It isn’t surprising that criminals would want to encrypt messages. Some need cipher systems to communicate securely with each other. Others who work alone may want to conceal the contents of diaries or notebooks they hope no one else will ever read. A few—think Batman’s adversary the Riddler—even use ciphers as puzzles to taunt police. The seven historic criminal cases discussed below involve ciphers that vary in sophistication: Some have been solved while others still puzzle the world’s leading cryptology experts to this day. But all are connected to the loss of life, most by murder and one a possible suicide.

This cryptogram by Henry Debosnys from the late 1880s includes his self portrait. (From the Collection of the Adirondack History Museum)

Henry Debosnys (1883) 

While serial killers are considered by some to be a new phenomenon, it’s likely they’ve always been among us and simply weren’t widely recognized until recent decades. One possible early example: Henry Debosnys. In 1883, he was convicted of murdering his wife, Betsey, after the jury deliberated for all of nine minutes. And as it turns out, Betsey, who was shot and had her throat slashed, wasn’t his only spouse who died under mysterious circumstances. His first drowned and his second died of starvation, after which Henry hired a cook to prepare all of his own meals. The connection between the three deaths has never been proven, but Henry left behind gloomy poems, figure sketches and enciphered messages combining mathematical-looking symbols with pictograms, all neatly rendered in ink. One page shows Henry’s self-portrait, while some include sketches of pretty women who may have been his victims. Could these still-unsolved ciphers conceal Henry’s darkest secrets?

This typewritten cipher was taped to the abdomen of a man found dead in a ditch outside Philadelphia. (Credit: The Federal Bureau of Investigation)

Paul Rubin (1953)

In 1953, a large male body was found dead in a ditch by the Philadelphia airport with enough potassium cyanide in his system to kill 10 men—and a cipher taped to his abdomen. He also had a fountain-pen gun and a fake alligator-skin wallet containing a picture of a Nazi aircraft. His glasses, thick enough to be compared to magnifying glasses, could have been worn as a disguise. Was this man a hardened Cold War spy up to no good in America—or staged to look like one? The typewritten cipher found taped to his belly included a pair of undisguised names—Dulles and Conant—tucked into strings of indecipherable words and numbers. John Foster Dulles was, at the time, designated by President Eisenhower to become the new Secretary of State, and Dr. James B. Conant, then President of Harvard University, was slated to become America’s first High Commissioner in West Germany. Such name-dropping earned the case the attention of the F.B.I. and pointed to potential high intrigue—until the victim was identified as Paul Rubin, whiz-kid college student from an Orthodox Jewish neighborhood in Brooklyn. No progress has been made cracking the cipher, which may unlock the details behind his mysterious demise.

The Zodiac killer sent his Z340 cipher to The San Francisco Chronicle in November 1969 along with a greeting card and a shirt fragment of one of his victims. (Credit: The Federal Bureau of Investigation)

Zodiac (late 1960s and early 1970s)

In all likelihood, the serial killer calling himself Zodiac carried out his crimes alone. He created encrypted messages, apparently for the sole purpose of taunting the authorities. Like Jack the Ripper in London and the Black Dahlia Avenger in Los Angeles, he courted publicity with nasty letters and was never caught. Although he included ciphers with his letters to newspapers on four occasions, there is only one for which a widely accepted solution has been found. The others remain mysterious, especially one known as the Z340, a crudely handwritten grid of letters and symbols that has challenged cryptology experts for nearly 50 years. While Zodiac is known to have killed three women and two men—and left two other men for dead—in one missive he alludes to a tally of more than 30 victims. He has claimed the Z340 offers clues to his identity. And while it may just be the cruel tease of a twisted mind, cryptologists—including a team associated with HISTORY Channel’s series “The Hunt for the Zodiac Killer”—continue to chip away at it, hoping to shed new light on this cold case.

John Walsh of ‘America’s Most Wanted’ television program was taunted with threatening letters that included Zodiac-like ciphers. (Credit: Chip Somodevilla/Getty Images)

Scorpion (1991)

In nature, predators select the most vulnerable animals as their prey, and most human predators do the same. However, in 1991 a predator calling himself Scorpion took on a particularly tough target: John Walsh, a man whose TV program, “America’s Most Wanted,” has led to the apprehension of over 1,000 fugitives. Walsh’s work continues today on “The Hunt” and through various organizations. In this instance, the predator apparently kept a safe distance, simply mailing Walsh threatening letters and taunting him with ciphers. Years later, his team made some of the communications public in the hope of generating leads. While the letters and ciphers resembled those of Zodiac, they seem to be more inspired than by the same hand. The distinctive phrases they contain (e.g., “see you in hell amigo”) may reveal the identity of this criminal, in the same way the content and style of the Unabomber’s manifesto led David Kaczynski to identify his brother Ted as a possible author—and ultimately led to his apprehension. The ciphers themselves, while amateurish in appearance, may prove very difficult to crack, as there are a large number of distinct symbols appearing in the relatively short texts.

A code sheet belonging to Ted Kaczynski, the Unabomber. (Credit: David Goldman/AP/REX/Shutterstock)

Unabomber (arrested in 1996)

Although the Unabomber’s brother David Kaczynski led authorities to him, much more than suspicion was needed to make an arrest. Ted Kaczynski maintained journals that contained detailed records of his manufacturing, testing and mailing of bombs. But Kaczynski knew better than to have such incriminating material recorded in plain English. Having scored 167 on an IQ test he took back in fifth grade, Kaczynski enrolled at Harvard at age 16, graduated at 20, went on to earn a Ph.D in mathematics and land a faculty position with the prestigious math program at University of California, Berkeley. However, he quit after two years and it was downhill from then on. In addition to eluding the authorities when carrying out his murders, he devised fiendishly difficult methods of encryption to conceal the documentation of his crimes. But despite his great intelligence, Kaczynski made two major errors, to the benefit of investigators. First, he kept the key to his journals with the journals. All the cryptanalysts had to do was follow his instructions to convert the volumes back to English. His other major mistake: keeping so much incriminating evidence in his cabin that his deciphered journals weren’t even needed at the trial. But what if Kaczynski had disposed of all of the other evidence, along with the keys to his ciphers? If conviction depended entirely on being able to break this genius’s encryption, would the FBI have been able to pull it off?

Aryan Brotherhood gang leaders Tyler Davis Bingham and Barry ‘The Baron’ Mills led a plot to incite a race war inside prisons across the U.S. Some of their messages to fellow gang members were encoded with a cipher system used 400 years ago by British philosopher Sir Francis Bacon. (Credit: Orange County Register/AP Photo)

Aryan Brotherhood (1997)

When a gang member goes to prison, he’s often in the company of fellow gang brothers—and rather than being reformed, his gang ties are strengthened. In an attempt to avoid this, federal authorities decided to redistribute members of a Washington, D.C. gang, the DC Blacks, from a central location to prisons across the nation. Imprisoned members of the white supremacist gang Aryan Brotherhood saw this as an opportunity to send a message about who was in charge. The plan, communicated through a network that spanned the federal prison system, was to kill the DC Blacks in each of the multiple prisons where they were being distributed, effectively starting a race war. This was all to be done in a coordinated attack on a single day, so it could not be taken as coincidental. Part of the plan was successfully communicated using a binary-alphabet cipher system that goes back over 400 years to the English philosopher and author Sir Francis Bacon. But one member of the Brotherhood got his dates mixed up and carried out his murder just prior to the intended date. After authorities quickly launched an investigation, the plan was exposed and further murders prevented.

A sketch of Germany’s Masked Man, whose encrypted computer drives were suspected to contain evidence about his molestation and murder of children. (Credit: Public Domain)

Germany’s Masked Man (arrested in 2011)

In today’s information age, criminals the world over have easy access to methods of enciphering far stronger than anything they’d be likely to devise on their own. The result: The number of unsolved ciphers associated with major crimes is skyrocketing. In the case of serial child molester and murderer Martin Ney, who was arrested in 2011 and sentenced to life in prison, he may have committed additional murders for which he has not yet been charged. Police suspected that one of the encrypted computer drives owned by the German killer might hold the answer, but Ney refused to divulge the keys to his encryption for five years before finally giving it up.

In other cases, however, the criminals are keeping their mouths shut tight and their ciphers are holding. Such cases will continue to pile up until a new technological or cryptological breakthrough allows the authorities to begin working their way through the backlog. One convicted serial killer, Joseph E. Duncan III, blogged the following prior to his apprehension:

“I wish I could be more honest about my feelings, but those demons made sure I’d never be able to do that. I might not know if it matters, but just in case, I am working on an encrypted journal that is hundreds of times more frank than this blog could ever be (that’s why I keep it encrypted). I figure in 30 years or more we will have the technology to easily crack the encryption (currently very un-crackable, PGP) and then the world will know who I really was, and what I really did, and what I really thought.”

Craig Bauer, professor of mathematics at York College of Pennsylvania and editor in chief of the journal Cryptologia, has served as a scholar in residence at the NSA’s Center for Cryptologic History. He is the author of Unsolved! The History and Mystery of the World’s Greatest Ciphers from Ancient Egypt to Online Secret Societies.