The Zodiac Killer murdered five or more people and sent four encrypted messages to newspapers. The case and three of the cryptograms have never been solved.

Henry Debosnys (1836-1883), a convicted murderer, left behind four cryptograms, which  are unsolved to date. Breaking them could shed light on the many mysteries that surround this case.

Alession Vinci, an Italian student, died in 2019 for unexplained reasons. He left behind an unsolved cryptogram.

It’s probably not a Christmas card, but at least an encrypted card that was sent on the occasion of a celebration. Can a reader decipher this cryptogram?

Here’s one of the oldest encrypted postcards I have ever seen. Can a reader decipher it?

Belgian scholar Michael Florent van Langren (1598-1675) proposed a naval navigation method that didn’t work. A second method he developed might be described a cryptogram that has never been solved.

On Cocos Island in the eastern Pacific, a “treasure” consisting of works created by 40 modern artists is buried. The exact location is described in an encrypted message. To my knowlege, this cryptogram has never been published.

In 1891, an unknown person published an advertisement in the Morning Post. Five words in this message are encrypted. Can a reader break this cryptogram?

Crypto expert Frode Weierud has published a collection of unsolved cryptograms from 1969. If you’re looking for a challenging codebreaking project, here you go.

In 1916, the USA bought the West Indian islands from Denmark. An encrypted telegram about this purchase, sent by the Danish ambassador, is available on a website.