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diff --git a/articles/ui/ui.bib b/articles/ui/ui.bib
index 21613536c..487b3b812 100644
--- a/articles/ui/ui.bib
+++ b/articles/ui/ui.bib
@@ -195,3 +195,15 @@ improvements},
   urldate = {2016-02-10},
   note = {[Online; Accessed: 2016-02-10]}
 }
+
+
+@article{chaum1981untraceable,
+  title={Untraceable electronic mail, return addresses, and digital pseudonyms},
+  author={Chaum, David L},
+  journal={Communications of the ACM},
+  volume={24},
+  number={2},
+  pages={84--90},
+  year={1981},
+  publisher={ACM}
+}