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How we can measure key space in transposition algorithms? Should we specify the method, like rail fence?

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Measuring the size of the key space in transposition algorithms is not important, because their security is far less than the size of the key space would suggest. Therefore, any measure of effective key length will be misleading and will not give an accurate picture of the true security of the scheme.

In general the standard way to compute the effective key length of a cryptographic algorithm is as follows: we let $K$ denote the set of all possible keys for that algorithm; then the effective key length is $\lg |K|$, where $|K|$ denotes the size of the set $K$ and $\lg$ denotes the logarithm to base 2.

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Thanks for your answer. What about measuring the time complexity of exhaustive key search in these algorithms? –  S Kh Mar 18 '13 at 7:36
    
Glad you found it helpful, @SKh! If it answered your question, please feel free to click the checkmark under the score to mark it as your accepted answer. As for time complexity, that's a different question and should be asked in a separate question, to keep things clean. –  D.W. Mar 18 '13 at 16:07
    
Ok. But can you please answer this question, too? Thanks. –  S Kh Mar 18 '13 at 17:20
    
Hi @SKh, Thanks for your interest. For your question about time complexity, you should post a separate question about that, if you want an answer to it. I'm afraid I'm not going to answer that question here. I'm not trying to be rude; it's just how this site works. Please read the FAQ for an explanation of why -- this is a question-and-answer site that attempts to provide archival-quality answers (one single, specific question per page). It is not a general discussion site. –  D.W. Mar 18 '13 at 23:07
    
Hi @D.W. Ok, Thanks. –  S Kh Mar 19 '13 at 7:26

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