Invisible Ink: Spycraft of the American Revolution


During the American Revolution, espionage was critical to the successes and failures of both Continental and British efforts, and those employed in cloakand- dagger operations always risked death. While the most notorious episode of spying during the war—the Benedict Arnold affair—was a failure, most intelligence operations succeeded. Spycraft was no more wholly embraced than by the American commander-in-chief, George Washington. Washington relied on a vast spy network and personally designed sophisticated battle...

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Invisible Ink: Spycraft of the American Revolution

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During the American Revolution, espionage was critical to the successes and failures of both Continental and British efforts, and those employed in cloakand- dagger operations always risked death. While the most notorious episode of spying during the war—the Benedict Arnold affair—was a failure, most intelligence operations succeeded. Spycraft was no more wholly embraced than by the American commander-in-chief, George Washington. Washington relied on a vast spy network and personally designed sophisticated battle plan deceptions and counterintelligence efforts, some surprisingly modern in form. In Invisible Ink: Spycraft of the American Revolution, award-winning author John A. Nagy briefly traces the history of spy techniques from ancient China through Elizabethan England before embarking on the various techniques used by spies on both sides of the war to exchange secret information. These methods included dictionary codes, diplomatic ciphers, dead drops, hidden compartments (such as a hollowed-out bullet or a woman’s garter), and even musical notation, as well as efforts of counterintelligence, including “Black Chambers,” where postal correspondence was read by cryptologists. Throughout, the author provides examples of the various codes and ciphers employed, many of which have not been previously described. In addition, the author analyzes some of the key spy rings operating during the war, most notably the Culper ring that provided information to Washington from inside British-controlled New York City. Based on nearly two decades of primary research, including the author’s discovery of previously unrecognized spies and methods, Invisible Ink is a major contribution to the history of conflict and technology.

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Product Details

  • ISBN-13: 9781594160974
  • Publisher: Westholme Publishing
  • Publication date: 12/8/2009
  • Edition description: 1st Edition
  • Pages: 384
  • Product dimensions: 6.00 (w) x 9.30 (h) x 1.40 (d)

Meet the Author

John A. Nagy is scholar-in-residence at St. Francis University, Pennsylvania, and a founder of the American Revolution Round Table of Philadelphia. He is author of Rebellion in the Ranks: Mutinies of the American Revolution and Spies in the Continental Capital, also available from Westholme Publishing.

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Read an Excerpt

Invisible Ink

Spycraft of the American Revolution
By John A. Nagy


Copyright © 2010 John A. Nagy
All right reserved.

ISBN: 978-1-59416-097-4

Chapter One

Early Spycraft: Codes, Ciphers, and Steganography

Spies have been around forever and have always used the latest tools to aid their mission. Codes and ciphers are among their earliest tools. In a code, a number or character represents entire words and requires that both the sender and the receiver have identical numerical listings, such as a codebook. A familiar code is the United States Postal Service's Zip Code system. A cipher uses a letter to represent another letter. Ciphers can be traced back to the substitutions made for proper names in the cuneiform writing of the Sumerians, the hieroglyphics of ancient Egypt, and the Mayan civilization. The oldest codebook, from Susa in present-day Iran, is a cuneiform tablet with a list of numbers in one column and its cuneiform signs in an adjacent column. Hebrew scribes writing down the book of Jeremiah used a reversed alphabet simple substitution cipher known as the Atbash cipher. It splits the alphabet into two equal halves and substitutes the first letter of the alphabet for the last and the second letter for the second to last, etc. Many names of people and places are believed to have been deliberately obscured in the Hebrew Bible using the Atbash cipher.

The Spartans of early Greece were the first to use cryptography in military operations in the fifth century B.C. using two cylinders of equal size. One cylinder was retained by the headquarters and the other given to the general going into battle. A slip of parchment, inscribed by being wrapped around the cylinder, is then sent to the general in the field. The commander takes the parchment, wraps it around the cylinder, and sees the message by reading down the cylinder. This method was still being used in seventeenth-century England.

Both Charlemagne and Alfred the Great of Wessex used substitution ciphers for secret communications. In the ninth century Rabanus Maurus, Abbot of Fulda and Archbishop of Mayence, used a cipher that replaced the vowels with dots. It used one dot for "i," two dots for "a," three dots for "e," four dots for "o," and five dots for "u." One of the weaknesses of a single or mono-alphabetic substitution, however, is that every letter in the plaintext message is represented by another letter, and always the same letter. A cipher used in Venice in 1411 improved upon earlier substitution ciphers by employing arbitrary symbols, multiple equivalents for vowels, and nulls.

Leon Battista Alberti, whom many people consider the quintessential Renaissance man, invented the cipher disk and cryptographic key in 1466. Alberti's cipher disk was polyalphabetic, meaning that a new alphabet could be created each time by turning the disk. It consisted of a fixed outer disk containing the plain text and a movable inner disk with the corresponding cipher text. To encipher a message you have to decide on the starting position of the inner disc; this forms part of the key. The other part of the key is a rule for the movement of the inner disk; for example, it could be moved nine positions counterclockwise every fourth letter. Alberti's disc had twenty-four positions containing twenty letters and four numbers in the outer disk. The numbers could represent anything decided between the users-letters or words. His system thus allowed the use of codes. The first printed book on cryptology, the science of analyzing and deciphering codes and ciphers, was Polygraphiae written by Abbott Johannes Trithemius at the Abbey of Saint Jacob in Würzburg and published in 1518. The second book, Opus Novum, followed in 1526. It was written by Jacopo Silvestri, who may have been a cipher clerk at the Vatican. These were followed by Subtilitas de Subtilitate Rerum (The Subtlety of Matter) in 1554 by Girolamo Cardano, the first cryptologist to propose the autokey method of encipherment. He also became famous for the Cardano grille, a sheet of stiff material with irregularly spaced rectangular holes which was placed over the writing paper. The secret message was then written in the holes, the grille or mask was removed from the writing paper, and a harmless message was filled in around the secret message to camouflage its being there. To read the message, an identical grille or mask was placed over the writing. A major fault in the system was the awkwardness in phrasing the surrounding message often led to suspicions of a secret message within. Cardano grilles or masks are in the General Sir Henry Clinton Papers from the American Revolution.

John Wilkins in his book Mercury, or the Secret and Swift Messenger published in 1641 reminds us that the letters of the alphabet we use for a word really represent an idea. By the use of a different language the same idea is represented by a different combination of letters. The use of ciphers is just a different language used to represent the same idea. Wilkins told of many of the uses of secret writing at this early period. In this first English-language book on cryptography, he tells about the use of codes and ciphers that used double alphabets, hieroglyphics, musical notes, the Bible, and the transposition of letters and words. Also in use at the time were abbreviations, dropping letters, adding extra letters, and invisible inks.

Antoine Rossignol and his son Bonaventure developed the "Great Cipher" after 1626 to encrypt Louis XIV's most secret documents. In the Great Cipher each of 587 numbers stood for a French syllable rather than a single letter. It also used codes to instruct the decoder to ignore the previous section. It was not broken until around 1893 when Commandant Etienne Bazeries, who worked at the French Ministry of Foreign Affairs Sales Office, realized that each number stood for a French syllable. He broke the code when he realized a particular sequence of numbers 124, 22, 125, 46, and 345 stood for les ennemis, that is, the enemies.

The British House of Lords was sufficiently familiar with ciphers that it allowed the introduction of deciphered writings in the 1723 trial of Bishop Francis Atterbury.

Shorthand is another cipher that had been around for quite some time. The first work in shorthand written in English was by Dr. Timothy Bright in 1586 when he wrote the Book of Titus in what was called "charactery." In 1588 he published Characterie: An Arte of Shorte, Swifte, and Secrete Writing by Character. Many books, such as La Plume Volante: The Flying Pen-Man or the Art of Short Writing by William Hopkins in 1674, were in common use in England a long time before the American Revolution. The Marquis of Worcester's book A Century of the Names and Scantlings of Such Inventions with its sections on ciphers had been printed five times before the American Revolution and twice during the war. A Century of the Names and other shorthand writing books would have been readily available to anyone interested. British General Sir Henry Clinton used his own version of shorthand for his personal notes to keep them from inquisitive eyes. He even devised a different shorthand system for filing his documents. Shorthand has progressed from the nonletter forms to text messages as LOL (laugh out loud) and CU (see you).

John Wilkins stated the obvious, that any writing to remain a secret must be devoid of suspicion and difficult to interpret. Once detected, a message may be interpreted, but if it can remain hidden, it cannot be deciphered. Under Queen Elizabeth I, Sir Francis Walsingham set up an elaborate intelligence system. In 1586, his operatives intercepted a coded correspondence between Mary Queen of Scots and Anthony Babington. The correspondence when deciphered by Thomas Phelippes exposed her involvement in Anthony Babington's plot to murder Elizabeth. What made the discovery special was not the coded correspondence itself but the fact that the messages were hidden inside a beer barrel bung to make their detection even harder. Indeed, the beer barrels were allowed to pass into Mary's prison. The art of hiding a message in such a way that no one but the sender and the intended receiver knows of its existence is called steganography. Today the term would include the hiding of messages in digital computer files and images.

Steganography is an important feature of secret communications, because while a cipher or code hides the meaning of a message, it does not conceal the fact that a message exists. Steganography does not necessarily have to involve a code or cipher, however. For instance, Histiaeus while at the Persian court wanted to send a message to Aristagoras, who was in Greece to instigate a revolt. Histiaeus shaved the head of his most trusted slave and tattooed a message there. When the slave's hair grew back, he was sent to deliver the message. When the messenger arrived, Aristagoras was to shave his head and reveal the message.

Another example occurred during the English Civil War. Royalist Sir John Trevanion was being held in Colchester Castle awaiting his execution. He was saved by a letter from a friend. It read:

Worthie Sir John-Hope, that is ye beste comfort of ye afflictyd, cannot much, I fear me, help you now. That I wolde saye to you, is this only: if ever I may be able to requite that I do owe you, stand not upon asking of me. 'Tis not much I can do: but what I can do, bee you verie sure I wille. I knowe that, if dethe comes, if ordinary men fear it, it frights not you, accounting it for a high honour, to have such a rewarde of your loyalty. Pray yet that you may be spared this soe bitter, cup. I fear not that you will grudge any sufferings: only if bie submission you can turn them away, 'tis the part of a wise man. Tell me, an if you can, to do for you any thinge that you wolde have done. The general goes back Wednesday. Restinge your servant to command. R.T.

The letter contained a secret message. By taking the third letter after each punctuation mark, a message is revealed. It reads, "Panel at east end of chapel slides." After his petition to his jailers to pray in the chapel was granted, he made his escape through the moveable panel.

In the fifteenth century Italian scientist Giovanni Porta described how to conceal a message in a hard-boiled egg. An ink is made with an ounce of alum and a pint of vinegar. This special penetrating ink is then used to write on the hard boiled egg shell. The solution penetrates the shell leaving no visible trace and is deposited on the surfaced of the hardened egg. When the shell is removed, the message can be read.

The use of invisible ink between the lines of visible writing is another example of steganography. As far back as the first century AD, Pliny the Elder explained how the "milk" of the thithymallus plant could be used as an invisible ink. Although transparent after drying, the ink turns brown upon gentle heating. Many organic fluids behave in a similar way, because they are rich in carbon and therefore char easily. Indeed, it was not unknown for twentieth-century spies who ran out of manufactured invisible ink to improvise using their own urine.

Steganography, hiding a message in such a way that it is apparent to no one but the sender and the intended receiver, has a distinct advantage over cryptography, which hides the meaning of a message but does not conceal that a message exists. Truly secret messages do not attract attention to themselves, to the messengers, or to the recipients. A visible coded message will arouse suspicion and may in itself be incriminating.

The Spanish at San Roque, a high point near Gibraltar, used a system of fire lights to convey messages of the number of warships anchored in the Bay of Gibraltar and the number that sailed away to the Governor of Cádiz. "The lights they did show, and the distance of time between each light or lights" represented the alphabet. They also used the lights to signify numbers. When the entire fleet was involved, one constant fire was maintained.

In the eighteenth century each general ran his own intelligence service. Any military officer on a general's staff or someone intent on moving up the ranks needed to know at least the basics of cryptography and steganography. Codes and ciphers were not an unknown subject. Businessmen needed to converse secretly with each other and their agents. Affairs of the heart also needed some privacy as letters passed through the hands of travelers who carried them to their destinations. John and Mary Winthrop of Massachusetts, for instance, corresponded before 1700 in a private cipher regarding intimate matters. 17 A young Thomas Jefferson kept his courting of Rebecca Burwell veiled in his correspondence with John Page. Secret writing was not just a man's practice. Catherine W. Livingston writing to Sarah Livingston Jay used a cipher in parts of her letter of July 10, 1780, to keep it from prying eyes.

But the effectiveness of each intelligence operation varied greatly due to the skill of the spies and the general running the operation. Prior to George Washington taking command of the assembled troops at Boston, the American intelligence system was, as expected, a decentralized operation. Various self-appointed groups would gather what information they thought might be useful. Each group had its own leader but no central keystone to direct its operations. It was intelligence by committee. The closest person to a central clearinghouse for the collected information in Boston at the time was Dr. Joseph Warren.

As with higher commanders of the period, General George Washington personally established and then controlled his own military intelligence service. He did use case agents to control groups of spies in the field. Although he knew of operators in the field he did not always know the names of the individual spies working for his case agents. He knew the importance of secrecy and scolded his case agents when they became careless. Going back to his early career, twenty-three-year-old George Washington had served in the French and Indian War under British Major General Edward Braddock, who had use of a cipher, but it is not known if Washington had any experience with it. Washington's expense account for military service during the American Revolution recorded expenditures for military intelligence of $7,617 (dollars) and $55,145 lawful money (originally paid in state or foreign currency but converted to United States dollars). Washington's adept selection of spy methods and ability to exploit British vulnerabilities to deception would be crucial in the Revolutionary War.


Excerpted from Invisible Ink by John A. Nagy Copyright © 2010 by John A. Nagy . Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Table of Contents


1. Early Spycraft: Codes, Ciphers, and Steganography....................1
2. Secret Communication....................9
3. Black Chambers....................21
4. Invisible Ink....................27
5. Ciphers and Codes....................42
6. Dictionary Codes....................69
7. Diplomatic Ciphers....................76
8. Dead Drops....................91
9. Hidden Compartments....................99
10. Flags of Truce....................121
11. The Culper Spy Ring....................134
12. "Traitor of the Blackest Dye"....................149
13. Deceptions and Smugglers....................168
14. Going Incognito....................189
15. European Subterfuge....................199
16. Deceptive Battle Plans....................211
A. McLane-Rivington Communications....................245
B. Culper Spy Ring Code....................247
C. Marquis de Lafayette's Numerical Code....................256
D. British Headquarters Code Book (Undated)....................258
E. British Headquarters Code Book, 1779....................264
F. Paul Wentworth's Code Book....................270
G. Baron von Steuben Cipher....................273
H. Comte de Rochambeau's Cipher....................274
I. Key of Walpole Cypher in the Hand of William Eden, 1777....................275
J. An Attempt to Decipher a Code of Unknown Origin by William Eden, 1777....................276
K. Description of Counterfeit Paper Money....................278
L. General Nathanael Greene and Colonel Henry Lee Jr. Code, 1781....................281
M. Major General Edward Braddock's Cipher....................282
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  • Posted December 21, 2009

    more from this reviewer

    Fascinating Read!

    This expert piece of historical research from John Nagy author of the critically acclaimed "Rebellion In The Ranks" is an important work on the Revolutionary War. Fascinating accounts are told with clarity and can be enjoyed by everyone and not just hard-core fans of the era. Nagy brings to life a largely forgotten but important aspect of the war. He visits forgotten places and identifies many unsung heros of our struggle for indeoendence. My favorite passage-the capture of a redcoat spy who swallows his message filled bullet (twice) and is then threatened with sugery! "Invisibe Ink" is like eating potato chips, you just can't stop reading it. Robert A. Mayers author of "The War Man"

    2 out of 2 people found this review helpful.

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    Posted December 28, 2010

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