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Challenge "Recycled One-Time Pad"

Challenge "Recycled One-Time Pad"  

  By: admin on Nov. 20, 2012, 4:51 p.m.

Can you decrypt the three military messages?
Read more...

 Last edited by: admin on Oct. 31, 2021, 2:55 a.m., edited 1 time in total.

Re: Challenge "Recycled One-Time Pad"  

  By: Bart13 on Nov. 24, 2012, 2:34 a.m.

mmm… very difficult for a level I challenge.
Well, not really difficult in itself, but I just seem to be unable to find a suitable 'crib'.

Re: Challenge "Recycled One-Time Pad"  

  By: GeorgH on Nov. 26, 2012, 2:20 p.m.

Sorry, I have spotted a mistake on the slides. I have notified the MTC3 team, but for now, the corrects ciphertexts are:

PERLSTXZDBFONKKYTQPQJFDEKKJODP

QHNNFIJNWSSOTGUJRNOPLMHRESMHVP

OERZTOHXANSASLKWELZBAOJNKSOUOV

Sorry for the inconvenience caused!!!

Re: Challenge "Recycled One-Time Pad"  

  By: Bart13 on Nov. 26, 2012, 5:19 p.m.

Well, this is not the odd typo…, but messages 2 and 3 are completely different!

I think it would be wise that challenges are checked better before they are posted.
Both by the author and the MTC3-team.

But it seems they are not checked at all as this is not the first time this has happened.
A way too large proportion of challenges has errors, either in the challenge itself or in the solution.

This is frustrating as all the time trying to solve the challenge is wasted.
There is also the score to consider which is now almost down to 100, so the incentive to be quick with a solution is lost as well.

I hope the MTC3-team will be more careful in future, but didn't they promise that a while ago….?

Re: Challenge "Recycled One-Time Pad"  

  By: Veselovský on Nov. 26, 2012, 6:11 p.m.

And also a week to realize that an erroneous challenge has been published seems to be too much.

Re: Challenge "Recycled One-Time Pad"  

  By: Veselovský on Nov. 26, 2012, 11:42 p.m.

Perhaps the author will explain to us the method he used to solve the challenge even without correct ciphertexts ;-)

Re: Challenge "Recycled One-Time Pad"  

  By: aurelie on Nov. 27, 2012, 2:27 p.m.

I checked the ciphertexts before we published the challenge for typos but the problem that we have here is that, in the first version, the one-time pad key did not have the same length as the sentences. That's why the ciphertext of the 2nd and 3rd sentence changed completely.

We apologize for the inconvenience and will fix the pdfs as soon as possible!

Re: Challenge "Recycled One-Time Pad"  

  By: Yokozuna on Nov. 28, 2012, 8:35 a.m.

This shows that only checking for typos is not enough. In this case some simple additional checks (for instance decrypting each ciphertext separately) would have revealed the error immediately. Such an additional check takes only a few minutes but could have saved many hours of work spent in vain by the solvers, which is very annoying as has been already said.
Moreover I do not understand why it takes so long time to fix the pdf. There still may be people who try to solve this challenge with the wrong ciphertexts.
Without the error this would have been a very nice challenge and we have to be thankful to the people, who undertake many efforts to give us such beautiful challenges, but such errors spoils their efforts. Thats a great pity.
Therefor I hope, that the challenges will be checked more carefully in the future before publishing them.

Re: Challenge "Recycled One-Time Pad"  

  By: Bart13 on Nov. 28, 2012, 6:22 p.m.

I checked the ciphertexts before we published the challenge for typos but the problem that we have here is that, in the first version, the one-time pad key did not have the same length as the sentences. That's why the ciphertext of the 2nd and 3rd sentence changed completely.

The 'new' ciphertexts, with length 30, were encrypted with a key with length 30.
That's probably what the author intended.

But the published ciphertexts were encrypted with a key with length 29.
The same key as the intended one, but with the 26th character missing.

That caused a wrap-around where the last character of the first message was encrypted with the first character of the key and the first character of the second message with the second character of the key.
At the end of the second message the same happens again, but now with an offset of 2.

So what we where trying to solve was a Vigenère Cipher with key length 29.
Not impossible at all, just a little bit more work trying to figure out the length of the key to compensate for the offset.
This would have been a nice challenge in itself, but perhaps this was already Part 2 of the challenge…..

So it seems a Vigenèr Cipher is not only susceptible to a frequency analysis attack with a short key, but also to a crib attack if the key is long, but less than half the length of the text.

Re: Challenge "Recycled One-Time Pad"  

  By: Veselovský on Nov. 28, 2012, 8:24 p.m.

Bart,

and now everything is as it should be. :-D
Since now combining the first triple of ciphertexts with second triple of ciphertexts with additional information you gave above users only need to check 26 different keys to solve the challenge.
And this final information makes it a challenge that perfectly fits into level 1 with its difficulty.

Finally everything is fine :-)
Except that users that tried the challenge from the time it was published needed more effort to solve it than the later solvers.

I hope all the posts remain as they are now as users can learn a lot from it, so please do not edit.

Re: Challenge "Recycled One-Time Pad"  

  By: Bart13 on Nov. 28, 2012, 8:48 p.m.

I must admit that it was only curious to find out what went wrong with this challlenge.
And when I did, I didn't realise that posting my findings would simplify things (too much).
But then, … if you can understand all of the above you would also be capable of solving the challenge as it is.
You're right if you say that (new) users can learn a lot from it.

All this reminded me of the German radio operator who made the mistake of sending the same message twice with different settings:

In December 1941, before the change had been made official (from using three rotors to four), a U-boat sent a message using the four-rotor machine. To compound the mistake, the same message was retransmitted using only three rotors. From this seemingly innocuous error, the cryptanalysts at BP (Bletchley Park) determined the wiring of the fourth rotor.

Re: Challenge "Recycled One-Time Pad"  

  By: Javex on Nov. 30, 2012, 1:08 a.m.

The challenge files have been updated and now the ciphertexts and key are as intended.

Sorry for the inconvenience. We also experienced some technical issues which is why this update took so long.

Regards,
Your MTC3 Team

Re: Challenge "Recycled One-Time Pad"  

  By: kelp on Nov. 30, 2012, 6:42 p.m.

Hey,

I'm new to this, so please excuse me if my question seems too amateurish! I guess I really have two questions.

  1. Since in M1 and M3, we have the characters "E" and "R" repeated in position 2 and 3, we know that these two letters are the same in the plaintext. So, would this be the way to guess a crib? That is, we can try and come up with two-letter combinations that would occupy the second and third positions in the word of a plaintext. And we'd obviously additionally know that we'd have to find combinations where the first letter is different.

EDIT: I guess we'd also know that the first letter of the first word would have to be separated by one? Since "P" is in M1 and "O" is in M3?

  1. Following from this, though, I'm still confused. We'd know, maybe, the first word. But if the key is truly random (can I assume this, if it's an OTP?), then, it wouldn't really help, since we'd have to figure out the next word, and so on. You could continue to guess at cribs, I guess?

Any helpful hints? I appreciate your thoughts, thank you!

Re: Challenge "Recycled One-Time Pad"  

  By: Bart13 on Dec. 1, 2012, 2:22 a.m.

Hello Kelp,

there are no amateurish questions, just amateurish answers…

  1. Your idea is ok but unpractical. There are thousands of words that have the same letters at the 2nd and 3rd position. Trying out all combinations would be too much work.

  2. You're assuming that both words have the same length. They probably are not!

Sometimes it's a good idea to replicate the challenge with your own plaintext and cipher.
That way you can test the program you want to use on the challenge.
It also gives you more insight in the problem.

So, make your own three senctences and encrypt them with a otp.
Then try 'guessing' one of the words and see what happens in the other two sentences….

Re: Challenge "Recycled One-Time Pad"  

  By: helmi on Dec. 14, 2012, 3:51 p.m.

It seems a bit strange to me that the author of this challenge is awarded 200 points as well
as the top position in the hall of fame for solving (or testing) his own challenge, whereas the
rest of us did not have any chance to get more than 114 points.

This makes no sense to me and I think the MTC3 team should care about it.

Moreover, those who try to solve the challenges without help from the forum (that's what
I did too, when I was a newbie here) are even worse off…

Well, I for myself pretty soon recognized how useful this forum is and I like to thank
everyone who contributes to it with his posts to make it such a success.


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