This time your friend encrypted his plaintext within several iterations (rounds). Are you able to decrypt the ciphertext anyway?
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Is the friend german? or does he writes in English? I should know since he is my friend :-)

The previous messages were in English but the hint says:

Hint:
You know that your friend is momentarily interested in
Fukushima und Chernobyl

using "und" and not "and" so this is not clear.

Does the use of the word "und" indicate that the text is in German, or was it just a mistake in the hint sentence?

Thx
George

the text contains english and german passages.

it is a concatenation of texts. some of them are in german, some in english.

best regards,
jomandi

BTW, in part 6 you say that more iterations don't increase security much. I would argue that for this cipher, they decrease security. With more iterations, the chances that the key contains short cycles increases. In itself, this weakens security, and it also means there are more likely to be plaintext characters showing through.

With 10 iterations and each key 256 in size, this is not such a problem.