Famous Unsolved Codes and Ciphers

This is the top 10 well-known unsolved codes and ciphers. A couple of the better-known unsolved ancient historical scripts are also thrown in, since they tend to come up during any discussion of unsolved codes. There has also been an attempt to sort this list by "fame", as defined by a loose formula involving the number of times that a particular cipher has been written about, and/or how many hits it pulls up on a moderately-sorted web search.

1. Beale Ciphers

Beale Ciphers - In 1885, a small pamphlet was published in Virginia containing a story and three encrypted messages. According to the pamphlet, around 1820 a man named Beale buried two wagons-full of treasure at a secret location in Bedford County, Virginia. He then left a small locked box with a local innkeeper, and left town, never to be seen again. The pamphlet went on to state that the innkeeper, after having not heard from Beale for many years, opened the box and discovered encrypted messages. Never able to read them, he eventually passed them along to a young friend shortly before the innkeeper's death in 1863. According to the pamphlet, the friend spent the next 20 years trying to decrypt the messages, solving only one which detailed the tons of gold, silver and jewels that were buried, along with a general location. The still unsolved messages supposedly give exact directions, and a list of who the treasure belongs to. According to the story, the friend finally decided to walk away from the quest, and publish everything they knew about the situation in the (anonymous) pamphlet, which was supposedly published by another friend of the innkeeper. There have been many exhaustive searches for the treasure, and much effort spent on decoding the other messages, without (confirmed) success. There are many claimed solutions , usually bannered in combination with a book that someone is trying to sell, but no one has ever been able to produce a duplicatable decryption method. There have also been some pretty compelling arguments that the entire original story was a hoax . There are several inconsistencies in the pamphlet's text, and even speculation that the story was a parable related to Masonic rituals . More information can be found here .

2. Voynich Manuscript

Voynich Manuscript - At least 400 years old, this is a 232-page illuminated manuscript entirely written in a secret script. It is filled with copious drawings of unidentified plants, herbal recipes of some sort, astrological diagrams, and many small human figures in strange plumbing-like contraptions. The script is unlike anything else in existence, but is written in a confident style, seemingly by someone who was very comfortable with it. In 2004 there were some compelling arguments which described a technique that would seemingly prove that the manuscript was a hoax , but to date, none of the described techniques have been able to replicate a single section of the Manuscript, so speculations continue. More information about the hoax possibility is here and more information about the Manuscript can be found at voynich.net and crystalinks.com .

3. Dorabella Cipher

Dorabella Cipher - In 1897, the well-known composer Edward Elgar (of "Pomp and Circumstance" fame) sent an encrypted message to a 23-year-old friend, Miss Dora Penny . To this day, it still has not been solved.

4. Zodiac Killer ciphers

Zodiac Killer ciphers - From 1966 to 1974, the Zodiac serial killer sent more than 20 written communications to police officials. Most of these messages have been cracked, but there are still some that remain unsolved. The killer was never caught.

5. D'Agapeyeff

D'Agapeyeff - Alexander d'Agapeyeff wrote an elementary book on cryptography in 1939, entitled "Codes and Ciphers." In the first edition, he included a challenge cipher. Nobody's solved it, and he embarrassedly admitted later that he no longer knew how he'd encrypted it. It was left out of the second and later editions. Some think it was botched, and many think it could still be solved despite that. It has lots of "phenomena" noted, but nothing close to a crack.

6. Linear A

Linear A - In 1900, a large number of clay tablets dating back to 1800 BC were discovered in Crete. The tablets appear to use two different types of scripts, which were named "Linear A" and "Linear B." Linear B was finally deciphered in the 1950s. Linear A remains unsolved.

7. The Phaistos Disk

The Phaistos Disk - A circular clay tablet about six inches across, discovered in Crete in the early 1900s, and believed to date back to 1800 BC. With an "alphabet" of 45 different symbols, 241 signs are stamped into both sides in spiral patterns. There has been much speculation about its meaning, with wildly variant claimed solutions so far. It's also been suggested that the disk might turn out to be a Rosetta Stone to help decipher Linear A, since it was discovered near a fragment of a Linear A tablet. More information here .

8. Chaocipher

Chaocipher - In 1918, J.F. Byrne created a machine-based cryptographic system. In 1953, he used it to create a code challenge as part of his autobiography "Silent Years". There are at least three people who know how the system works: his son, and two editors of Cryptologia who were let in on the secret in 1990. There's a lot of known plaintext available, as well as some hints, but no break yet. The mechanism used to generate it fits in a cigar box.

9. Chinese "Gold Bar" ciphers

Chinese "Gold Bar" ciphers - In 1933, seven gold bars were allegedly issued to a General Wang in Shanghai, China. These gold bars appear to represent metal certificates related to a bank deposit with a U.S. Bank. The gold bars themselves have pictures, Chinese writing, some form of script writing, and cryptograms in latin letters.

10. RSA Challenges

RSA Challenges - There are a number of modern computer-based challenges, including several factoring challenges from RSA Labs that have implications for the strength of public-key systems, and some equivalently difficult elliptic curve challenges , also relating to public-key cracking ( check here for Bruce Schneier's high-math analysis of the RSA/Elliptic Curve debate ). As of this writing, the most recently-cracked RSA Challenge was in November 2005, when RSA-640 , a 193-digit number, was successfully factored (which won a $20,000 prize). There are several more on the list, with prizes up to $200,000, which have not yet been cracked. Many fly-by-night snake-oil crypto companies also put out challenges that are arguably famous because the media sometimes pick up the challenge uncritically , but they are usually not worth mentioning on this list.