In April 1862 Civil War general Robert E. Lee received an encrypted note from a Joseph E. Johnston. According to the library that owns this document, the encryption is unsolved.

A postcard from 1909 bears an encrypted message and an unusual spelling of a remarkable rock. Can a reader solve these mysteries?

The best-selling novel “The Gold Bug Variations” from 1991 contains an encrypted dedication. It has never been deciphered.

Daniel Kammen, energy advisor of US president Donald Trump, has hidden an acrostic in his letter of resignation. He’s not the first one to do so.

My blog post about acrostics (especially about one in a letter of resignation from a presidential advisory committee) has been quite popular among my readers. Here are a few more acrostics.

Acrostics are a data hiding technique that has been in use for centuries. The latest example is contained in a letter of resignation written by a US committee last week.

The SG-41, also known as Hitler Mill, was the successor of the Enigma encryption machine. Detectorists have now found a specimen in a Bavarian forest.

For decades, members of the American Cryptogram Association have tried to decipher an encrypted note found in Denmark – but to no avail.

In 2006 a couple from Erba, Italy, murdered four neighbors. An encrypted letter from this criminal case is still waiting to be solved.

“Fringe”, a successful US TV series, contains a hidden code. It has been solved by fans of the series.