Bacon, Shakespeare, and the NSA's 'Cut the Ends!' Reflex

The well-known history of cryptography, much like the history of humanity as a whole, is densely filled with distortions and falsehoods. For this deception to work, it's necessary to hide and destroy those facts and documents that expose the lie, showing how it really was. Since it's impossible to destroy all evidence of this kind, the hints and indicators that help find what is being concealed are removed. For those who would like to know the truth, it's useful and instructive to see the mechanics of such clean-ups. How it was done in the past – and how it's being done right now.

For the vast majority of people interested in history but not familiar with the materials on the kiwi arXiv website, the topics investigated here may seem extremely strange and far-fetched. Indeed, what a wild idea it is, for example, to suggest close connections between the debates over the real author of Shakespeare's works and the activities of the NSA?

After all, this is the National Security Agency, the world's largest intelligence agency specializing in decryption and signals intelligence. Do they really have no more important tasks and interests than being concerned with the topic of Bacon as Shakespeare?

The NSA certainly has more pressing concerns. However, the Bacon-Shakespeare topic is not merely a subject of operational interest for this agency but remains an area where lies are actively spread to obscure the true picture. The activity here is constant, starting essentially from the moment the spy agency was founded in the 1950s and continuing right up to the 2020s, when a book revealing uncomfortable truths, "4in1: Mask of Shakespeare and Mysteries of Bacon, Book by Cartier and Secrets of the NSA" was published.

Such a strong statement can indeed be proven — by relying on facts and documents whose reliability is beyond dispute. However, to ensure the persuasiveness of this evidence is clear not only to those deeply familiar with the subject but also to anyone simply interested, it's more convenient to start from a different angle.

In 1967, the United States saw the release of a book, exceptional in every respect, "The Codebreakers" [1] by David Kahn, a journalist passionately interested in cryptography. This impressive work, spanning over a thousand pages, managed to interestingly and substantively cover the history of secret writing and codebreaking — "from Ancient Egypt to the present day", so to say.

The detailed accounts by the meticulous author about modern ciphers and their breaking inevitably led the book to discuss the activities of the National Security Agency, a super-secret intelligence service at the time. Consequently, the NSA, having learned in advance about the plans to publish "The Codebreakers," was greatly concerned.

They attempted to prevent the book's release, to discredit both the author and his book before publication, or, at the very least, to remove from the work any facts whose disclosure particularly worried the spies. In the end, little came of these schemes, but at the insistence of the NSA (and their British counterparts from GCHQ), the Macmillan publishing house agreed to remove a number of particularly sensitive sections from the monograph.

Despite these omissions, David Kahn's book remained extremely informative and earned well-deserved success among readers. It has maintained its reputation as a "monumental and unmatched" masterpiece of historical literature dedicated to cryptography, up to the present day.

A decade later, in 1977, amidst the simultaneous spread of computers and growing public interest in cryptography, the CRYPTOLOGIA magazine was born. Dedicated, in general, to all aspects of the science of ciphers and the secrets of their decryption, it paid special attention to the history of cryptography. David Kahn became one of the five founding editors of this journal.

Over the next fifteen years, the relationship between the NSA and the open cryptographic research movement remained quite tense. The community of scientists and cryptography enthusiasts was very persistent in piecing together and systematizing any information related to ciphers, their resilience, and cryptanalysis, mostly gathering crumbs of information. The NSA, being the primary expert in the state on all such matters, actively tried to hinder this process. They hid everything they could, not only about their top-secret spy activities but also about the secrets of cryptography in general.

In the early 1990s, however, with the unexpected collapse of the USSR and the disappearance of the United States' main adversary on the world stage, the previously generous funding for spy agencies began to quickly and significantly decrease. Naturally, this caused concern. They decided to remind politicians and the public of their past achievements, so people understood whose secret successes had contributed to the downfall of the USSR. In other words, carefully measured and authorized leaks of super-secret information about the past feats of government cryptanalysts started coming directly from the NSA to the press.

In the new conditions of the 1990s, both David Kahn's active research and the work of the closely associated Cryptologia magazine began to be seen not as an annoying hindrance to the NSA's secret work, but quite the opposite. As convenient "state-independent" mechanisms for instilling in the public mind ideas that were definitely beneficial for both strengthening the NSA's reputation and for portraying the history of cryptography in a favorable light.

In short, in 1995, David Kahn was given the official status of NSA scholar-in-residence. That same year, the Cryptologia editorial office moved from the private educational institution (Rose-Hulman Institute of Technology) to the much more comfortable conditions of the United States Military Academy, better known as West Point. Deeply touched by such radical changes in the state's attitude towards his work, David Kahn eventually donated his extensive cryptographic collection, which he had been gathering throughout his life, to the NSA (more precisely, to its National Cryptologic Museum)...

The shared stance on the remarkable role of William F. Friedman played a key role in strengthening the long-term alliance between the NSA and David Kahn. Friedman, characterized by NSA historians as the "father of American scientific cryptology" and simply "the greatest of cryptologists" according to his passionate admirer David Kahn, held a central place in their mutual respect.

Kahn's enthusiastic, one-sided, and uncritical view of Friedman's talents and personality was clearly reflected not only in the content of "The Codebreakers," his main and still very influential book. But it also impacted the entire open history of cryptography as a whole, as well as the contents of the publications in the Cryptologia magazine in particular.

This biased approach to historical truth is particularly evident in the portrayal of the role and place of another outstanding cryptographer, General François Cartier. At the beginning of the 20th century and during World War I, Cartier led the Bureau of Ciphers and cryptanalytic work in the French armed forces, and in the post-war years, he became a professional opponent of William F. Friedman.

The beginning of the conflict between these two great cryptographers is described in Kahn's book [1] as follows:

Riverbank Publication No. 22, written in 1920 when Friedman was 28, must be regarded as the most important single publication in cryptology. Entitled The Index of Coincidence and Its Applications in Cryptography, it <...> took the science into a new world.

Fabyan had the pamphlet printed in France in 1922 to save money; General Cartier saw it and thought so highly of it that he had it translated and published forthwith false-dating it "1921" to make it appear as if the French work had come first!

It's difficult to tell where the completely false part of the story about saving on printing costs in France and about General Cartier supposedly trying to steal the major work of an American cryptologist originated from. Whether David Kahn himself invented this version, or whether he heard it from one of the Friedman cryptographer couple, with whom he was personally acquainted, is no longer of primary importance.

What is important, however, is that cryptology historians have long been well aware of how this misunderstanding with the first French version of the pamphlet by Cartier, the discrepancy in dates, and the key role in this whole intrigue played by the millionaire patron George Fabyan actually occurred. Fabyan, who owned not just a personal cryptographic Department at his estate, Riverbank, but also a state-of-the-art printing press, sent William Friedman's manuscript (by that time, Friedman had already fled Riverbank) as an anonymous work of the "Riverbank Laboratories" to General Cartier in France in 1921, to showcase the professionalism of his cryptographers. Their main field of work involved decryption of the ciphers of Francis Bacon found in ancient books from the Elizabethan era and proving that Bacon was the actual author of Shakespeare's works.

All these facts, well known to historians, are detailed in the text "The problem of Shakespearean authorship as an OSINT task." However, even in 1996, thirty years after the publication of David Kahn's "monumental work," when a new and expanded edition of The Codebreakers was being prepared, the author chose not to make any changes to his fundamentally incorrect original version of this episode. Even worse, in 2002, Kahn repeated it in an article [2] for Cryptologia. Thus, it's precisely this false version of history that, at the behest of David Kahn, continues to roam the pages of the authoritative journal [3][4].

It's understandable that if many historians repeatedly tell a story that never happened, while simultaneously hiding how it actually occurred, there must be compelling reasons. In this specific case, the insistence on falsehood is due to the figure of the general and cryptographer François Cartier. More precisely, Cartier's book "The Problem of Cryptography and History," [5] published in Paris in 1938, presents a significant inconvenience for contemporary science.

For instance, the search engine on the Cryptologia journal's website, over the nearly half-century of the journal's existence, finds NOT A SINGLE publication mentioning General Cartier's book. Similarly, there is not a single mention of it in David Kahn's voluminous monograph, The Codebreakers.

The most recent and clear explanation of the events, without any mention of Cartier's name, is provided by David Sherman. A former high-ranking NSA official and now, in retirement, a regular author for the Cryptologia magazine, Sherman is a knowledgeable historian of state cryptography and a great admirer of William F. Friedman.

In the first half of 2020, Sherman published another article in the series "Sources and methods [for information gathering] for cryptologic history," entirely dedicated to the the "William and Elizebeth Smith Friedman [cryptographic] collections." [6] Regarding the circumstances under which this famous cryptographic couple came into history, Sherman explains as follows:

William and Elizebeth Friedman each came to cryptology by happenstance, under the auspices of the wealthy Chicago businessman George Fabyan. William, having been recruited from his graduate studies in agriculture at Cornell University, joined Fabyan’s Riverbank Laboratories in 1915 and established its genetics program.

Elizebeth came to Riverbank a year later following a chance meeting at a Chicago library with Fabyan, who offered her a job on the spot. She was assigned to a Riverbank effort that aimed to find enciphered messages thought (wrongly) to have been embedded in the texts of William Shakespeare’s plays.

Elizebeth soon became disillusioned with her work. William, however, was drawn into it through his talent with a camera, which allowed him to photograph for Fabyan those pages in early editions of Shakespeare’s works that were deemed (again, wrongly) to contain examples of encrypted text. He and Elizebeth also drew closer together. The two married in May 1917.

The special and repeated emphasis on the "mistakenness" of the ideas about encrypted texts in the works of Shakespeare is not made without reason, of course. After World War II, the cryptographic couple decided to revisit this issue. Quoting David Sherman further:

[The pace at which he acquired new items accelerated significantly thereafter,] and beginning in the late 1940s the collection expanded rapidly. The most significant additions were made in support of William and Elizebeth’s renewed work on whether encrypted messages could be found in the works of William Shakespeare, a question which they definitively resolved in the negative in The Shakespearean Ciphers Examined, published in 1957 [7].

The next very important aspect for historians and other researchers is the Guide to W.F. Friedman's Cryptographic Collection.

About this work, D. Sherman's article states:

Dr. Rose Mary Sheldon, a historian and former professor at the Virginia Military Institute, has performed an invaluable service for all students of cryptology by creating a 600-page guide to the numbered items in the William Friedman Collection. Extensively annotated and including descriptions of the nature of contents of individual entries, Professor Sheldon’s guide is an essential companion for anyone interested in this collection.

The Marshall Library also has produced a finding aid at the box and folder level for the Elizebeth Friedman Collection. While not as detailed as Professor Sheldon’s guide to William’s, the aid’s description of each folder generally is sufficiently specific to direct researchers to individual items relevant to their needs.

The guide to William’s collection and the finding aid for Elizebeth’s are available from the Marshall Library (https://www.marshallfoundation.org/library/wp-content/uploads/sites/16/2020/01/Friedman_Collection_Guide_September_2014.pdf and https://www.marshallfoundation.org/library/wp-content/uploads/sites/16/2020/01/ESF-collection-summary-sheet.pdf).

The reason for citing all this information from David Sherman's article in such detail will become clear very soon. In the meantime, the following quote from the same work is needed — about what other substantive materials can be found on the topic of interest:

There are also a number of useful videos on William and Elizebeth posted to the YouTube Channel of the Marshall Library’s parent organization, the Marshall Foundation. Narrated by historians Rose Mary Sheldon, Bill Sherman, and Betsy Rohaly Smoot, these videos can be accessed easily via the YouTube icon on the Marshall Foundation’s webpage, then scrolling to the section “William & Elizebeth Friedman & Cryptology.”

One of the video stories in this series, which is of particular interest to us and entirely dedicated to the Friedmans' book "The Shakespearean Ciphers Examined," is worth discussing in more detail.

On the Marshall Foundation's website, the video of interest is located in the same section where the Friedman's book "The Shakespearean Ciphers Examined" is available for download. Additionally, this video material can also be found on YouTube.

In a nearly complete [slightly condensed for compactness], the transcription of the story from two history professors looks like this:

(Bill Sherman:) The little we know about this figure. William Shakespeare doesn't really fit the kind of profile that you imagine is necessary. The kind of learning that's necessary, the life experience. You know Shakespeare never as far as we know were very far from London or Stratford. So how would he know about Italy? How would he know about these historical sources that lie behind the plays that he writes? Where did it come from? And that mystery opens up all kinds of speculation, all kinds of passionate inquiry and mystery, that people have to plunge into and try to crack in various ways.

(Rose M. Sheldon:) There's an entire field of literature on Shakespearean studies concerned with who wrote Shakespeare. Was it Bacon? Was it Edward de Vere? Did Shakespeare write it himself? Could a man from his low social status have written the kinds of plays?

(Bill Sherman:) So there's almost always been some air of mystery around the works. And at times it has grown into a really vexing problem. In fact by 1915 a book could be published -- on the war on the question of Shakespeare's authorship -- called the greatest problem in history. It's the thing that everybody is worried about at that point.

In fact, such books not only "could have been" but were actually written at that time, particularly in Fabyan's Riverbank Laboratories. And there, in 1916, they were printed as well — in the form of several brochures under the general title "The Greatest Work of Sir Francis Bacon." Notably, the author of two of these Riverbank brochures, which delved more deeply into cryptography — "Hints for …" and "Keys to Deciphering the Greatest Work of Sir F.B." — was none other than William F. Friedman [for details on this, see here].

Yet, for modern history professors, all this remains somewhat unknown. And although descriptions of the listed — formally anonymous under Fabyan's rules — Riverbank brochures are contained in her "Guide to the Friedman Collection," Rose Mary Sheldon attributes their authorship to another person. And about William Friedman's entrance onto this scene, she narrates in the video as follows:

(Rose M. Sheldon:) And Friedman comes into this controversy via the Baconian society. Those people, who believe that Bacon wrote the plays, formed a society. And there were a number of theories about what kind of code, and how you read it, and where it was found. Friedman never bought the idea from the beginning. He did it as a pastime, it was a hobby.

It should immediately be clarified that, in fact, as early as 1916, William Friedman became the head of the ciphers department at the Riverbank Laboratories. Moreover, during that period, before the United States entered the war in 1917, the Department's staff (where both Friedman and his future wife Elizebeth worked) exclusively dealt with tasks surrounding the Bacon cipher. Thus, claims about it being a "hobby and entertainment" are simply untrue... But let's continue quoting the professors:

(Bill Sherman:) Say they [Friedmans] simply were never persuaded as let's say literary scientists, that the methods people were using were persuasive. They simply felt that Elizabeth Wells Gallup and her successors were setting up rules for finding secret messages that were either flawed as methods or imperfect in their findings. And I think they ended up simply debunking attempt after attempt after attempt.

And that led ultimately to this extraordinary hammer blow of a book the Shakespearean ciphers examined. Which was not their favorite title and in fact it was not their title at all by the way. They didn't like the fact that the book was called the "Shakespearean ciphers examined" because they thought that implied that there was a Shakespearean cipher to be examined.

For these two paragraphs of quotes, several comments need to be made. First of all, the work of Elizabeth Wells Gallup on the Bacon cipher, sponsored by George Fabyan and supported by the Friedmans at Riverbank, was later — in the 1920s and 1930s — checked and confirmed by the highly authoritative French cryptographer, General Cartier. But this is completely omitted in the dialogue of contemporary history professors.

Furthermore, the Friedman couple themselves kept silent about their "complete disagreement" with Gallup's methods and Cartier's conclusions for nearly forty years. They only decided to speak out loudly after General Cartier died in 1953 (George Fabyan and Elizabeth Gallup died significantly earlier, in the 1930s).

Finally, even the claim that the Friedmans (according to Elizebeth after William's death) always disliked the publisher-given title for their book, implicitly suggesting the existence of ciphers where there are none, is also contradicted by documents.

In the "Analytical Guide..." by Sheldon, "inventory number 63" is described, in particular. Collection items were numbered as the collection grew, so number 63 dates back to the early 1930s. It contains folders with clippings and copies of articles from those years about "encrypted meanings" in modernist poetry. Interestingly, a folder — as "item 63.2" — from a completely different, post-war period, was also included. And in this folder, among clippings about poetry, are pages with the theses of a lecture by William Friedman, which he gave on January 19, 1949, at the Folger Shakespeare Library. Notably, this document is titled: “Notes on a talk on the Shakespeare-Bacon Ciphers”.

Why the Friedmans placed this document in a folder entirely unsuitable for it, both in theme and time period? And what does it say in these "notes on ciphers" (in Friedman's own words)? Sheldon, in her "Guide," does not mention a word about this... However, let's continue quoting the video:

(Rose M. Sheldon:) They had been involved with the question for so long and he [William Friedman] was finally retired from the government. And so they could dedicate full-time to these other little projects none of which had anything to do with their government work.

Why does military historian and Colonel R.M. Sheldon state such an obvious falsehood here? Let's not speculate but rather clarify easily verifiable facts. William F. Friedman was preparing a book "on the absence of Bacon-Shakespearean ciphers" throughout 1954, while holding a high-ranking position at the NSA. In the spring of 1955, when his and his wife's work was declared the winner of the Folger Shakespeare Library contest, Friedman's health sharply deteriorated, leading to his resignation from government service in the fall of 1955. However, his top-secret work for the NSA continued thereafter... (but that is a completely different story). But let's conclude the quotation of the video featuring Sheldon and Sherman:

(Rose M. Sheldon:) Well he wrote a book on it because there had been 30 years of correspondence with these people and many many books written about it. And he came out with the different -- what I consider the definitive -- work, saying: there is no cipher. These things are all subjective, and this should be the end of the story. Did this stop people from writing to him? -- NO.

From the video quotes and accompanying comments provided here, it should be clear that authoritative and knowledgeable historians likely know more about the subject than they discuss. However, they choose to omit known facts important to the topic but "not fitting the concept." Worse, instead of inconvenient facts, deliberate falsehoods are presented.

However, what's particularly interesting here is not these manipulations but what has been happening with the "Friedman Cryptographic Collection" in the recent 2020s...

Whether coincidental or not remains unknown, but immediately after former NSA official and now cryptographic historian David Sherman published his methodological recommendations for researchers of the Friedman Collection in the CRYPTOLOGIA magazine [6], a long-forgotten book by General Cartier, "The Problem of Cryptography and History," suddenly resurfaced on the kiwi arXiv website.

As the translation of this fascinating book progressed, along with the collection of accompanying documents, kiwi arXiv's investigation inevitably led to Rose Mary Sheldon's highly informative "Analytical Guide to the Friedman Collection." It was precisely through this Guide that a very little-known and deeply hidden article by General Cartier from 1939 (inventory number 1378 of the Friedman Collection) was discovered. It provided especially valuable recommendations for stable and verifiable decryption of secret messages in ancient books.

These messages tell not only about the true author of Shakespeare's works and the secret pages of England's history but also about Bacon's astonishing – Rosicrucian-style – experiments with Nature... In short, an interesting path was found to significantly new facts and evidences from the history of the 16th and 17th centuries, not yet explored by modern science.

What do you think happened next to the "Guide to the Friedman Collection" on the Marshall Foundation's website, where this archive is stored?

Now, if you try to use the guide link from David Sherman's article, you'll get a "404 File not found" error. Moreover, if you try to search for this document by the author's name, Rose Mary Sheldon, through the site's search engine, you'll find that no such Guide exists there at all. Furthermore, any mentions of the Friedmans and their archives have now completely disappeared from the Marshall Foundation's main page. Indeed, no special pages previously dedicated entirely to the Friedmans and their cryptographic collections can be found anymore. Neither at the old addresses nor in any other way...

This turn of events might surprise some. But for those familiar with the history of the issue, it's rather "business as usual."

In both the article by David Sherman cited above [6] and especially in his earlier 2017 article "The National Security Agency and the William F. Friedman Collection" [4], one can find numerous details about the extremely nervous attitude of the National Security Agency's leadership towards this cryptographic archive.

For the first time, all even slightly sensitive "cryptosecrets" from the Friedman collection were withdrawn into the NSA's closed archives back in 1958 (when a large and super-secret spy-op to weaken Hagelin cipher machines began). William Friedman died in 1969, but even after his death, NSA experts thinned out the cryptologist's archives at the Marshall Foundation several times, especially in connection with the early 1980s publication of "The Puzzle Palace" by James Bamford. He found many previously non-secret, intriguing details in the Friedman archives, which the NSA immediately wanted to hide more deeply.

In light of all these known stories, it's hardly surprising that as soon as new excavations of the Friedman legacy highlighted the NSA's role in the great cryptographic deception around the Bacon-Shakespeare cipher topic, someone immediately wanted to make all documents of the Friedman collection as difficult to access as possible. The detailed Guide to this archive — as an "indispensable companion" for researchers — was simply cut out altogether.

It was, one might say, a kind of "cut the ends!" reflex action...

In 1996, renowned cryptography historian David Kahn prepared a "revised and expanded" edition of his landmark book The Codebreakers. Indeed, a lot had happened in the world of cryptography in the three decades since the first edition was published.

Two significant additions to the new version stood out. One was the story of the decryption successes against the German "Enigma" machine, which in the 1960s was still a major intelligence secret. The other was a new — final — chapter on the invention and adoption of public-key cryptography by the academic community.

In an ironic twist of fate, in the very last paragraph of his revised book, David Kahn vividly demonstrated that he remained for life an "devoted amateur" (William F. Friedman's description, incidentally). Despite his lifelong zeal in studying the history of cryptography, Kahn never fully grasped the profound essence of breaking ciphers.

He concluded his updated work with the following reflection:

Near the end of his ever-fascinating book The American Black Chamber, published in 1931, Herbert O. Yardley, speaking about the Vernam one-time tape system, the oldest practically and theoretically unbreakable system, wrote, “Sooner or later all governments, all wireless companies, will adopt some such system. And when they do, cryptography [meaning cryptology], as a profession, will die.”

His prediction is coming true.

Of course, this is not the place to detail why the main task of powerful cryptographic agencies like the NSA has always been and remains not the attempt to crack difficult and unbreakable ciphers. Their primary task is the secret weakening of any ciphers they can reach, to make every theoretically robust cipher successfully readable. The National Security Agency, in particular, actively engaged in these affairs from its very inception — with the active participation of William F. Friedman.

David Kahn, despite a lifetime of studying the history of cryptography, somehow failed to understand this. Perhaps due to overly romantic notions of this spy profession, which is very cynical at its core, boiling down to reading others' letters by any means necessary.

Let's not speculate on whether this or some other reason led to the almost complete absence in Kahn's works of any mention of the very potent cryptographic technique known as TEMPEST by spies. Consequently, in Kahn's view, there's absolutely no connection between this NSA term and the famous play The Tempest, which opens Shakespeare's First Folio.

It was this very collection of plays, the First Folio, without which the literary world might have remained without Shakespeare at all, that served as the main source of Bacon-encoded messages, from which Elizabeth Gallup began her decryption work. Based on this book, the First Folio, William F. Friedman in Riverbank Labs produced for Fabyan visual aids and test examples, teaching novices how to crack Bacon's cipher in ancient books. And a little later, Elizebeth Smith (later Friedman), working with these aids, herself cracked Bacon's cipher to pass the test and become Gallup's assistant.

Such documents from the 1916-1920 period from the Fabyan archives — both aids made and signed by William Friedman, and test sheets with decryptions from Shakespeare, signed by Elizebeth Smith — have all safely survived to the present day in the New York Public Library, where the "Bacon Cipher Collection" from Riverbank is kept (Bacon Cipher Collection, Manuscript and Rare Books Division, New York Public Library).

In the early 2000s, the indefatigable David Kahn, working on yet another of his historical books, discovered this collection in the New York Library, about which he previously knew nothing. However, since the above-mentioned documents clearly demonstrated the truth about how the Friedmans actually worked with Bacon's ciphers in ancient books in their youth, historian Kahn chose to completely ignore all these documents.

And in 2002, he told only about how he found a "true treasure" in this collection on the pages of Cryptologia [2] — one of the first drafts of Friedman's famous work on the Index of Coincidence. Incidentally, he reiterated the old falsehood about General Cartier allegedly attempting to appropriate this work.

In Sheldon's "Guide...", by the way, the description of Inventory Number 167 tells a story much closer to the truth of this misunderstanding. But now, the Guide itself has been hidden from readers (though diligent readers can easily find a copy of this PDF file [8] in the Internet Archive).

And now, the NSA historian David Sherman we're familiar with has just published his latest work in Cryptologia [9]: about David Kahn's old book The Codebreakers – as a "Masterpiece of Cryptological History"...

It seems that the NSA prefers David Kahn's version of cryptographic history much more than the reality of how it was and is.

Main Sources

[1] David Kahn (1967). The Codebreakers – The Story of Secret Writing. Macmillan

[2] David Kahn (2002): A Riverbank Trove, Cryptologia, 26(3):161–164

[3] Chris Christensen (2013). Review of Memories of My Work at the Cipher Bureau of the General Staff Second Department 1930–1945 by Marian Rejewski, Cryptologia, 37:2, 167–174

[4] David Sherman (2017): The National Security Agency and the William F. Friedman Collection, Cryptologia, 41(3):195–238

[5] François Cartier (1938), Un problème de Cryptographie et d’Histoire. Paris: Editions du Mercure de France

[6] David Sherman (2020): Sources and methods for cryptologic history: the William and Elizebeth Smith Friedman collections, Cryptologia, 44(3):267–279

[7] W. Friedman, and E. Friedman (1957). The Shakespearean ciphers examined: An analysis of cryptographic systems used as evidence that some author other than William Shakespeare wrote the plays commonly attributed to him. Cambridge, UK: Cambridge University Press.

[8] Sheldon, R.M. (2014) The Friedman Collection: An Analytical Guide

[9] David Sherman (2023) The Codebreakers war: David Kahn, Macmillan, the government, and the making of a cryptologic history masterpiece. Cryptologia, 47(3):205–226