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Challenge "Kaskade-S/T — Part 4"  

  By: admin on May 5, 2011, 10:30 a.m.

This time your friend packed the plaintext in a ZIP archive before encrypting it. Are you able to get the archive from the given ciphertext file, and then reveal the wanted plaintext?
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 Last edited by: admin on Oct. 31, 2021, 2:54 a.m., edited 1 time in total.

Re: Challenge  

  By: Norbert on Sept. 22, 2016, 12:02 p.m.

I could gather some useful information about the key by studying the overall structure of a zip file - but as it seems to me, this is by far not sufficient to really get a foot in the door … So, as I saw this challenge has not been solved yet for five years, I thought it might be the right time to kindly ask for hints … Here are some questions, maybe the answer to one or several of them could give us some support:

1.) Which program was used to pack the plaintext into a zip file?
2.) More specific, which zip version is needed at least to extract it?
3.) Which compression method has been used? (deflate?)
4.) Has the zip program applied by itself any sort of encryption to it, I mean prior to the Kaskade-S/T? (I do not hope so …)
5.) Does the zip file contain any "file comments" or "extra fields" (apparently no, but to be sure)?
6.) When decrypted and unzipped, which filename will the plaintext have? (maybe "plaintext" without suffix?)
7.) Does our friend who is working on "power grids" and "Stromversorgung", prefer specific sources like Wikipedia?
8.) First the English section, or first the German one?
9.) Does the author of the challenge recommend specific papers on the compression method used here or more generally, on recognition/statistics of a zip file?

Re: Challenge  

  By: be on Oct. 1, 2016, 2:30 a.m.

So, as I saw this challenge has not been solved yet for five years, I thought it might be the right time to kindly ask for hints

I am currently abroad. I'll give some answers when I am back. Best regards, be

Re: Challenge  

  By: be on Oct. 20, 2016, 7:02 p.m.

So, as I saw this challenge has not been solved yet for five years, I thought it might be the right time to kindly ask for hints

Thanks for gathering these questions.

Maybe, I explain a bit about the context: The (slightly changed) series was part of the training material built in order to let develop students their own feeling, what changes make ciphers stronger and what changes don't. This example from a seminar should show four things (which only apply in general, but not mandatorily in each specific case):
a) Applying the two principles substitution and transposition to bytes (object of length 8 bit) is still a good start.
b) However, even their combination doesn't hide enough structure to avoid being easily broken (see challenges part #1, #2, and #3).
c) Compressing the cleartext before applying substitution and
transposition makes the cipher very much harder
(PGP implementations also compress before performing the encryption; and this is not only done to have shorter ciphertexts).
d) Comparing the three ciphertext-only challenges #3, #4, and #5 shows,
that appending more rounds (like in #5) doesn't add as much
additional strength as appending compression.
The result until now seems to affirm these assumptions.

We still don't want to reveal the plaintext or the key, because then the solution of the challenge is known. However, we can answer most of your questions (as the way, the sample was built, is for sure public information).

–> 1+9) As far as I remember, the zip-compressed file was built using the compress tool from the context menu of the File Explorer of Windows 7. So the result should be a typical zip file: It should start with "PK" and it should contain the original filename which had the extension "txt". No specific paper on compression methods is recommended or needed.

–> 2) I hope that the zip format is standardized so well that you can unzip it with any zip version.
I remember we had some tests under Linux using zip and unzip which worked well too.

–> 3+5) All is standard zip. No parameters had been set.

–> 4) No, the zip file was built purely by compressing the original text file.

–> 6) We don't want to reveal the name of the plaintext file right now. Maybe it helps to know the size of the original text file: 13631 bytes.

–> 7) This is a very good idea :-)
(The student's who solved the task also analyzed context information; they didn't find a purely generic analyzer.)

–> 8) English text first.

I hope this helps a bit despite it still leaves the main task up to you.


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