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The original CPA game:

  1. The adversary gives two arbitrary messages $m_0$ and $m_1$ to a challenger;
  2. The challenger encrypts an arbitrary message $m_u$, $u∈\{0,1\}$ , and sends the ciphertext to the adversary;
  3. The adversary outputs the guess $u$.

Now I give a revised version of the above CPA game:

  1. The adversary gives two arbitrary sets of messages $M_0$ and $M_1$ to a challenger, where $|M_0|=|M_1|$, and each pair of counterparts are of equal size;
  2. The challenger encrypts an arbitrary set of message $M_u$, $u∈\{0,1\}$, and sends the ciphertexts to the adversary;
  3. The adversary outputs the guess $u$.

If a public-key encryption scheme is CPA-secure is it also secure in this revised version of CPA game?

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