Timeline for What is the limit of plaintext required to break the Vigenère encryption?
Current License: CC BY-SA 3.0
9 events
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| Jun 16, 2019 at 23:41 | comment | added | mdittmer | Perhaps this is what I'm getting at: We (the holder of the key) can know how much plaintext an attacker needs to encrypt to recover the key. What's interesting to me is that we know the attacker has recovered the entire key, but the attacker does not. (It's easy to construct a key sufficiently long that would refute the attacker's claim "I, with certainty, have recovered the entire key".) | |
| Jun 16, 2019 at 23:37 | comment | added | mdittmer | Thanks for the link. It provides a solid explanation of how to use index of coincidence.I'm not sure the method solves the problem I have in mind, though. The problem arises from the fact that any, say, 30-letter ciphertext where K=ABC{repeated 10 times}{100 characters selected uniformly at random} is identical to a 30-letter ciphertext where K=ABC{repeated 10 times}. | |
| Jun 10, 2019 at 18:16 | comment | added | John Deters | @mdittmer , that’s what the index of coincidence is supposed to reveal. Check out this paper for more info: nku.edu/~christensen/1402%20Friedman%20test%202.pdf | |
| Jun 10, 2019 at 15:14 | comment | added | mdittmer | That's true. I guess what I'm trying to point out is that not knowing the length of the key poses a problem for the attacker. If, instead, the key is ABC{repeated 10 times}{100 characters selected uniformly at random}, the attacker may be confident after processing a ciphertext of length 30, but will be unpleasantly surprised when faced with the task of decrypting a 130-character ciphertext. (Obviously, I've chosen a particularly diabolical example.) | |
| Jun 9, 2019 at 14:45 | comment | added | John Deters | @mdittmer , let’s say the key was ABCABCABX. Assuming average distributions, you would see two very strong correlations and one would be weaker, but you’d clearly accept it was a length 3 key of ABC. The reveal would come when you decrypt, and every 9th character would be wrong. However, that’s plenty of correct context to figure out the right key. | |
| Jun 8, 2019 at 21:18 | comment | added | mdittmer | You mention "you will spot the repeating sequence after the second repeat." While true, how can the attacker be certain that the key doesn't contain repetitions followed by a deviation from the repeated pattern? | |
| Nov 1, 2013 at 14:18 | vote | accept | Bush | ||
| Oct 28, 2013 at 13:57 | history | edited | e-sushi | CC BY-SA 3.0 |
Mini-typo correction.
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| Oct 28, 2013 at 0:43 | history | answered | John Deters | CC BY-SA 3.0 |