Definition
An encryption scheme $\Pi = (\Gen, \Enc, \Dec)$ is IND-CPA secure if for all PPT adversaries $A:=(A_1, A_2)$, $\Game_{\Pi, A}^{\CPA}( \lambda, 0) \approx_c \Game^{\CPA}_{\Pi, A}(\lambda, 1)$, where the game is the following:
\[\begin{aligned} &&& \underline{\Game_{\Pi, A}^{\CPA}( \lambda, b)}: \\ & \\ &1. && (pk, sk) \samples \Gen(1^\lambda) \\ &2. && (m_0, m_1, z) \samples A_1(pk) \\ &3. && c \samples \Enc(pk, m_b) \\ &4. && b' \samples A_2(c, z) \end{aligned}\]