The 20 Most Difficult Codes and Ciphers to Crack in History

The Enigma machine became legendary because it turned German military communications into a real puzzle. It took years of work by some of the brightest minds in the Allied forces to decipher it. When the colossal efforts of the British at Bletchley Park finally succeeded in cracking it, the impact was so significant that it helped turn the tide of the war. 

The Vigenère cipher was created by Blaise de Vigenère, a French diplomat, in the 16th century. It long enjoyed a formidable reputation because it used multiple substitution alphabets instead of just one. This made traditional frequency analysis much less effective and helped it appear virtually indecipherable to those accustomed to simpler systems. It was eventually deciphered, of course, but not until 200 years later.

The Zimmermann Telegram is significant not because its method was the most elegant ever devised, but because its decryption had major geopolitical consequences. British cryptanalysts managed to decipher enough of the German message to uncover the planned alliance involving Mexico, which helped push the United States into World War I. A code becomes particularly memorable when its decryption helps change the course of a war. 

The Japanese “Purple” cipher machine posed a major challenge to Allied cryptanalysts before and during World War II. American cryptanalysts eventually figured out how it worked without having a physical copy of the machine, by identifying recurring patterns. Deciphering this code allowed Allied forces to obtain crucial information about German plans in Europe, particularly regarding the diversionary operations planned for D-Day.

The Navajo code talkers did not use an encryption machine in the traditional sense, but their communications were, in practice, extremely secure. This system combined the Navajo language with specially developed military terminology, and it proved so effective that the Japanese never managed to decipher it during the war. 

The Playfair cipher was invented in the 19th century to simplify the encoding of telegrams. By encrypting pairs of letters rather than individual letters, it complicated the usual patterns that code breakers loved to exploit. It was possible to decipher it with enough text and expertise, but it remained much more difficult to crack than the more rudimentary systems of the time. 

These German campaign codes from World War I were designed to make them extremely difficult to intercept by combining fragmentation and transposition. These systems were so robust that deciphering them required true ingenuity, particularly that of the famous French cryptanalyst Georges Painvin. They are not as well known as Enigma today, but they have certainly earned a reputation as formidable adversaries.

Thomas Jefferson’s wheel cipher, sometimes referred to as the “Bazeries cylinder” in certain variants, was ingenious because it transformed encryption into a problem of physical alignment between several rotating discs, making it much more difficult to decipher than a simple handwritten substitution cipher.

The “Grand Chiffre,” developed in the 17th century, protected French state secrets so effectively that, even after the political context had changed, it remained indecipherable for centuries. It relied on thousands of numerical elements and was far more complex than the codes one typically imagines for that era. A code capable of outlasting the government that used it was clearly well designed. 

The Pigpen cipher seems deceptively simple, as it replaces letters with geometric symbols rather than traditional alphabetical characters. It has been particularly associated with Freemasons and other groups who appreciated the fact that it appeared mysterious to anyone unfamiliar with the system. Once you understand the principle behind it, it’s not the most difficult cipher in the world, but for those who encounter it without any preparation, it can prove surprisingly difficult to decipher and easy to misinterpret.

The Venona Project, a top-secret U.S. counterintelligence program conducted in the mid-20th century, made it possible to decrypt Soviet telegrams. Although the Soviet messages used a one-time pad encryption system considered “unbreakable,” procedural errors made by the Soviets allowed American cryptanalysts to crack the code. Venona played a crucial role in uncovering a large-scale Soviet infiltration of the Manhattan Project, the State Department, and other government agencies.

The Beale Codes owe their fame partly to the promise of buried treasure and partly to the fact that two of the three texts have still not been deciphered. One of them is said to have been deciphered using the Declaration of Independence as a key, while the others continue to inspire skepticism, obsession, and numerous efforts on the part of amateurs. Whether they hold real secrets or are simply a very convincing legend, they have certainly defied any simplistic explanation. 

The Voynich Manuscript, an illustrated 15th-century codex considered the world’s most mysterious book, falls into that exasperating category where one doesn’t even know exactly what kind of problem one is dealing with. It features a strange script and baffling illustrations, and for centuries, researchers and cryptologists have made unsuccessful attempts to provide a definitive interpretation of it. 

Linear A is not a secret code in the sense that spies understand it, but it is undoubtedly one of the most difficult writing systems in history to decipher. It is the writing system used in ancient times by the Minoan civilization on Crete. Researchers have managed to identify certain signs and a few structural features, but the underlying language remains undeciphered. 

The Zodiac Killer sent a series of four encrypted messages between the late 1960s and early 1970s, only two of which have been deciphered. This 340-character code withstood all attempts at decipherment for decades before finally being cracked in 2020. It is particularly frustrating that one of the undeciphered messages appears to reveal the Zodiac Killer’s identity. 

Kryptos may be a modern work of art, but it has carved out a historical reputation all its own. It is a sculpture located at CIA headquarters that consists of four encrypted sections; while the first three have been deciphered, the fourth remains a mystery despite decades of effort and even clues provided by the artist. 

The “Copiale Cipher” is an 18th-century German manuscript that baffled researchers for years before it was finally deciphered in 2011. It turned out to describe the rituals of a secret society known as the “Order of the Oculists,” which seems entirely plausible. 

Marie’s coded letters, which she wrote from prison, could not remain indecipherable forever, but they remained encrypted long enough for their secrets to remain hidden for centuries. Modern researchers have succeeded in identifying and decoding dozens of them, thereby demonstrating just how sophisticated her encryption system was. The decoded letters revealed many details about the last decade of Marie’s life, including her poor health and her efforts to negotiate her release.

Edward Elgar’s “Dorabella Cipher” is an encrypted letter that the composer wrote to Dora Penny, one of his close friends. It is short, strange, and remains undeciphered to this day. Its brevity is part of the problem, because with so little text, we lack the statistical clues that generally make it easier to solve other cipher puzzles. Sometimes, the most difficult code isn’t the most sophisticated one, but the one that gives us virtually no clues to work with. 

The SIGABA was an American encryption machine used during World War II that earned a solid reputation. Unlike other well-known systems, it was designed to make common cryptanalytic attacks much more difficult, and it was never cracked by the enemy during the war. It was so secure that it was not declassified until 1996.