From 15e78da19ec83cf2e4fd6f5185eabe998d5ae901 Mon Sep 17 00:00:00 2001
From: Lars Wirzenius <liw@liw.fi>
Date: Sat, 28 Dec 2019 19:04:29 +0200
Subject: Add: link to hashcash

---
 email2.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/email2.md b/email2.md
index a1bcd2d..942f980 100644
--- a/email2.md
+++ b/email2.md
@@ -343,3 +343,7 @@ Links to more information, sources, etc.
   not have originated there.
   - <https://github.com/emacsen/rwot9-prague/blob/ap-unwanted-messages/topics-and-advance-readings/ap-unwanted-messages.md#postage>
   - <https://podbay.fm/podcast/252333772/e/1503662400>
+
+* Hashcash is a system where the sender proves they do some work
+  before the recipient accepts a message.
+  - http://hashcash.org/
-- 
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