In 2013 George Lasry broke a ciphertext many had considered unbreakable. In the wake of this success, a few crypto experts created a similar challenge with an even higher level of difficulty. Is this one unbreakable? So far, nobody has solved it.

The Codex Seraphinianus is an unusual book, a beautiful artwork, and an unsolved crypto mystery. It’s the Voynich Manuscript of the 20th century.

This is the shortest cryptogram I have ever introduced on this blog: 46, 9, 4-57, 3, 5. Can a reader decipher it?

A new internet mystery game, named “Red Triangle”, has surfaced. Meanwhile, six chapters with crypto puzzles and other challenges are known to exist. Is the Red Triangle the new Cicada 3301?

A few years after the Zodiac murders, an unknown person sent a letter to an address in Albany, NY, claiming that he was the Zodiac Killer and that he intended to start killing again. A few lines of this letter are encrypted. The cleartext is unknown.

During the Second World War, an unknown person stuck sheets with seemingly random letter sequences into a book. The meaning of these letters is unknown.

A Reddit user has found an encrypted message written in a dictionary. Can a reader break this cryptogram?

In May 2014 the NSA published four crypto challenges via Twitter. To my knowledge, the fourth one is still unsolved.

In 1916 an unknown man robbed a ticket counter at the train station of Lima, Ohio. According to a local newspaper he left behind an encrypted message, the purpose of which is unknown. The message has never been deciphered.

20 years after the last message of the infamous Zodiac Killer an unknown person sent a Zodiac-style postcard to a California newspaper. As far as I know, it has never been deciphered.