Perfect secrecy

Definition

An encryption scheme is perfectly secret if for all distributions/random variables $\mathbf{M}$ over $\mathcal{M}$, $\forall m \in \mathcal{M}$, $\forall c \in \operatorname{supp}\mathcal{C}$ where $\mathcal{C} = \operatorname{Enc}_\mathbf{K}(\mathbf{M})$, $\mathbf{K}$ uniform over $\mathcal{K}$, $$\Pr[M=m] = \Pr[M=m | C = c]$$ (a posteriori, a priori)

References

1. https://www.khoury.northeastern.edu/home/wichs/class/crypto-fall17/lecture1.pdf
2. https://people.cs.uchicago.edu/~davidcash/284-autumn-19/03-perfect-secrecy.pdf