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| jdege | Apr 16 2008, 04:11 AM |
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I'd call it a progressive polyalphabetic substitution. You're using substitution on alternating letters, using two different alphabets, then adding a fixed number sequence to it. The trick of reversing the text is arguably a transposition, but it's such a trivial one as to hardly be worth the effort. I don't want to discourage you, playing around with new ideas is always worth the effort, but most of your primitives seem to do more to add to the work of encoding and decoding, than to making the systems harder to break. It's easy to design a complex system, but you only want to add complexity when it adds to security, otherwise you're making the lives of your cipher clerks more difficult for no gain. I mentioned the Affine because you seemed focused on systems that involve arithmetic manipulation of the ciphertext, and the Affine is the exemplar of those. One of the difficulties of your system is that in a lenthy message the numbers will grow very large, making the ciphertext much larger than the plaintext. Most of the classical ciphers did their math mod-26, to avoid that, and to increase security - the distribution of the modulo sum of two integers is flat, the ordinary sum is not. The simplest mod-26 cipher is the Caesar shift - every letter of the plaintext is treated as a number between 0 and 25. to which is added mod-26 a constant between 0 and 25. The result is converted back to a letter. This is a pointlessly simple cipher, with only 26 keys. An alternative is to multiply each letter of the plaintext by a constant. Also simple, but with even fewer possible keys. You can only multiply by the numbers for which there are inverses mod-26, which means numbers between 0 and 25 that are relatively prime to 26. Of which there are 12 - the odd numbers other than 13. But suppose you did both - multiply by one number and then add another? That's the Affine cipher. With 12*26 = 312 keys. Still nothing more than a toy, but an interesting exercise. The next step, I suppose, would be the Hill cipher. Hill proposed using matrix multiplication. Using, for example, a 2x2 matrix - containing four numbers, he'd take pairs of letters from the plaintext, treat them as a 2 element vector, multiply that vector by the matrix, and use the resulting vector as the ciphertext. Still no more than a schoolbook exercise, easy to break. But what was fascinating about the Hill cipher was that it could be expanded to use 3x3 matrices, 4x4 matrices, atc. As the Playfair was the first cipher to operate on digraphs, instead of on single characters, the Hill cipher was the first to operate on more than two letters at the time. |
| When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl. | |
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| My New Cipher · Challenges | |
11:22 AM Nov 25
