To be a master cryptanalyst like the legendary Abe Sinkov and Alan Turing you’ll need to sharpen your skills. Let’s get cracking with a countdown of some of the world's most difficult and amusing codes and ciphers. 

10. Sherlock Holmes: The Dancing Men Cipher

Given Sherlock Holmes’ love of encrypted personal messages buried in The Times’ ‘agony column’, it was perhaps inevitable that Sir Arthur Conan Doyle would invent his own secret alphabet in The Adventure of the Dancing Men. Can you solve Conan Doyle’s substitution cipher? ‍

Hint: Look for single-letter words and count the number of times each symbol appears. Also, keep an eye out for apostrophes or repeated letter patterns. 

Solution: You will find more of Conan Doyle’s cipher and its solution in The Return of Sherlock Holmes, The Adventure of the Dancing Men.
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9. China's Yuan Dynasty Coin Inscriptions

Chinese numismatists are baffled by six centuries-old Indian coins discovered in Hunan province in the 1960s - so much so, they offered a $1,500 reward to anyone who can decipher the inscriptions and shed light on the etchings. The coins were found inside a small, glazed pot which arrived at Jinshi City's museum in the 1980s. The front of the coins are believed to bear the name of a king, written in a rare form of Arabic, but archeologists are puzzled about the etchings on the back of the coins.

Hint: The coins were manufactured in the Delhi Sultanate, the main Muslim sultanate in northern India, around the late 13th century during China's Yuan dynasty.

‍Solution: You’ll need to discover the solution to claim the prize money.
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8. Australia’s Somerton Man

Australia is so obsessed with Somerton Man the authorities exhumed his body in 2021. The well-dressed man was found on an Adelaide beach, slumped against a seawall in 1948. He carried no ID. His clothing labels were removed. His pocket held a ripped piece of paper - torn from a poetry book, the Rubaiyat of Omar Khayyam - with the Persian words Tamam Shud, which means ‘it is finished’. The book itself revealed another clue: a handwritten message or code that has never been deciphered. The coroner suspected Somerton Man was poisoned but couldn’t be certain. Australia hopes DNA tests will finally reveal his identity and cause of death but what about the code?

Hints: There are no hints, just a few curious coincidences. The code was written in the Rubaiyat of Omar Khayyam, a poetry book that was not found at the crime scene. John Freeman, a chemist, later handed the book over to the police. Its pages also revealed a phone number for Jessie Thomson, a nurse who claimed she did not know Somerton Man.

‍Solution: Unsolved.
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7. The MIT Cryptographic ‘Time-Lock’ Puzzle - LCS35

Ron Rivest, co-inventor of the RSA algorithm - an asymmetric cryptography algorithm for encrypting online communications - devised a ‘time-lock’ puzzle in 1999. Rivest estimated it would take 35 years to solve but Bernard Fabrot, a self-taught Belgian programmer, came up with the solution 15 years early. Rivest's problem is this: compute 2^(2^t) (mod n) for specified values of t and n.

Hint: The puzzle is an example of a verifiable delay function, which means the answer can only be solved after a certain number of steps. It uses the ideas described in the paper Time-lock puzzles and timed-release Crypto.

Solution: You will find a description of the solution here. 

6. Dorabella Cipher

Edward Elgar, the composer of the Enigma Variations, wrote coded notes to Dora Penny in the late 1800s. She was the daughter of a British vicar and 17 years his junior. If the correspondence was an expression of love, however, his deepest desires were lost on Dora. She never did decipher the contents. Mark Pitt, a member of Cleveland Police's specialist operations unit, later claimed he cracked the code using a revolving cipher and a musical cipher but hasn’t revealed the answer.

Hint: The cipher appears to be made up of 24 symbols, each symbol consisting of 1, 2, or 3 approximate semicircles oriented in one of eight directions. 

Solution: Pitt believes the cipher is a romantic note and he is writing a book on the topic.

5. The Voynich Manuscript

Cryptographers still haven’t been able to crack the Voynich code, leading some to believe it may be a hoax. Yale University Press released the first authorized copy of the mysterious, centuries-old puzzle in 2015. The original, illustrated manuscript, believed to date back to the 14th century, is handwritten in a script unrelated to European languages. The alphabet has up to 28 characters, used without punctuation throughout the text.

Hint: There is strong evidence that many of the book's bifolios were reordered at various points in its history.

Solution: Unsolved.
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4. The Code Book

A team of Swedish computer buffs fought off thousands of rivals in 2000 to crack what was billed as the toughest code challenge ever set. The Swedes took the equivalent of 70 years of computer time to decipher 10 codes ranging from ancient Greece ciphers to a German WWII Enigma code laid out in Simon Singh’s The Code Book. The book also discusses the Man in the Iron Mask, Arabic cryptography, Charles Babbage, and public-key cryptography. The Swedish team was awarded £10,000 ($13,600).
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Hint: Pace yourself. It took the Swedes more than a year to resolve the 10 puzzles.

Solution: You will find the solutions here. 

3. Kryptos at the CIA HQ

The Kryptos puzzle at the CIA’s Langley, Virginia HQ confounds agents, academics, computer scientists, and anyone else who dreams about solving it. The 12-foot copper statue, erected in 1991, seems to display a collection of random letters but it is a riddle for the ages. Only three of the four passages have been decrypted. Can you solve it? Here’s the Kryptos puzzle in full:

Hints: Northeast, Berlin, and clock are hints to the Kryptos riddle.

Solution: Here is the partial solution solved by the NSA.

2. Zodiac Killer

French engineer Fayçal Ziraoui claims to have cracked the Zodiac killer’s last two ciphers which have puzzled California police for more than 50 years. Not everyone is convinced, but that doesn’t mean he’s wrong. Ziraoui also claims to know the killer’s name and sent his findings to the FBI and San Francisco Police Department. Neither has commented, citing the ongoing investigation.
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Hint: Ziraoui used the Z340 key and translated the remaining letters into numerical digits, using A for 1, B for 2, C for 3, and so on. He ended up with a set of 13 numbers.

Solution: Read on for the solution and - if Ziraoui is correct - the name of the Zodiac killer. 

1. The Beale Papers

Are The Beale Papers a map to a hidden treasure or a hoax? Whoever cracks the 19th-century code could uncover $50m in gold, silver, and jewels - or waste a lot of time. As the story goes, Thomas J. Beale rode into Lynchburg, Virginia in 1820 where he remained for a short time - never speaking about his background or the purpose of his visit. He left a strongbox with a hotel owner and disappeared. Eventually, in 1845, the locked box was opened and sheets of paper filled with numbers were found inside. They supposedly revealed information about a buried treasure, with a promise of a key to unlock the ciphers but the key was never received.

Hint - The second Beale cipher contains about 800 numbers, beginning with the sequence; 115, 73, 24, 807, 37 … Each number corresponded to a word in the Declaration of Independence.

Solution: Unsolved. Progress has been made on Beale’s second cryptogram, however. Click here for the three Beale cipher texts and background.
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