Dead Letters to Alice - Reachability of E-Mail Addresses in the PGP Web   of Trust
  Over the last 25 years four million e-mail addresses have accumulated in the PGP web of trust. In a study each of them was tested for vitality with the result of 40% being unreachable. Of the mailboxes proven to be reachable, 46.77% turn out to be operated by one of three organizations. In this article, the authors share their results and challenges during the study. 
Benjamin Leiding and Andreas D\"ahn
cs.CR