unconditional security

an information theoretic definition of security.

An encryption scheme over message space $\mathcal{M}$ is perfectly secret if for every probability distribution over $\mathcal{M}$, message $M$ and ciphertext $C$ with $P(C)>0$, we have $P(M|C)=P(M).$

this express that knowing the ciphertext leads to no new information relative to the priors over $M$. even with unlimited computational power, no new information exists to be had.