"Computational flattening algorithms have been successfully applied to X-ray microtomography scans of damaged historical documents, but have so far been limited to scrolls, books, and documents with one or two folds. The challenge tackled here is to reconstruct the intricate folds, tucks, and slits of unopened letters secured shut with “letterlocking,” a practice—systematized in this paper—which underpinned global communications security for centuries before modern envelopes. We present a fully automatic computational approach for reconstructing and virtually unfolding volumetric scans of a locked letter with complex internal folding, producing legible images of the letter’s contents and crease pattern while preserving letterlocking evidence. We demonstrate our method on four letterpackets from Renaissance Europe, reading the contents of one unopened letter for the first time. Using the results of virtual unfolding, we situate our findings within a novel letterlocking categorization chart based on our study of 250,000 historical letters.
"Introduction
"The letter is one of the most important communication technologies in human history. Before the proliferation of mass-produced envelopes in the 1830s, most letters were sent via letterlocking, the process of folding and securing writing substrates to become their own envelopes. Letterlocking was an everyday activity for centuries, across cultures, borders, and social classes, and plays an integral role in the history of secrecy systems as the missing link between physical communications security techniques from the ancient world and modern digital cryptography1,2,3. While our attention is naturally drawn to a letter’s written contents, the material evidence on surviving opened letters, such as crease marks and wax seals, testifies to thousands of folding techniques used over the centuries to turn a flat sheet of paper into a secure letterpacket. With careful study, this evidence can be used to reverse-engineer historical letterpackets, which themselves become a key dataset for the study of historical communications security methods.
"Our study of 250,000 historical letters has produced the first systematization of letterlocking techniques (discussed below in “Results” section). We observe built-in tamper-evident locking mechanisms that deter potential interceptors by irreversibly damaging a letterpacket on opening, enabling the intended recipient to detect so-called “man-in-the-middle” attacks. We use these mechanisms to help assign security scores. However, since letters are designed to be opened at their intended destination, our system necessarily relies on inference drawn from surviving opened documents. A European postmaster’s trunk preserving 300-year-old undelivered post, the Brienne Collection4, provides a rare opportunity to study sealed locked letters. The trunk contains 3148 cataloged items, including 2571 opened letters, fragments, and other documents, and 577 letterpackets that have never been opened. Figure 1 shows four Brienne letterpackets studied in this paper (each measuring ~50 mm×80 mm), illustrating a variety of locking techniques despite similar exterior forms.
"Until now, our analysis of letterlocking has been limited by the standard archival practice of cutting open sealed letters on request, compromising the physical integrity of the unopened letterpacket. We propose an alternate conservation approach grounded in computational analysis, where letters remain intact in their locked state, yet researchers can still read their contents and identify their letterlocking mechanisms. Drawing on high-resolution volumetric scans, produced by high-contrast time delay integration X-ray microtomography (XMT), we developed virtual unfolding to prove our letterlocking theories, and elucidate a historically vital—but long underappreciated—form of physical cryptography...."
