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 Post subject: Challenge "Beaver Code"
PostPosted: Mon Aug 13, 2012 20:15 
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Joined: Wed Sep 30, 2009 14:10
Posts: 241
The Beaver Code is a transposition cipher. Apart from the solution of this challenge, can you find a formula for any given ciphertext length, which generates the permutation needed to decrypt the ciphertext?
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Last edited by admin on Wed Jul 31, 2013 15:14, edited 1 time in total.

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 Post subject: Re: Challenge
PostPosted: Mon Aug 13, 2012 23:54 

Joined: Wed Jul 27, 2011 17:13
Posts: 238
Quote:
We would like to know whether there is an algorithm that can
generate the permutation which is needed to decrypt the ciphertext
for messages of any given length.
Why it shouldn't?

To code a message we need s steps
Image
(n is the length of message, || is the ceiling function)

2^(k-1) divisions are needed in each k-th step, so then

Total number of divisions for the whole coding process is exactly:
Image

Same number of steps and divisions is needed for the decoding process.

This is of course the most straightforward algorithm so a better question would be if there is a more efficient/better/faster algorithm.


Attachments:
File comment: total number of divisions
bea2.png
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File comment: steps
bea.png
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 Post subject: Re: Challenge
PostPosted: Wed Aug 15, 2012 02:06 

Joined: Wed Jun 15, 2011 03:29
Posts: 1
Quote:
Uns würde interessieren, ob es eine Formel gibt, die für alle
Text-Längen direkt die Permutation erzeugen kann, mit der man
aus einem Geheimtext den entsprechenden Klartext erzeugen kann.

The following is a more or less handy description of the permutation:

If the length L of the text is a power of two, say L=2^n, then the encryption permutation P is given by bit reversal P=BR_n, in particular it is an involution then. For general L choose n with 2^n >= L. Then P(i)=BR_n(k_i) where k_i is the (i+1)-th element in 0,1,...,2^n-1 such that BR_n(k_i) < L.

[ Encryption: (a_0,...,a_{L-1}) -> (a_{P(0)},...,a_{P(L-1)}) ]


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 Post subject: Re: Challenge
PostPosted: Wed Aug 15, 2012 03:07 

Joined: Wed Jul 27, 2011 17:13
Posts: 238
:-)
It seems that the English version of the challenge is not an exact translation of the German one.
In the appendix of English version they asked for an algorithm that can generate the permutation and in German one they asked for a formula.


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 Post subject: Re: Challenge
PostPosted: Wed Aug 15, 2012 05:24 

Joined: Tue Jun 14, 2011 00:18
Posts: 75
Quote:
We would like to know whether there is an algorithm that can
generate the permutation which is needed to decrypt the ciphertext
for messages of any given length.


Yes.


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 Post subject: Re: Challenge
PostPosted: Wed Aug 22, 2012 06:45 

Joined: Tue Mar 01, 2011 11:56
Posts: 53
Veselovský wrote:
:-)
It seems that the English version of the challenge is not an exact translation of the German one.
In the appendix of English version they asked for an algorithm that can generate the permutation and in German one they asked for a formula.


Indeed, the English translation was imprecise. We are interested in a formula to solve this transpostion cipher.
A new version of the pdf file is going to be uploaded within the next days.


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 Post subject: Re: Challenge
PostPosted: Sun Oct 21, 2012 17:01 

Joined: Mon Dec 05, 2011 19:43
Posts: 1
Im pretty sure ive decoded the cipher but everytime i enter the answer it doesnt work. what should i do?


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 Post subject: Re: Challenge
PostPosted: Sun Oct 21, 2012 23:56 

Joined: Fri Dec 18, 2009 23:31
Posts: 167
barr306 wrote:
Im pretty sure ive decoded the cipher but everytime i enter the answer it doesnt work. what should i do?
Hello, According to the log you solved this challenge already (18:22). Could you please add a comment, how you was able to circumvent the problem? Thanks, be


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 Post subject: Re: Challenge
PostPosted: Wed Nov 14, 2012 20:23 

Joined: Sun Nov 11, 2012 16:33
Posts: 2
hello,
little help please :)
so if i got 10 letters of the alphabet

a b c d e f g h i j
it will go like this ?
acegi | bdfhj
aei | cg | bfj dh
ai e |cg |bj f |dh

==> aiecgbjfdh ?


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 Post subject: Re: Challenge
PostPosted: Thu Nov 15, 2012 12:57 

Joined: Tue Mar 01, 2011 11:56
Posts: 53
Hello i2gh0st,

i2gh0st wrote:
a b c d e f g h i j
it will go like this ?
acegi | bdfhj
aei | cg | bfj dh
ai e |cg |bj f |dh
==> aiecgbjfdh ?


yes, you're right. There is also an example on slide 3/6.

Good luck!


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