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As part of an assignment, I have been given a cipher text of 

FFFXXXFGXAGFAFDAFXADDAGAD

with the following description, 

A zebra could unlock this German, 1st World War cipher.

At first, I thought that the description could contain a Playfair keyword that could be used to crack the ciphertext. However, the ciphertext is 25 characters long - shouldn't the plaintext be padded before encryption to create an even number of characters? Also, I don't see how Playfair encryption could yield repeating characters in a pair like "FF" or "XX".

I was hoping someone could validate my reasoning behind this and maybe point me in the right direction (in terms of determining which encryption method has been used).

Patriot
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MPol
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  • Welcome to Crypto.SE! First up, your question is a bit borderlining as "Requests for analyzing ciphertext or reviewing full cryptographic designs are off-topic, ...." but that's just an aside and (from my point of view) not really applicable in this case. Anyway, the answers to the following might help - How to attack a classical cipher using known partial plaintext? – e-sushi Nov 23 '16 at 12:53
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    A zebra has black and white lines. In the Netherlands a "zebrapad" is a pedestrian crossing (due to the white and black lines). People tend to walk only on the white lines. "Zebra" therefore often means that you have to skip each other element or character. – Maarten Bodewes Nov 23 '16 at 13:04
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    And as far as I know, the playfair cipher was exclusively used by the British and not the Germans... – SEJPM Nov 23 '16 at 13:19
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    You want to look at the ADFGVX cipher, a German, 1st World War cipher. – fgrieu Nov 23 '16 at 13:25
  • @fgrieu It looks like this might be the right answer. The cipher text clearly shows its characteristics. Too bad we didn't actually learn about this in class. – MPol Nov 23 '16 at 13:28
  • @MPol, I would hope your instructor is willing to introduce new things in homework to stretch the students. If homework were just a regurgitation of lectures, you aren't getting your money's worth. – mikeazo Nov 23 '16 at 15:03
  • The person who found this question interesting and decided to edit it, me, learned a bitter lesson: some questions are best left alone, and they can be found by those who are interested. And an unappealing title might just kill an otherwise valuable and interesting question. – Patriot Jul 21 '19 at 13:04
  • I'm just wondering. .. what part of the question made it worthwhile to you? In the end it is just finding the algorithm with the ciphertext, right? Or is it just the fact that the classic cipher exists? Not a critique, I'm just curious. – Maarten Bodewes Jul 21 '19 at 20:49
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    @MaartenBodewes Just the fact that this cipher existed, that many people at that time trusted it, and then that the U.S. Army wrote a manual on Cryptography in the 40s/50s which said it was trustworthy to use (except that it was not that hard to break). One can still download that manual from the NSA. Also, I like how it was optimized for Morse Code. – Patriot Aug 07 '19 at 12:26

1 Answers1

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As it turns out, the encryption method used was "ADFGX", an earlier version of the proposed "ADFGVX" method suggested in a comment by fgrieu:

You want to look at the ADFGVX cipher, a German, 1st World War cipher.

This made me search and find ADFGX - a German cipher based on a combination of the Polybius checkerboard and ciphers using key words. It was known as ADFGX, because those were the only letters used in the cipher. The Germans chose these letters because their Morse code equivalents are difficult to confuse, reducing the chance of errors.

This answers my question and shows that even a little hint can be really helpful.

Community
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MPol
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  • Presumably ZEBRA is the 5 letter transposition key used (we have a nice 5x5 grid we can fill) and then you're left with a monoalphabetic substitution to solve. – Henno Brandsma Jul 19 '19 at 22:07