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Running Key Challenge
Topic Started: Dec 4 2007, 02:28 PM (376 Views)
jdege Dec 4 2007, 02:28 PM Post #1
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 I've been musing about running key ciphers, in particular Vigenere's original add-the-plaintext-shifted auto-key variant:

Code:
 
attackatdawn
xyzattackatd
------------
xrsavdavnapq


I noticed something that I expect everybody already knew, but it was new to me.

But I thought I'd kick off the discussion with a simple challenge:

Quote:
 
tikvm lapwe again wshyi rngwf qbwph


(It's an easily-broken system, so you shouldn't need much.)
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.
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Revelation Dec 9 2007, 04:24 PM Post #2
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 That was a cool challenge!

My method:

After doing some reading on the Vigenere autokey, I thought I knew what to do. First I swept the text for "the", yielding 'his' and 'did' as the two best options.

Then I tried filling up the beginning with abc, and that gave me 'this'. Then I knew that my 'his' was 'this'. This also gave away the shift. After getting this critical information, the rest was easy :)


The result:
thisisthewinterofourdiscontent

and the key:
abcdethisisthewinterofourdisco
RRRREJMEEEEEPVKLWENFNVJKEEEEEAOLKAFKLXCFZAASDJXZTTTTTTTLSIOWJXMOKLAFJNNKFNXN
RAGRBAQEMHIGDJVDSEOXVIYCELFHWLELJFIENXLRATALSJFSLCYTKLASJDKMHGOVOKAJDNMNUITN
RRRRLJVEEEEECLYVYHNVPFTAEEEEEMWLMEIRNGLARWJAKJDFLWNTIERJMIPQWOTZEOCXKNUBNXCN
RJIRPOWEANFUSNCZVDVZNMSFEKLOEPZLDKDJWSAAAAAAAOERHJCTNCKFRIMVKSOFOMKMANREWNBN
RZUDRGXEEEEENFQIDVLQNCKNEEEEEDGLLLLLLAWIOSNCDARLODMTOEJXMILDFJROTKJSDNLVCZNN
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jdege Dec 9 2007, 11:44 PM Post #3
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Dec 9 2007, 10:24 AM
That was a cool challenge!

Seems to me that we've all been too busy to deal with really challenging challenges, lately.

There's a good explanation on breaking autokeys on wikipedia:
http://en.wikipedia.org/wiki/Autokey_cipher
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.
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Revelation Dec 10 2007, 12:07 PM Post #4
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Quote:
 
Seems to me that we've all been too busy to deal with really challenging challenges, lately.


Unfortunately, yes :( It has been a bit quiet here lately...
RRRREJMEEEEEPVKLWENFNVJKEEEEEAOLKAFKLXCFZAASDJXZTTTTTTTLSIOWJXMOKLAFJNNKFNXN
RAGRBAQEMHIGDJVDSEOXVIYCELFHWLELJFIENXLRATALSJFSLCYTKLASJDKMHGOVOKAJDNMNUITN
RRRRLJVEEEEECLYVYHNVPFTAEEEEEMWLMEIRNGLARWJAKJDFLWNTIERJMIPQWOTZEOCXKNUBNXCN
RJIRPOWEANFUSNCZVDVZNMSFEKLOEPZLDKDJWSAAAAAAAOERHJCTNCKFRIMVKSOFOMKMANREWNBN
RZUDRGXEEEEENFQIDVLQNCKNEEEEEDGLLLLLLAWIOSNCDARLODMTOEJXMILDFJROTKJSDNLVCZNN
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