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The "security" of an encryption scheme is commonly defined through a so called indistinguishability games, i.e. the attacker picks two messages of same length. You pick one of those two at random, encrypt it, and give it to the attacker. If the scheme is "secure", then the attacker's advantage of guessing which messages was encrypted should not be ...


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Does re-encrypting the same value with multiple keys reduce security? The answer is "it depends"; there are some attack models and encryption methods where the security is reduced, there are other cases where there appears to be no security reduction. Let us go through some models where we actually see a security reduction: Plaintext guessing attack ...


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If by "encrypted" you mean generating a keystream, then what you propose is to use in the CTR mode $$ C_i = P_i \oplus F_K(IV||i) $$ the following function $F$: $$ F_{K_1||K_2||K_3} = E_{K_1}\oplus E_{K_2} \oplus E_{K_3}. $$ This is secure as long as you ensure that for each key all the used IVs are different (i.e. are nonces). As mentioned in another ...



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