Historic Letterlocking: the Art and Security of Letter Writing

Abstract

What is letterlocking? Letterlocking refers to the process by which a substrate such as paper, parchment, or papyrus has been folded and secured shut to function as its own envelope. It is part of a 10,000 year-old information security tradition, variations of which have been used in cultures throughout the world on formats ranging from Mesopotamian clay bullae to Bitcoin. Locked paper documents, the focus of this article, have been used since the late Middle Ages by regents, their secretaries, spymasters, soldiers, and the general public. Often times the same person used more than one letterlocking format; some variations were more secure than others. In order to be considered to have the highest level of built-in security, a locked paper document must meet the following criteria: it must have a paper lock cut from the letter itself, as well as a secondary locking system of an adhesive substance such as wax, and one must be required to tear or cut the paper lock to gain access to the information inside.