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I understand that if a block cipher has $k$-bit keys and $n$-bit input/output blocks, then if $k>n$, we can expect one message-ciphertext pair to narrow us down (I think?) to $2^{k−n}$ possible keys, right? That is approximately correct (if the block cipher with the wrong key acts like a random permutation; this is generally a safe assumption); if ...


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You're missing a component : a padding convention. Yes, if you're trying to reduce a block size, it will reduce the cipher strength. That's why the less-sized blocks are padded/filled to fit the exact size. What to do : pad or fill or both - that is a question. First you need to understand, that the more predictible the message, the less secure the ...



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