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Running Key cipher
Topic Started: Mar 29 2014, 02:14 PM (246 Views)
novice
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A very simple cipher is the Running Key. Alice writes out the message and below it writes some text that she has agreed with Bob as the key. Then she adds the two texts to get the ciphertext:

Pt: thereisataverninthetown
Ky: niceguysneverfinishlast
Ct: GPGVKCQSGEQIISQABZLEOOG

which she sends to Bob. Bob subtracts the key and gets the message.

Admittedly the cipher can be solved by Eve if she has one or more probable words. It's not an easy task if the texts are a bit obscure -- as I can say with feeling, having just solved such a cipher published by the ACA after expending quite some energy.

But what if Alice and Bob are Navajo Indians? Eve might then be completely stymied, as were the Japanese during WW2 when confronted by the 'Navajo Talkers'.

On the other hand if Eve is bright, couldn't she drag dictionary words of the foreign language through the ciphertext and look out for recognisable plain words in that language? It would be rather boring by hand but 70 years after WW2 couldn't she get her computer to help out?

So maybe something along these lines would provide a neat challenge for this Forum. Watch this space!
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jdege
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If you're getting your running key from a book, some books are better than others:

http://www.amazon.com/dp/0833030477

(I've always loved the customer reviews on this one ...)
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.
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novice
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The reviews themselves strike me as better source material than the random numbers (and a lot cheaper ;)
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mok-kong shen
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The random material of Rand Corp. is very well-known (but very old). More ecoomical than buying the book is to download digits of math constants (at no cost).
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JOE.TEKK1
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Truly, this topic is thought provoking. How about using the words to your National Anthem.

Interesting concepts.

Joe.

// Saturday, 29 March 2014; 13.34HRS //
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mok-kong shen
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English language texts are estimated to have 0.6 - 1.6 bits of entropy per character. To get better key streams in this sense one could combine several text streams but of course at the cost (and inconvenience) of corresponding computer processing. (An attempt of mine in that direction is http://s13.zetaboards.com/Crypto/topic/6936496/1/.)
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jdege
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mok-kong shen
Mar 29 2014, 03:51 PM
The random material of Rand Corp. is very well-known (but very old). More ecoomical than buying the book is to download digits of math constants (at no cost).
You can't leave the downloaded file sitting out on your coffee table.
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.
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mok-kong shen
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jdege
Mar 30 2014, 01:34 AM
You can't leave the downloaded file sitting out on your coffee table.
True to some extent, I suppose. Having a smartphone would provide a possiblity. (I don't know exactly, for my cellphone is old-fashioned.)
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