A telegram sent by a British colonel in 1916 still waits to be solved. The encryption method used might be a letter-pair substitution.

The encrypted inscription on the Kryptos sculpture is one of the most famous crypto mysteries in the world. Recently, a TV documentary about Kryptos was made. Here’s my making-of report.

In 1910, a man living in Bedford, Ohio, received an encrypted postcard. Can a reader decipher it?

In 1909, a woman living in Toledo, Ohio, received an encrypted postcard. Can a reader decipher it?

A Reddit user has posted scans of something that looks like a small codebook. It was found in a prison. Can a reader say more about it?

Five weeks ago, I blogged about an encrypted diary kept by a French pedophile priest. Tony Gaffney has now broken the cipher. I hope, my readers can help to decrypt the two diary pages that are publicly available.

In 1931, codebreaker and book author Herbert Yardley published an encrypted message without providing the plaintext. Can a reader decipher it?

Years ago, Otto Leiberich, the former president of the West German cipher authority, told me about a little known encryption system developed by his team. Can you break a challenge cryptogram I have created with this system?

The encrypted inscription on the Kryptos sculpture located in front of the CIA building in Langley, Virginia, is the world’s most famous crypto puzzle. In March, a Kryptos gathering organized by Elonka Dunin will take place.

In the early 19th century, an unknown person sent an encrypted postcard from Munich to Berlin. It is not hard to decipher.