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The idea you have is: lets say we have a known plaintext/ciphertext pair; can we use a precomputed table to speed up the recovery of the key? Well, the main problem with that precomputed tables don't actually speed up the search time, if you count the time taken to generate the precomputed table. What the table precomputation does is (for example) generate ...

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For simple XOR-based encryption algorithms such as OTP, the key size must be the same as the message size. If you choose a smaller key and try to divide the message into chunks, you would not have a perfectly secure scheme anymore. Now, since you tagged java, I'm assuming that this increase in time for smaller key sizes is due to the code trying to divide ...

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Yes, we need symmetric cryptosystems, for many reasons; to give three of these: We need a hash function to make most asymmetric cryptosystems secure (e.g. we simply do not have a secure signature system based on RSA without a hash), and current hash functions are (or are built from) symmetric cryptosystems. All asymmetric encryption cryptosystems are bound ...

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Scroll to the end for tl;dr. Regarding your contrived example: Alice doesn't have Bob's keypair, but sends a message in such a way that only Bob can read it, eg. puts it in a dead-drop. So she takes out her pen and writes Hey Bob, could you sign and send me $X$ along with your public key? Here's mine: $P_A$ Signed, Alice Bob has no way of knowing if ...

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