A Reddit user has posted a postcard from Sweden written in 1910. Can a reader decipher it?


An advertisement poster that is currently shown in London underground trains contains an encrypted message. It is not very hard to solve.


In World War II German secret agents used a little known cipher device named Schlüsselrad (Cipher Wheel). An old document blog reader Ralph Erskine made me aware of contains the first photographs I have ever seen of this device, as well as some additional information.


“Chief Security Officer”, my 24th book, has just been published. Contrary to my other publications, this one is a cartoon book. It tells funny, yet true stories about IT security.


According to a Reddit post, a needlework made in the 1980s contains a hidden message. The details are not known any more. Can you find this message?


From 1884-1887 a series of 25 encrypted newspaper advertisements was published in the “Daily Telegraph”. The key is known. Can you help me to decrypt these messages?


Four years ago I blogged about an encrypted (?) text a blog reader had found in an old school book. It is still unsolved. Meanwhile I have better scans and some additional information.


On Wikimedia, photographs of a number of lesser-known encryption devices from Switzerland are available. Can a reader tell me more about them?


The Siemens & Halske Geheimschreiber (T-52) was the second-most im portant encryption machine of the Germans in World War II. George Lasry has recently published a few computer-based attacks on this device.


A Rubik’s Cube can be used to define a crypto system that cannot be broken with quantum computers. Here’s a puzzle that shows the concept this system is built on. Can you solve it?