GoKawiilScienceblogs.de, a German science blogging portal, includes a relatively famous list of 50 unsolved ciphers, which range from cryptograms published by serial killers to the famous Voynich manuscript.
Among these ciphers is a set of German radio messages from World War I that were encoded using the ADFGVX method.
This method is illustrated by the following example using the word “HOUSE” as the key:
A D F G V X A H O U S E A D B C D F G I F J K L M N P G Q R T V W X V Y Z 0 1 2 3 X 4 5 6 7 8 9
As you can see, ADFGVX is used both horizontally and vertically to give each “cell” in the table a value. For example, in this text, “AA” corresponds to the letter H, “AD” corresponds to the letter O, “DA” corresponds to the letter B, and so on. And so, the word “PRINZ” would be encoded as:
FX GD DX FV VD
Using an encryption word other than “HOUSE” would result in a completely different table.
There is a list of known keys used by the Germans to encrypt these radio.messages, and hundreds of these messages have already been decoded, including by codebreaking expert George Lasry. Still, over a dozen have thus far eluded efforts to solve them, including (to my knowledge) this one, originally transmitted on November 27, 1918 (pg. 217):
GPT-6 Astra solved this cipher, and believes that the original message was as follows:
EIN ENGLISCHER KREUZER EINLIEG X SEWASTOPOL X S4STEN X EIN GESCHWADER DER X ALLIIERTEN FOLGT 26STEN X
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