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| jdege | Apr 21 2008, 12:13 PM |
Elite member
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We're getting there. Yes, simple substitution yields a cipher that is simple to break. But using a fixed substitution yields a cipher that can simply be read. Your "Catastrophe" cipher has some interesting ideas. You're layering a number of different cipher approaches on top of each other, and if you were implementing each layer in a fairly secure way, you'd have a quite secure result. To break the cipher, you'd need to attack each level individually, and if the underlying layer had any kind of strength, it'd be very difficult to determine when you had a proper solution to the top layer. So if you're substitution layer is done well, figuring out when you've stripped the additive layer would be difficult. So, next step. You're currently using two substitution alphabets. Why stop there? Why not have three? Five? A variable number? The standard Vigenere cipher uses multiple alphabets, one for each letter of the keyword, each of those alphabets being the standard alphabet shifted by the value of the letter in the keyword. A mixed-alphabet Vigenre does the same, only it shifts a mixed alphabet rather than the standard. The standard Vig is considered more secure the longer the keyword is, using a mixed alphabet is about the same as adding five more characters to the keyword. Cracking a standard Vig isn't hard. It's actually easier than cracking a simple substitution. Cracking a mixed alphabet Vig involves figuring out how long the key is, then shifting the various alphabets against each other until they line up, which then gives you a simple substitution to solve. But the point isn't that the Vig can't be cracked, but that it hides the statistical qualities of the plaintext far better than does a simple substitution, and the longer the key the better it does so, which makes the task of recognizing when you've stripped off the additives correctly far more difficult. The Incidence of Coincidence of a standard Vig with keylength 10 or greater, or a mixed-alphabet Vig with keylength five or greater, is indistinguishable from random text. So, when you're trying to strip off the additives, you don't have an easy way of recognizing that the result is correct. |
| When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl. | |
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| My New Cipher · Challenges | |
6:12 PM Nov 27
