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Challenge "Flawed Usage of a OTP Cipher by the BND"

Challenge "Flawed Usage of a OTP Cipher by the BND"  

  By: admin on Aug. 23, 2015, 7 p.m.

During the cold war the foreign intelligence agency of Germany established so-called stay-behind units. Their radio messages were encrypted by a OTP method, but the keys were used multiple times.

This challenge is about the effect of this multiple usage on the security of the method. You are given 5 messages, all encrypted with the same key.
Read more...

 Last edited by: admin on Oct. 31, 2021, 2:55 a.m., edited 2 times in total.

Re: Challenge  

  By: Integral on Sept. 3, 2015, 5:10 p.m.

Hi All,

I think I have found the first 4 words of message 1.
In addition, 3 of the other messages give me a nice plain-text with the OTP.
However, my solution is rejected… (capitals, no spaces) . What did I do wrong ?

Can anyone PM me ?

Many thanks in advance,

Integral

Re: Challenge  

  By: Yokozuna on Sept. 3, 2015, 8:41 p.m.

To all solvers of this challenge,

some of you may encounter the same problems as Integral. It is obviously not possible to decrypt all five messages correctly with the same key. This may be due to encryption errors some of the secret agents made during encryption of their message (encryption errors made during an encryption by hand seems to me a very realistic scenario). If for instance the messages 2 and 3 will be decrypted correctly, then message 1 contains an error, but its only one wrong character. If you adapt the key in such a way, that message 1 will be decrypted correctly, messages 2 and 3 will show up some decryption errors. Its difficult to decide, who made the errors.

I assumed, that the encryption errors are due to the secret agents, who encrypted the messages. But it would not be the first time in the history of MTC3, that some errors in the description of a challenge have been made during the generation of the pdf file of the challenge. Can the author of this challenge confirm, that the errors have been already in the original messages?

Kind regards

Yokozuna

Re: Challenge  

  By: Yokozuna on Sept. 4, 2015, 8:24 a.m.

In my last post I said the following:

"If for instance the messages 2 and 3 will be decrypted correctly, then message 1 contains an error, but its only one wrong character."

This wrong character is in the range of the first 4 words of message 1, required for the solution. I don't know, if the part of message 1 after the 4 words will be deciphered correctly or not.

Kind regards

Yokozuna

Re: Challenge  

  By: Scatha on Sept. 6, 2015, 9:24 p.m.

Hello,

first, I want to thank you for your comments.
I looked into the challenge and found some faults, starting with the end of text 3, whose last ten characters contain two wrong ones. Text 1 and 2 are correct.
Right now I'm waiting for the response of the author for final proofreading. As soon as that is done, I will update the challenge at once and inform you here.
I'm sorry for the inconvenience.

Best regards,
Patricia

Edit: Thanks to Yokozuna I found a typo in the template:
The beginning of message 1 should be:
'053 22 15315 04033 19214 20480 …'
Please excuse that fault, it will be corrected soon together with others we found.

———————————
Edit: Dank Yokozunas Hilfe konnte ich einen Tippfehler im Template finden. Der Anfang von Spruch 1 sollte richtig lauten:
'053 22 15315 04033 19214 20480 …'
Bitte entschuldigen Sie diesen Fehler, er wird zusammen mit den restlichen gefundenen Fehlern bald korrigiert.

Re: Challenge  

  By: Scatha on Sept. 14, 2015, 9:20 a.m.

Hello,

the challenge has been updated and all ciphertexts should now be correct.

Best regards,
Scatha

————————————————–

Hallo,

die Challenge wurde aktualisiert und alle Geheimtexte sollten nun korrekt sein.

Beste Grüße,
Scatha

Re: Challenge  

  By: Bart13 on Sept. 14, 2015, 10:15 a.m.

I'm pretty sure the ciphertexts are still not ok.
They're good enough to get a solution, i.e. the first for words of message 1, but it is impossible to get 5 complete messages that are all correct.

I has been stated by other members on this forum before, but I would like to repeat it, that too many challenges are put online without being thoroughly checked.

Re: Challenge  

  By: Scatha on Sept. 14, 2015, 10:56 p.m.

Hello,

there seems to be an information missing:
If the plaintext contains numbers, these will be tripled to mark them.
For example, if you substitute '13. Mai' with the matrix given in the challenge, you will get '111333 4882'.
('.' has been left out, because it is not necessary for understanding that this is a date.)
Thus, if you find a tripled number in your decrypted number sequence, you don't have to use the matrix. Just make a single number out of it.

Best regards,
Scatha

——————————————————–

Hallo,

offenbar fehlt noch eine Information:
Falls der Klartext Zahlen enthält, so werden diese verdreifacht um sie zu kennzeichnen.
Falls Sie z.B. '13. Mai' mit Hilfe der Matrix aus der Challenge ersetzen, so erhalten sie '111333 4882'.
(Der '.' wird weggelassen, weil er nicht nötig ist um zu verstehen, dass es sich um ein Datum handelt.)
Wenn sie also in Ihrer entschlüsselten Zahlenfolge eine Zahl dreimal hintereinander vorfinden, müssen Sie diese nicht mit der Matrix ersetzen. Sie machen einfach eine daraus.

Beste Grüße,
Scatha


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