Definition and meaning of "semantic security"
What if it could? What does this definition mean in practice?


Consider $M_0=$ attack and $M_1=$ don't attack. If the adversary can distinguish which message you are sending to your troops, they can optimize their strategy to defeat you.

Another example. Say you are casting a yes ($1$) no ($0$) vote for a proposed piece of legislation. If the adversary can distinguish between yes and no votes, they might be able to throw the election (by discarding votes that don't go their way) or persecute you according to your vote.