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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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What is the functional role they play? They exist to identify the SA. That is, when an IKE implementation receives an IKE packet, it is able to use the cookies to identify the SA that the packet corresponds to. Why not simply use an SPI value like IPsec does? Why do you think there is some fundamental difference between the cookies that IKE uses, ...



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