Playfair cipher or ADFGX?

Question

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).

Answer 1

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.