Burr's cipher, sir: The 1807 treason case that featured in the Apple/FBI conflict

The damned fool who shot Alexander Hamilton in a duel also, some historians believe, wanted to set himself up as the Emperor of Mexico. Coded dispatches led to his downfall, but not conviction.

vanderlyn burr cropped
Credit: John Vanderlyn/Wikicommons

How does a cipher and code, used by a failed politician, dropped into the middle of a trial for treason in 1807, involving then-president Thomas Jefferson and Chief Justice John Marshall, lead to a citation and a counter-citation in a legal brief in 2016? It all leads back to Aaron Burr’s cipher, certainly. He did not throw away his key.

The trial is well known to students of American history. Burr, depicted accurately in the musical Hamilton as having few convictions except a desire to come out on top, had run in 1800 as Jefferson’s vice president on a ticket against incumbent John Adams. Constitutional peculiarities of the day led to Burr and Jefferson receiving equal electoral votes, in turn leading to 35 ballots in the House of Representatives until Jefferson prevailed with a majority of the states’ votes.

Burr had served reportedly admirably as veep, presiding over the Senate, especially in deciding an impeachment case in favor of Supreme Court Justice Samuel Chase against Jefferson’s attempts to weaken the judiciary. The case is a foundation for judicial independence, and has provided a long-standing bulwark for which he’s still commended. This and other matters did not endear Burr to Jefferson, who wanted to weaken the courts and federal control, and who booted Burr from the ticket in his successful re-election campaign of 1804. (Before the election that year, the constitutional oddity regarding electoral votes was resolved with the 12th amendment.)

While the election season was still underway in 1804, Burr, stung by Alexander Hamilton’s private and public attacks against him and Hamilton’s ultimate refusal of Burr’s demand for him to recant 15 years’ worth of statements, challenged him to a duel. We all know the outcome of that. Following Hamilton’s death, Burr fled charges in both New York and New Jersey, though they were ultimately dropped.

Burr completed his vice-presidential term in early 1805, then headed to the “west,” a so-called frontier that then stretched from much of Montana in a ragged line southeast to all but the southwesternmost portion of modern Louisiana. War with Spain seemed imminent. And this is where encryption catches up with Burr during a legendary treason trial.

Codes, ciphers, and substitutions

I’ll get into the nitty-gritty of Apple and the Department of Justice’s brief in the next section, and how it relates to Burr’s 1807 treason trial. But let’s look first at the encryption employed.

Several kinds of encoding systems were already popular in the 1600s in various countries, and some were used extensively by Americans in the Revolutionary War. Jefferson used them to secure messages during his ambassadorship to France (from 1784 to 1789) because, as the Monticello site notes, “Codes were an essential part of his correspondence because European postmasters routinely opened and read all diplomatic and any suspect letters passing through their command.” The posts were not safe in America, either—nor anywhere—whether for commercial or governmental purposes.

aaron burr encoded letter 1

The so-called "treason letter," written in at least three kinds of code, maybe more.

Burr was known to use code in his letters as early as 1774 at the age of 18. (He was a prodigy, like Hamilton.) The letter between Burr and the governor of the Lousiana Territory, James Wilkinson, that prompted treason charges contained three distinct methods:

  • Codes or what were termed “hieroglyphics” at the time: Symbols that referred to a specific thing or person for which each party had a codebook with those correspondences. It also included some numeric codes for certain terms and for people. (At least two separate code systems may have been mixed together, or possibly three.)

  • A book or dictionary cipher, in which numbers referred to pages and columns in a standard reference work. This is an “out-of-band” element, something parties who encoded and decoded could have on hand that might not be unique, but didn’t pass over the same channel as the message. In this case, the 1800 edition of Entick’s Spelling Dictionary was the source.

  • A substitution cipher, in which each letter has a replacement with a symbol, apparently used as a fallback when parties couldn’t access the spelling dictionary, and for speed. Many ciphers swap one letter for another, instead. A more complicated substitution cipher was used by others who corresponded with Wilkinson, and relied on a commonly known word, like “cuba” or “france,” each letter of which seeded a separate substitution cipher which was used in sequence. One had to know the seed word to rotate correctly through associated ciphers and restore the text. (Jefferson used a more advanced and user friendly multiple-cipher version that relied on wooden disks.)

A codebook is the most resistent of all methods, because it is entirely arbitrary. Without the association of codes and meanings being discovered, a codebreaker would need to assemble enough material to find patterns.

The book-based method could be seen as foolhardy, because both this particular book (across multiple editions) had already been used, and those intercepting a message would recognize the form in which references to pages and columns were made. Obtaining a copy of the book on demand or trying multiple potential books would be time consuming, however.

aaron burr code part 1

The alleged code table and substitution cipher used in the "treason letter."

Substitution ciphers are the easiest to break, because letter-frequency analysis of text of any modest length reveals most of the letters. It can be performed by hand. Even the rotation method (not employed in the so-called “treason letter”) isn’t resistent to frequency analysis, because patterns still emerge that can be broken down.

The greatest flaw in this and similar letters is mixing code types, as once the simpler encryptions are broken, this often reveals enough context to break the harder ones, such as the codebook method.

In the actual event, however, no early NSA cracker had to go at it. Wilkinson, as the recipient of the letter, provided an ostensibly accurate decoded copy to Jefferson, which formed the basis of his federal charges.

You’ve got mail

It’s a fascinating time in America: Aaron Burr has returned to the limelight as an antagonist in the hottest ticket on Broadway, and a case in which he was involved became an issue of contention in the most fraught battle over the future of strong encryption. Having explained the encryption used in the letter at issue, let’s look at the actual trial.

In a brief filed March 10 by the Department of Justice in the now-settled battle to determine whether Apple had to create a custom operating system to allow the FBI a better ability to decrypt a phone owned by one of the terrorists in the San Bernardino tragedy, this remarkable statement appeared:

Contrary to Apple’s argument, the Order does not require it to “provide decryption services” to the government. But that would not be novel, either. Indeed, no less an authority than Chief Justice Marshall held that Aaron Burr’s clerk could be forced to decipher a coded letter of Burr’s, provided that doing so would not incriminate the clerk. See United States v. Burr, 25 F. Cas. 38, 39-40 (C.C. Va. 1807).

Apple’s rejoinder appeared as a footnote in its filing six days later (the term “cipher” here refers to the encoded message):

The government contends that Chief Justice Marshall once ordered a third party to “provide decryption services” to the government. Opp. 20 (citing United States v. Burr, 25 F. Cas. 38 (C.C.D. Va. 1807) (No. 14692E)). He did nothing of the sort, and the All Writs Act was not even at issue in Burr. In that case, Aaron Burr’s secretary declined to state whether he “understood” the contents of a certain letter written in cipher, on the ground that he might incriminate himself. 25 F. Cas. at 39. The Court held that the clerk’s answer as to whether he understood the cipher could not incriminate him, and the Court thus held that “the witness may answer the question now propounded”—i.e., whether he understood the letter. Id. at 40. The Court did not require the clerk to decipher the letter.

The case at hand was a trial over whether Burr and some accomplices were attempting to raise an army to wage war either within or without the bounds of the United States. The charges were treason and “high misdemeanor.” Jefferson in pursuing the charges and a grand jury in indicting Burr covered both treason defined narrowly in the Constitution (waging war against the United States) and the misdemeanor charge, which would cover violation of a neutrality treaty to engage in military operations against another nation (here, Spain) if the United States wasn’t itself in a state of war.

It’s key to note that though Marshall was the Chief Justice of the Supreme Court, at the time it was customary for justices to handle other criminal matters in districts that corresponded to their position. Because we live in the future, we can easily pull up the transcript of the testimony and Marshall’s response. It’s a complicated back and forth (abbreviated in the link; the full transcripts are even denser), but Mr. Willie (whose first name I cannot determine) has stipulated he didn’t know the cipher. One of the prosecutors asks:

He has sworn in his deposition that he did not understand the cipher of this letter. How then can his merely copying it implicate him in a crime when he does not know its contents?

Ultimately, Marshall says he’ll consider the question, and then a few days later issues a lengthy ruling. In it, he never addresses whether the secretary has stated that he knows the decoding key or keys at all; in fact, no one challenges Willie on that point. He asserts he didn’t understand the coded portion of the letter at all. Marshall doesn’t ask him for the keys nor to ask him decrypt the letter. Rather, the ruling only allowed that Willie could refuse to answer certain legitimate questions only if he swore it would incriminate him:

…if he say on oath that he cannot answer without accusing himself, he cannot be compelled to answer.

The letter itself, incidentally, was seen as less than credible, even decoded by its recipient, James Wilkson, who turned it over to Jefferson out of fear of implication. Charles F. Hobson, an expert on John Marshall, writing for the “Federal Trials and Great Debates in United States History” project, notes:

His testimony before the grand jury was undermined by indications that he had altered the famous cipher letter in ways to conceal his own relationship with Burr.

Wilkinson, who Theodore Roosevelt later labeled one of American history’s most despicable characters, was removed from his position, and later faced court martial, but was exonerated. Years after his death, evidence of his own treason became more clear.

There are still questions raised today about whether Burr intended to challenge Spain, seize a huge amount of territory, and style himself Emperor Aaron I of Mexico; tried to set up a more modest secession of territory; or wanted to secure a part of the country in which he planned to live.

The thing that’s clear is that he relied on the wrong secret keeper.


Google Books and other digitized forms of primary documents are incredibly helpful, and some are referenced above. In addition:

I also referred to “War of Two” by John Sedgwick, a book about the relationship and feuds between Hamilton and Burr, and “An Artist in Treason: The Extraordinary Double Life of General James Wilkinson” By Andro Linklater.

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