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Let's suppose I encrypt something with a stream cipher (CSPRNG) two times using two 256-bit keys and different IVs.

Will I get 512-bit encryption strength?

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    $\begingroup$ You're right. The meet in the middle attack will apply and you can attack with about 512-bits of matched plain and cipher (and a whole lot of memory) using a little over 256-bits of work. $\endgroup$ – Daniel Shiu Apr 19 at 5:59
  • $\begingroup$ Nobody needs 512 bits of security (or 256 bits of security against a quantum computer) as it is far outside analysis / brute-forcing. What you may need is to remain at 256 bits of security if one of the ciphers is broken. In that case using the same cipher twice may not be the way to go - even if it would provide 512 bits of security without a feasible attack. $\endgroup$ – Maarten Bodewes Apr 19 at 9:43

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