factor
them before accepting the message. ") not quite a year ago in message
https://www1.ietf.org/mail-archive/working-
groups/asrg/current/msg00989.html
There apparently is some reason why this solution won't work, however
(I'm not sure what the issue was, but I received private replies that
insisted that the solution was unworkable, and the proposal itself
was generally ignored).
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